KR20140062202A - Sulfur-doped graphene-based nanosheets for lithium-ion battery anodes - Google Patents

Abstract

The present invention relates to a sulfur-doped graphene nanosheet (S-GNS) for a lithium ion battery and, more specifically, to an S-GNS having an improved electrochemical function by introducing sulfur (elemental sulfur) in graphene oxide (GO) and to a method for producing same. According to the present invention, provided are a graphene nanosheet that is heated after being mixed with GO and sulfur and a method for producing same. The S-GNS of the present invention has excellent capacity and reversibility and an improved electrochemical function by means of a stable periodic lifespan so as to be utilized as an anode material of a lithium ion battery so that a product with improved properties of the secondary battery can be provided.

Description

TECHNICAL FIELD [0001] The present invention relates to a sulfur-doped graphene nanosheet for a lithium ion battery cathode,

The present invention relates to a sulfur-doped graphene nanosheet (S-GNS) for a lithium ion battery anode, and more particularly, to an electrochemical cell having an electrochemical performance by introducing elemental sulfur into graphene oxide (GO) To an improved sulfur-doped graphene nanosheet and a method of manufacturing the same.

Lithium-ion batteries are small in size, light in weight and energy-efficient, and account for most of the rechargeable batteries currently used due to their chemical and physical properties not being degraded due to external or internal influences as compared to other batteries. It is used as a main energy storage device of portable electronic devices and hybrid vehicles. It has been attracting great attention from scientific and industry sectors. The rapidly increasing demand in various applications is due to its excellent reversible capacity, rate capability, (Long cycle life), and the like. Renewable energy sources are becoming increasingly important in modern society, and the need for efficient and sustainable energy storage technologies is increasing.

The carbon electrode, which is the anode material of a lithium ion battery, greatly changes the lifetime and capacity of the lithium ion battery depending on its structure. The existing cathode materials using graphite have a Li-storage capacity of 372 mAh / g, which is limited to LiC ₆ in the lithium ion storage site in the sp ² carbon hexahedron structure, (Guo, B .; Wang, X .; Fulvio, PF; Chi, M .; Mahurin, SM; Sun, X.-G .; Dai, S. Adv. Mater., 2011 , 23, 4661-4666).

Due to its excellent physical properties, graphene composed of a single layer of sp ² hybrid carbon atoms, which has received much attention in various applications, can form Li ₂ C ₆ structures on both sides to store lithium ions, The inter-lithium bond can further improve the capacity.

Recently, research has been reported on improving the electrochemical performance of a lithium ion battery using nitrogen-introduced graphene nanosheets (N-GNS) as a negative electrode material. The reversible capacity of N-GNS over graphene nanosheets (GNS) appears to be due to the large number of defects formed due to nitrogen incorporation. The introduction of nitrogen improves the insertion characteristics of lithium ions by forming a large amount of defects on the graphene surface and an amorphous carbon structure. The spaces formed in the hexagonal carbon structure of the graphene as pyridinic nitrogen is formed are lithium The capacity of the N-GNS cathode to which nitrogen is introduced is increased as a result.

Sulfur is also a material that can be introduced into graphene. It has electronegativity lower than that of nitrogen, has large atomic size, has metal ions and good attraction, but graphene (S-GNS) It has not been studied as a material. Only a few studies have reported the use of amorphous carbon in which sulfur has been introduced as a cathode material for lithium ion batteries to improve charge / discharge capacity.

However, the lithium ion storage mechanism of amorphous carbon and crystalline carbon is distinctly different. Crystalline carbon, such as graphite, is a structure of LiC ₆ that stores one lithium ion per hexagonal carbon ring and reversibly stores lithium ions. In contrast, in the case of amorphous carbon, absorption and desorption of lithium ions or hydrogen The lithium ion storage characteristics are irreversible. So it has a theoretical capacity of two of the 372 mAh / g theoretical capacity of the graphite times 744 mAh / g as lithium can be stored, and Li ₂ C ₆ structures on both sides than in the case of graphite pins. In addition, since the crystalline carbon has a relatively small irreversible characteristic, the cyclic characteristic is superior to that of amorphous carbon.

The conventional technique of introducing sulfur into amorphous carbon involves a simple increase in the capacity. In the manufacturing method, 4 at% of sulfur is uniformly injected into the graphene nanosheet by using the same elemental sulfur as in the present invention. There is no case applied.

Korean prior art 10-2012-0064980 (method for producing nitrogen-doped graphene and nitrogen-doped graphene produced thereby), Korea Patent No. 10-0508149 (Lithium ion battery cathode Korean Patent No. 10-1173629 (a method for producing a graphene nanosheet and a graphene nanosheet manufactured by the above method).

It is an object of the present invention to provide a sulfur-doped graphene nanosheet (S-GNS) having improved electrochemical performance by heat-treating a mixture of graphene oxide (GO) and sulfur (elemental sulfur) A large amount of sulfur is introduced into a graphene nanosheet to provide a graphene nanosheet that can be utilized as an anode material of an efficient lithium ion battery due to its stable, excellent capacity and reversibility and long cycle life.

In order to achieve the above object, the present invention provides a graphene nanosheet in which elemental sulfur is introduced into graphene oxide (GO), wherein the graphene nanosheet has a graphene oxide and sulfur in a ratio of 1: 0.01 to 1 : ≪ / RTI > 10 by weight, based on the weight of the graphene nanosheet.

The graphene nanosheet is prepared by mixing graphene oxide powder prepared by freeze-drying a graphene oxide aqueous solution and sulfur and then heat-treating the graphene oxide at 200 to 1500 ° C.

(1) preparing a graphene oxide powder by freezing and lyophilizing an aqueous solution of graphene oxide (GO); (2) preparing a mixture by mixing the graphene oxide powder and elemental sulfur produced in the step (1); And (3) heat-treating the mixture prepared in the step (2) at 200 to 1500 ° C. Doped graphene nanosheets, comprising the steps of:

And the freeze-drying is performed for 24 to 80 hours.

In the step (2), the graphene oxide powder and sulfur are mixed in a weight ratio of 1: 0.01 to 1:10.

In the step (3), the heat treatment is performed at a rate of 10 ° C / min to 200 ° C to 1500 ° C, followed by heat treatment at an isothermal temperature for 30 minutes to 24 hours.

According to the present invention, there is provided a graphene nanosheet obtained by heat-treating graphene oxide (GO) and sulfur (elemental sulfur), and a method for producing the graphene nanosheet, which has excellent capacity and reversibility and has a stable cycle life Doped graphene nanosheet of the present invention having improved electrochemical performance can be used as a negative electrode material of a lithium ion battery to provide a product improved in characteristics of the secondary battery.

1 is the XPS S 2p spectrum of S-GNS, b is IR spectrum of GNS (black line) and S-GNS (red line), c is AFM image of GNS, d is AFM image of S- the Raman spectrum of the GNS (black line) and S-GNS (red) seen in the c and d, f is the Raman mapping of the GNS I _G / I _D (Raman mapping), g is the S-GNS I _G / I _D Raman mapping. The scale bar at c, d, f, and g represents 1 μm.
B is a dQ / dV analysis of the S-GNS at a current density of 372 mA / g (1C) to 11160 mA / g (30 C), c is the charge / discharge curve of the GNS D / dV analysis of GNS at 372 mA / g (1C) ~ 11160 mA / g (30C) current density, e is the charge / discharge cycle stability test results of GNS and S-GNS , and f is the charge / discharge cycle stability test result of S-GNS.
FIG. 3 is a graph of formation energy obtained from theoretical calculations of GNS and S-GNS.
FIG. 4 is a graph of Li intercalating voltages of lithium ions through a theoretical calculation of GNS and S-GNS.
FIG. 5 shows an EDX image of a TEM image showing the incorporation of a sulfur atom into the GNS, wherein a is an image of S-GNS and b is an image showing uniform distribution of sulfur atoms in S-GNS.

Hereinafter, the present invention will be described in detail.

In the present invention, GNS (graphene-based nanosheets) refers to graphene nanosheets, and S-GNS (S-graphene-based nanosheets) refers to graphene nanosheets incorporating sulfur.

The present invention relates to a graphene nanosheet having an elemental sulfur introduced into graphene oxide (GO), wherein the graphene nanosheet has a ratio of graphene oxide and sulfur of 1: 0.01 to 1:10, preferably 1: : 0.5 to 1: 1.5, and more preferably 1: 1. The graphene nanosheet is doped with sulfur. If the amount of introduced sulfur is too small, the amount of sulfur introduced into the GNS may not be sufficient. If the amount of introduced sulfur is too large, the cost increases. Therefore, it is preferable to mix the graphene oxide and the sulfur at the same weight ratio.

The graphene nanosheets are prepared by mixing graphene oxide powder prepared by freeze-drying a graphene oxide aqueous solution and sulfur and heat-treating the graphene oxide at 200 to 1500 ° C, preferably 600 to 700 ° C. Sulfur melting temperature is about 115.2 ℃ and vaporization temperature is about 444.6 ℃. Doping effect is shown at grazing point. Graphene oxide starts to remove oxide group at over 150 ℃ in heat treatment process. Accordingly, a substantial reaction takes place at a temperature of 200 ° C, and the reaction is completed at a temperature near the vaporization temperature. It was experimentally confirmed that the introduced sulfur maintained the introduced contents even at a temperature of 1000 ° C or higher.

(1) preparing a graphene oxide powder by freezing and lyophilizing an aqueous solution of graphene oxide (GO); (2) preparing a mixture by mixing the graphene oxide powder and elemental sulfur produced in the step (1); And (3) heat-treating the mixture prepared in the step (2) at 200 to 1500 ° C. Doped graphene nanosheets, comprising the steps of:

In this manner, sulfur-doped graphene nanosheets (S-GNS) can be prepared by applying 4 at% of sulfur uniformly to graphene nanosheets using elemental sulfur.

In the step (1), the graphene oxide aqueous solution can be produced by peeling off graphite by acid treatment using the Hummers method. More specifically, NaNO ₃ was dissolved in sulfuric acid, and graphite was added thereto. KMnO ₄ was added thereto, and the mixture was reacted for 2 hours. Then, the mixture was put into a mixed solution of H ₂ O ₂ and water. After filtering and washing, An aqueous solution of an oxide can be obtained.

In the above step (1), freeze-drying is performed for 24 to 80 hours, preferably 70 to 75 hours, for optimum effect.

In the step (2), the graphene oxide powder and sulfur are mixed at a weight ratio of 1: 0.01 to 1:10, preferably 1: 0.5 to 1: 1.5, more preferably 1: 1.

In the step (3), the heat treatment is preferably performed at a temperature of 10 to 200 ° C, preferably 600 to 700 ° C, and then heat-treated at an isothermal temperature for 30 minutes to 24 hours, preferably 2 to 3 hours. Effect.

FIG. 3 shows the structure of a stable S-GNS as a graph of formation energy of GNS and S-GNS. Generally, a sulfide requires higher formation energy than an oxide. However, oxygen and sulfur defects located at the edge of graphene cause the formation energy to be lowered. This can be seen through the fact that the S-O bond formation energy of the edge is lower than the other.

There are many defect sites in the graphene oxide plane after reduction, and the main properties of the defect sites are determined by the ratio of the hole and the amorphous portion. Since the energy barrier required to form pores due to carbon vacancies is higher than that of carbonyl, the formation of carbonyl thermodynamically takes precedence over the reduction by heat. In addition, holes with a pair of carbonyls form a relatively strain-free structure by forming a low strain of the graphene oxide plane. Therefore, the S-O bond is well formed on both the plane and the edge of the S-GNS and has a great influence on the electrochemical performance.

As shown in FIG. 4, the sulfur bound to the edge has a very low lithium ion insertion potential in all of E, F, G, and H, while in the case of edge oxygen, only the potential in G is low. These phenomena allow S-GNS to have more defects than GNS and to interpret the results of Raman spectroscopy. S-GNS has high specific capacity and excellent reversibility even at a high charge / discharge rate of 30 C because it has more stable defect sites and a low lithium ion insertion potential.

As a result of the preparation of graphene nanosheets incorporating sulfur by heat treatment of a mixture of graphene oxide and sulfur according to the present invention, sulfur was introduced as a whole on the surface of the S-GNS, resulting in an electrochemical Performance. The reversible capacities of S-GNS at 372 mAh / g (1C) and 11160 mA / g (30C) current density were 2 and 3 times higher than GNS, respectively. Electrochemical performance of S- The excellent reason is explained by theoretically calculating the formation energy. Theoretical calculations show that oxygen and sulfur defects at the edges of GNS and S-GNS have an effect of lowering the insertion potential of lithium ions and that sulfur is more effective than oxygen.

Hereinafter, the present invention will be described in more detail with reference to Examples. It is to be understood by those skilled in the art that these embodiments are merely illustrative of the present invention and that the scope of the present invention is not construed as being limited by these embodiments.

Examples. Manufacture of S-GNS

Graphite (Sigma-Aldrich) was removed by oxidation using the Hummers method.

After dissolving 1 g of NaNO ₃ in 140 ml of sulfuric acid, 2 g of graphite was added, and 6 g of KMnO ₄ was added. The mixture was reacted for 2 hours and then put into 1 L of a mixed solution of H ₂ O ₂ and water. To obtain an aqueous phosphorous oxide solution.

 The prepared graphene oxide aqueous solution was frozen with liquid nitrogen and lyophilized (LP3, Jouan, France) at -50 DEG C and 0.045 mbar for 72 hours to prepare graphene oxide (GO) powder. The freeze-dried graphene oxide was obtained as a powder having a low density.

Thereafter, a mixture of 100 mg of graphene oxide and 100 mg of sulfur (elemental sulfur, Sigma-Aldrich, 99.98%) in a mortar bowl was heated to 600 ° C at a rate of 10 ° C / min in an argon atmosphere of 200 ml / min S-GNS was obtained by heat treatment through an electric furnace under isothermal conditions for 2 hours.

The thus-obtained S-GNS was stored in an oven at 30 ° C.

Comparative Example. Manufacturing of GNS

GNS was prepared by the same procedure as in the above example except that sulfur was added.

Experimental Example 1. Physical Characteristic Analysis

The shapes of S-GNS and GNS prepared in the above Examples and Comparative Examples were measured by AFM (a Digital Instrument Nanoscope IVA). Cantilever was measured using semi-contact operation mode using a commonly used NSG-10 (NT-MDT, Russia).

Raman spectroscopy used 473 nm (2.62 eV) on-line, linearly polarized laser, 50 μm pinholes and 600-groove / mm gratings and a weak intensity laser (<300 μW) range for nondestructive measurement Respectively.

Monochromated Al K radiation (hv = 1486.6 eV) was used for X-ray photoelectron spectroscopy (XPS, PHI 5700 ESCA) analysis and IR spectroscopy was performed using VERTEX 80v (Bruker Optics, Germany).

The results are shown in Fig. Sulfur molecules attack the C-O bond of graphene oxide and are uniformly introduced to the surface and edge of the graphene.

In the XPS S 2p spectrum of S-GNS, CS bonds and C-SO _x bonds were observed at 164.4 eV and 167.6 eV, respectively. In the IR spectroscopy of S-GNS, CS stretching vibration was found at 877 cm ^-1 , and S = O, C = S and sulphone showed peaks at 1126, 1093 and 1048 cm ^-1 was, 1223 cm ^-1 appears largely at GNS (CO bond) in 1300 cm ^-1 or less, such as the peak does not appear clearly showed a further aspect the GNS. A large amount of sulfur was introduced throughout the S-GNS to form a stable structure with a defect site. The amount of sulfur introduced into the S-GNS was found to be 4.3 at% and 8.7 wt% in XPS and EA, respectively.

The GNS and S-GNS AFM images show that both GNS and S-GNS have a size of several μm and a thickness of about 10 nm.

In the Raman spectrum of S-GNS, D and G bands were found at 1368.0 and 1599.0, respectively. The ratio of G peak to D peak was 0.83 (I _G / I _D of GNS was 1.14). The D-peak size of S-GNS was larger than that of GNS. In addition, Raman mapping (Raman mapping) as a result, I on the surface of the S-and GNS edge _G / I _D of the difference is almost eopeotjiman GNS and GNS S-I _G / I _D measured by the difference was pronounced. These results indicate that sulfur is uniformly introduced over the entire surface of S-GNS. From the results of EDS mapping, it was confirmed that sulfur was uniformly introduced over the entire GNS surface. Based on the results of Raman spectroscopy and Raman mapping, it was found that sulfur was introduced as a defect in S-GNS.

Experimental Example 2: Electrochemical Characterization

When S-GNS and GNS prepared in the above examples and comparative examples were used as cathode materials for lithium ion batteries, electrochemical characteristics were measured by Wonatec automatic battery cycler and CR2016-type coin cell.

Coin cells were assembled in a glove box in an argon atmosphere and 1 M LiPF ₆ (Aldrich) was added to a solution of ethylene carbonate / dimethyl carbonate / diethyl carbonate (1: 2: 1 v / v) 99.99%) was melted and used as an electrolyte. The battery was measured by repeating charging / discharging process by constant current method while varying the current density in the range of 0.01 to 0.3 V and Li / Li + condition.

The results are shown in Fig.

The charging / discharging curves of S-GNS and GNS were similar. The charge / discharge voltage hysteresis was large, but the flat surface of the distinct potential region was not observed. This means that the stacking structure of the graphene nanosheets is not constant and consequently the point at which the lithium ions are inserted is not equivalent during the charge / discharge process, and these results indicate that the characteristics already reported for GNS Respectively. It is considered that the flat surface seen at 0.6 ~ 0.7 V is due to electrolyte decomposition and formation of solid electrolyte membrane (SEI) on the electrode surface.

The results of dQ / dV analysis of S-GNS and GNS were in agreement with those of charge / discharge test. The first charge / discharge test results of S-GNS under the current density of 372 mA / g (1C) showed a large specific capacity of about 1700 and 870 mAh / g, and the reversible capacity of S- Which is more than twice that of GNS (400 mAh / g), which is consistent with the value in the form of Li _2.4 C ₆ . The high reversibility capacity is due to the increased number of nano-sized holes that can be inserted with lithium ions and the edge interface of the GNS. These results were superior to those of N-GNS measured at similar conditions.

In addition, S-GNS exhibited a very stable capacity and reversibility of about 285 mAh / g even at a high speed of 30 C, which is more than three times higher than the GNS of the same condition. Even after lowering the current density to 8C and 10C after 40 cycles, S-GNS recovered its initial capacity and showed very stable reversibility. GNS also showed similar rate capability and reversibility as S-GNS, but the overall capacity was less than half of S-GNS. These results indicate that there are more points where lithium ions can be inserted into the S-GNS, and that they can effectively withstand the structural changes occurring during the repetitive charging / discharging process.

The charge / discharge stability test of the S-GNS was carried out for 10 cycles at a current density of 372 mA / g (1C) and then for 500 cycles at 1488 mA / g (4C). As a result, it showed stable performance over 500 cycles and maintained a specific capacity of 290 mAh / g at 4C.

Having described specific portions of the present invention in detail, it will be apparent to those skilled in the art that this specific description is only a preferred embodiment and that the scope of the present invention is not limited thereby. It will be obvious. Accordingly, the actual scope of the present invention will be defined by the appended claims and their equivalents.

Claims (6)

As graphene nanosheets in which elemental sulfur is introduced into graphene oxide (GO)
Wherein the graphene nanosheet is mixed with graphene oxide and sulfur in a weight ratio of 1: 0.01 to 1:10.
The method according to claim 1,
Wherein the graphene nanosheet is prepared by mixing graphene oxide powder prepared by freeze-drying a graphene oxide aqueous solution and sulfur, and then heat-treating the graphene nanosheet at 200 to 1500 ° C.
(1) freezing an aqueous solution of graphene oxide (GO) to prepare graphene oxide powder;
(2) preparing a mixture by mixing the graphene oxide powder and elemental sulfur produced in the step (1); And
(3) heat-treating the mixture prepared in the step (2) at 200 to 1500 ° C; Doped graphene nanosheet.
The method of claim 3,
Wherein the freeze-drying is performed for 24 to 80 hours in the step (1).
5. The method of claim 4,
Wherein the graphene oxide powder and sulfur are mixed at a weight ratio of 1: 0.01 to 1:10 in the step (2).
6. The method of claim 5,
In the step (3), the heat treatment is performed at a rate of 10 ° C / min to 200 to 1500 ° C, followed by heat treatment at an isothermal temperature for 30 minutes to 24 hours.

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