KR100673987B1 - Sulfur electrode using multiwalled carbon nanotubes and graphitic nanofibes for lithium/sulfur battery - Google Patents

Abstract

The present invention is a cathode additive material of carbon nanotubes (MWNT) and carbon nanofibers (graphitic nanofibes, GNF) manufactured directly by thermal CVD as an additive to improve cycle characteristics of lithium / sulfur secondary batteries. The present invention relates to a lithium / sulfur secondary battery having improved cycle characteristics at room temperature by using a furnace conductive material. The sulfur electrode of the present invention is added with an existing conductive material and MWNT or GNF, which are manufactured directly, in a weight ratio of 20: 5. By improving the surface area of the electrode active material to improve the cycle characteristics of the lithium / sulfur secondary battery.

In order to improve the cycle characteristics of the lithium / sulfur secondary battery of the present invention configured as described above, the sulfur electrode using MWNT and GNF as an additive is a sulfur-based MWNT and GNF additive manufactured by thermal CVD. Evenly dispersed in the anode to form a sulfur electrode having a structurally stable and high specific surface area, the sulfur electrode thus produced has a high initial capacity and improved cycle life can maximize the efficiency of the sulfur battery.

Sulfur Battery, MWNT, GNF, Conductive Material, Additive

Description

Sulfur electrode using multiwalled carbon nanotubes and graphitic nanofibes for lithium / sulfur battery as additives to improve cycle characteristics of lithium / sulfur secondary batteries

1 is a graph illustrating a discharge test result of a lithium / sulfur secondary battery,

2 is a photograph of a thermal CVD apparatus for manufacturing carbon nanotubes (MWNT) and carbon nanofibers (graphitic nanofibes, GNF) used according to the present invention.

FIG. 3 is a photograph of the surface shape of acetylene black (AB), which is a conductive material, and MWNT and GNF prepared by thermal CVD using a scanning electron microscope (SEM).

4 is a photograph of observing the surface shape of an electrode using only acetylene black as a conductive material and the electrode added with MWNT and GNF according to the present invention with a scanning electron microscope,

5 is a cross-sectional view of a lithium / sulfur secondary battery configured to investigate the performance of the manufactured sulfur electrode.

*** Explanation of symbols for the main parts of the drawing ***

a --- current collector of positive electrode b --- sulfur electrode

c --- liquid electrolyte d --- separator

e --- lithium metal as negative electrode f --- current collector of negative electrode

The present invention relates to a sulfur electrode using carbon nanotubes (MWNT) and carbon nanofibers (graphitic nanofibes GNF) as additives in order to improve cycle characteristics of a lithium / sulfur secondary battery. The present invention relates to the production of a lithium / sulfur secondary battery having improved cycle characteristics at room temperature by using MWNT and GNF prepared directly by CVD as a cathode additive together with a conductive material.

Recently, due to the rapid development in the fields of electricity, electronics and information communication, the demand for small portable devices that can realize high performance, light weight, and multifunctionality such as electronic devices such as camcorders, mobile phones, laptops, etc. is explosively increasing. As the necessity of automobiles emerges, the demand for improving the performance of secondary batteries used as a power source of these products is also greatly increased.

Lithium batteries using lithium as a negative electrode as one of secondary batteries can realize high voltage and high energy density due to the low density of lithium and the standard reduction potential. However, when lithium metal was used as the negative electrode, there was a risk of explosion due to battery shortage due to the formation of dendrite during charging and discharging. A lithium ion secondary battery for solving this problem is a battery system using an intercalation reaction of lithium ions using a lithium transition metal oxide as a cathode material instead of a carbon material instead of lithium metal. Currently, the cathode active material used in commercially available lithium ion batteries is LiCoO _2, and many studies have recently been conducted to replace LiNiO ₂ , LiMn ₂ O ₄ , V ₂ O ₅ , but the theoretical energy density is low. There is a limit to increasing the density. Therefore, in order to develop a battery of high energy density, research on a new electrode material having a high theoretical energy density is necessary.

The electrode material in the spotlight is elemental sulfur. Sulfur is an environmentally friendly and inexpensive material, which makes it suitable for high energy density batteries.

The lithium / sulfur secondary battery (Li / S) shows a theoretical capacity of 1675 mAh / g-sulfur (2600 Wh / kg) per active material, assuming that sulfur (Elemental sulfur, S ₈ ) completely reacts to Li ₂ S, a discharge product. . Due to these high theoretical capacities, many studies have been conducted on Li / S batteries, but the abrupt capacity reduction caused by the degeneration of sulfur electrodes generated during repeated charge and discharge cycles remains the biggest problem that degrades the performance of the batteries. .

In order to solve this problem, carbon nanotubes and carbon nanofibers are added to the electrode to prevent structural collapse of the electrode and to provide conductivity to sulfur as an active material. Nano-size carbon materials are receiving the most attention as core materials in the 21st century, and carbon nanotubes included in nanotechnology (NT) use the electric discharge method by Dr. Iijima of Japan Electric Company (NEC), who was studying new materials in 1991. The mass of carbon formed on the graphite negative electrode was first discovered by TEM analysis. Due to its excellent properties and structure, it is expected to be applied to the fields of electronic information communication, environment, energy and medicine, and in particular, it is expected to be widely applied in the next generation electronic information industry. In order to secure the competitiveness of materials, we are conducting extensive research on the synthesis application of carbon nanotubes with national support.

On the other hand, Han Sang-chul of the Korea Advanced Institute of Science and Technology has recently conducted studies to improve the cycle characteristics of lithium / sulfur secondary batteries by adding MWNTs in sulfur electrodes to stably form a base of carbon material as a conductive material. The cycle characteristics of did not show satisfactory levels.

Therefore, the present inventors, while diligently researching that the cycle characteristics of the lithium / sulfur secondary battery can exhibit satisfactory levels, the optimum of acetylene black widely used as the conductive material, carbon nanotubes and carbon nanofibers as additives The composition of the present invention was completed by improving the cycle characteristics of the lithium / sulfur secondary battery by finding the composition ratio of and increasing the surface area of the electrode active material.

Accordingly, an object of the present invention is to construct an acetylene black widely used as a conductive material, carbon nanotubes and carbon nanofibers with an optimum composition ratio, thereby improving the cycle characteristics by widening the surface area of the electrode active material and improving sulfur characteristics. It is for providing an electrode.

In general, in the case of a lithium / sulfur secondary battery, since a non-conductive sulfur is used as a positive electrode active material, in order to effectively provide conductivity, a carbon material as a conductive material should be uniform and distributed around the sulfur. However, in the case of widely used acetylene black, when the active material is agglomerated during charging / discharging, spherical fine acetylene black is also agglomerated, thereby making it difficult to impart conductivity to the active material at the same time.

Accordingly, the present invention improves the conductivity and mechanical strength of the sulfur electrode by adding acetylene black, MWNT, and GNF, which are widely used as conductive materials, to the sulfur electrode for lithium / sulfur secondary battery to improve the cycle characteristics of the lithium / sulfur secondary battery. Is to improve.

Sulfur electrode using MWNT and GNF as an additive to improve the cycle characteristics of the lithium / sulfur secondary battery for achieving the object of the present invention;

MWNT or GNF, which is an additive prepared directly with the existing conductive material, is added in a weight ratio of 20: 5 to increase the surface area of the electrode active material, thereby improving cycle characteristics of a lithium / sulfur secondary battery.

According to another configuration of the invention, the sulfur electrode is characterized in that consisting of S, NaS, MnS, FeS, FeS _2, NiS, CuS or a mixture thereof.

According to another configuration of the invention, the production of the sulfur electrode is characterized in that it is carried out by a magnetic stirring method, an atlit ball mill or a mixture of the two methods.

According to another configuration of the present invention, there is provided a sulfur battery which uses the sulfur electrode according to the present invention and consists of a metal of Li, Na, or a mixture thereof as a negative electrode material.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

MWNT and GNF used as an additive according to the present invention and a sulfur electrode using the same can be prepared as follows.

(1) Preparation of MWNT

Figure 2 is a thermal CVD equipment for manufacturing MWNT and GNF used in the present invention.

The production of MWNT and GNF consists of preparing a catalyst and then synthesizing it. Metal catalysts are prepared by coprecipitation using Fe nitrate and Mo nitrate. Synthesis of MWNT is carried out by charging the prepared metal catalyst into an electric furnace and using a mixture of C ₂ H ₄ gas and H ₂ at an appropriate temperature.

(2) manufacture of GNF

Metal catalysts are prepared by coprecipitation using Cu nitrate and Ni nitrate.

Spiral type GNF is synthesized by charging the prepared metal catalyst into an electric furnace and using a mixture of C ₂ H ₄ gas and H ₂ at a proper temperature.

(3) Preparation of Sulfur Electrode

In the pretreatment process, the raw material powder is vacuum dried, and sulfur, acetylene black, MWNT or GNF, and a weight ratio of the PEO polymer are preferably 60: 20: 5: 15 to prepare a sulfur electrode.

In the manufacturing method, sulfur and acetylene black, MWNT, or GNF among the components of the sulfur electrode were placed in a 30 ml Erlenmeyer flask, and 16 cc of ACN (acetonitrile, 99.5%, Junsei chemical Co.) was added, followed by ultrasonic wave for 1 hour. To disperse. PEO polymer as a binder is added to the dispersed sample and subjected to magnetic stirring for 2 hours. The stirred sample was placed in an attritor jar and ball milled for 2 hours to prepare a uniform slurry. At this time, the ball and the sample is preferably weight ratio 60: 1. After applying the prepared uniform slurry on a glass plate, possible solvents are completely removed at room temperature and vacuum dried at 60 ℃ for about 24 hours. The sulfur electrode thus prepared may be punched into a circular shape of about 0.825 cm ² and applied to a lithium / sulfur secondary battery.

Hereinafter, although an Example demonstrates this invention in more detail, this invention is not limited to this.

Examples 1-2

All raw materials used in this example were pretreated to remove moisture and impurities. MWNT (multi-walled carbon nanotubes) manufactured directly with sulfur (less than 100mesh, Aldrich Co.) used as active material of electrode and acetylene black (Acetylene Black, less than 1μm, more than 99.9%, Alfa Co.) used as conductive material And GNF (graphitic nanofibers) were vacuum dried at 120 ° C for 24 hours. And the binder PEO (polyethylene oxide, Aldrich Co.) was vacuum dried at about 50 ℃ for 24 hours, the solvent ACN (acetonitrile, 99.9%, Aldrich Co.) was not pretreated.

Sulfur electrodes of Examples 1 and 2 using MWNT and GNF as additives, respectively, were prepared as follows.

In the sulfur electrode of Example 1, sulfur, acetylene black, and MWNT, which were pretreated, were placed in a 30 ml Erlenmeyer flask, and 16 cc of ACN was added thereto, followed by dispersion for 1 hour using ultrasonic waves. PEO polymer as a binder was added to the dispersed sample, followed by magnetic stirring for 2 hours.

On the other hand, the sulfur electrode of Example 2 was the same as in Example 1 except for using GNF instead of MWNT.

The self-stirred sample was placed in an attritor and ball milled for 2 hours to produce a uniform slurry. At this time, the ratio of the ball and the sample was 60: 1 weight ratio. A uniform amount of the prepared slurry was coated on a glass plate, and then solvents were completely removed at room temperature, followed by vacuum drying at about 60 ° C. for about 24 hours.

The ratio of sulfur, acetylene black, MWNT, and PEO binder in each of the above examples was 60: 20: 5: 15 (Example 1), and the ratio of sulfur, acetylene black, GNF, and PEO binder was 60: 20: 5: 15 (Example 2) was set.

The electrolyte is a liquid electrolyte obtained by dissolving lithium trifluoromethanesulfonimide (LiTFSI) lithium salt in 1M concentration in a TEGDME (Tetraglyme, tetra (ethylene glycol) dimethyl ether) electrolyte.

The lithium foil used as the negative electrode did not undergo any other treatment.

The sulfur electrode, the liquid electrolyte, and the lithium foil were stacked in the order of the sulfur electrode, the liquid electrolyte, the separator, and the lithium foil in a glove box of an argon atmosphere to prepare a lithium / sulfur secondary battery. The prepared sulfur electrode was applied to a lithium / sulfur secondary battery to perform a charge / discharge experiment at a temperature of 25 ° C. During charging / discharging, the current density was 100 mA / g-sulfur, the end voltage of charging was 3.2V, and the end voltage of discharge was 1.5V.

Comparative example

After pre-treatment of each component in the same manner as above, sulfur and acetylene black, PEO polymer weight ratio of 60: 25: 15 and other manufacturing methods are the same as in Example 1. The prepared sulfur electrode was applied to a lithium / sulfur secondary battery to perform a charge / discharge experiment at a temperature of 25 ° C. During charging / discharging, the current density was 100 mA / g-sulfur, the end voltage of charging was 3.2V, and the end voltage of discharge was 1.5V.

Test Example

The cycle test performed by applying the sulfur electrode manufactured by the method of the said Example and a comparative example to a lithium / sulfur secondary battery is shown. In the case of the lithium / sulfur secondary battery prepared in Example, when the MWNT was used as an additive, the initial discharge amount was about 1200 mAh / g-sulfur, which was about 75% of the theoretical discharge rate, and 50 to 10 mAh / g per cycle at the beginning of the cycle. The capacity reduction of -sulfur occurred, but after 30 cycles, the capacity decrease was reduced, showing a high discharge capacity of about 600mAh / g-sulfur after 50 cycles. In case of lithium / sulfur secondary battery manufactured using GNF as an additive, the initial discharge capacity of 960mAh / g-sulfur was shown. As with the sulfur electrode using MWNT as an additive, a considerable amount of capacity reduction occurred at the beginning of the cycle, but after 30 cycles, the capacity decreases slowly and shows a high discharge capacity of about 500mAh / g-sulfur after 50 cycles. . However, as a result of testing the cycle characteristics of the lithium / sulfur secondary battery using the sulfur electrode manufactured in the comparative example, the initial discharge capacity of 870mAh / g-sulfur was shown, and the capacity decreases steadily during the cycle, which results in 50 cycles. While being shown a capacity of 400mAh / g-sulfur. Thus, it can be seen that the use of MWNT and GNF as additives delays the aggregation of constituents occurring in the sulfur electrode during charging and discharging, and improves cycle characteristics of the lithium / sulfur secondary battery by smoothly supplying electrons to the active material.

In order to improve the cycle characteristics of the lithium / sulfur secondary battery of the present invention configured as described above, the sulfur electrode using MWNT and GNF as an additive is a sulfur-based MWNT and GNF additive manufactured by thermal CVD. Evenly dispersed in the anode to form a sulfur electrode having a structurally stable and high specific surface area, the sulfur electrode thus produced has a high initial capacity and improved cycle life can maximize the efficiency of the sulfur battery.

Claims (6)

Dispersing ultrasonically by mixing carbon nanotubes or carbon nanofibers with acetonitrile as a sulfur cathode active material, an acetylene black conductive material, and an electrically conductive additive; Adding a PEO polymer to the dispersed mixture as a binder and magnetic stirring; Atrium ball milling it to prepare a slurry; And applying the slurry to a glass plate and drying the slurry. delete The method of claim 1, wherein the electrically conductive additive is added in an amount of 20 to 50% by weight based on the weight of the conductive material. The method of claim 1, wherein the cathode active material is S, NaS, MnS, FeS, FeS _2, NiS, CuS or a mixture thereof. delete A sulfur battery using the sulfur electrode prepared by the method of claim 1 and comprising a metal of Li, Na, or a mixture thereof as a negative electrode material.

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