JP2016115417A - Positive electrode used for lithium sulfur secondary battery, and lithium sulfur secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode used for a lithium sulfur secondary battery, which is high in energy density and excellent in cycle characteristics.SOLUTION: There are provided: (1) a positive electrode used for a lithium sulfur secondary battery, having a plurality of meso pores continuously arranged therein and including at least a binder and porous carbon in which meso pores adjacent to each other are communicated partly or entirely through micro pores and sulfur, serving as an active material, is contained in pores and around pores; (2) a positive electrode mentioned in (1), a phase of the porous carbon being amorphous; (3) a positive electrode mentioned in (1) or (2), vapor growth carbon fibers being added further thereto; and (4) a lithium sulfur secondary battery including one of the positive electrodes mentioned in (1)-(3) and a negative electrode using silicon monoxide (SiO) as an active material.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a positive electrode used for a lithium-sulfur secondary battery, and a lithium-sulfur secondary battery using the positive electrode.

Power generation using sunlight, solar heat, wave power, and geothermal heat has attracted attention due to CO ₂ emission control, coal, and oil resource depletion problems. And in order to use these energy efficiently, the demonstration experiment of energy management systems, such as a smart grid and a local grid united with IT technology, is developed worldwide. Under such circumstances, a hybrid vehicle loaded with gasoline and a secondary battery is attracting attention as an automobile subject to CO ₂ emission suppression, and its spread is accelerating. In the smart grid, for example, a secondary battery for storing surplus power generated by a solar battery plays an important role. Thus, in the future, demand for secondary batteries including those for mobile phones will increase, and higher energy density and output density will be required. In particular, it is said that electric vehicles are difficult to achieve the required weight (volume) energy density even with current lithium ion batteries and even high energy density lithium ion batteries that will be developed in the future.

  The weight energy density required for electric vehicles is also said to be 500 to 700 Wh / kg. Therefore, in order to use oxygen present in the air as an active material, a lithium metal air secondary battery that can be expected to have a very large energy density, a sulfur battery that uses sulfur as an active material, a polyvalent cation of magnesium or aluminum is used. Multivalent cation secondary batteries that increase the capacity and increase the energy by reaction of two electrons or three electrons at a time instead of one electron are attracting attention as a next-generation secondary battery and are being studied. Furthermore, if a solid electrolyte can be used in place of the organic electrolyte and an all-solid battery can be used to achieve a high energy density, an ideal secondary battery with high safety can be realized.

While metal-air batteries and multivalent cation batteries have many problems and are considered to be in the basic research stage, research on sulfur positive electrode using sulfur as an active material has been progressing very recently. Technology for suppressing elution of polysulfides produced during the oxidation-reduction reaction into organic electrolytes has been progressing.
For example, in Non-Patent Document 1, by confining sulfur in TiO ₂ nanoparticles having a hollow spherical shell structure and using a mixture electrode composed of a conductive additive and a binder as a positive electrode, the capacity deterioration after 1000 cycles is reduced by 60%. Suppressed to near. In addition, it aims to improve the number of cycles by supporting sulfur in the mesopores of mesoporous carbon, but it is only effective for all solid state batteries using solid electrolyte, and specific effect of secondary battery using organic electrolyte. Is not shown. Moreover, although there exists patent document 1 as a similar technique, it is not paying attention to the magnitude | size of the volume of a hole.

In the case of Non-Patent Document 1, since the hollow sphere shell structure is an oxide TiO ₂ and the electronic conductivity is lower than that of carbon, it is necessary to add a large amount of conductive additive for improving the conductivity. The proportion of the active material in the entire electrode is reduced. In this case, in order to flow a large amount of current at a time, that is, to increase the charge / discharge rate, it is necessary to coat the electrode thickly. However, if the electrode is thickened, the conductivity decreases and the rate characteristics deteriorate. Further, when the positive electrode contains oxygen and the negative electrode is made of lithium metal, a problem remains in safety (ignitability) as in the case of the lithium ion battery. Therefore, it is necessary to consider other active materials as the negative electrode.
Further, in the case of the sulfur positive electrode in which sulfur is supported on the hollow sphere shell structure carbon of Non-Patent Document 2, the material is carbon, but the carbon spheres are independent one by one and their contact area tends to be small. Therefore, there is a problem that the capacity becomes small for a high charge / discharge rate.

Lithium sulfur batteries have the potential to dramatically improve weight energy density and volumetric energy density compared to lithium ion batteries. However, in the positive electrode, there is a problem that the polysulfide formed in the process of sulfur producing the lithium sulfur compound Li ₂ S elutes into the electrolytic solution. Further, when lithium metal is used for the negative electrode, there are problems such as dendrite growth, decomposition of the electrolytic solution, and an overcharge phenomenon caused by the exchange of lithium electrons from the eluted polysulfide ions. Therefore, it is necessary to create a secondary battery configuration that solves these problems and achieves both safety and high energy density.
In the sulfur positive electrode, the area where sulfur comes into contact with the electrolyte solution is as small as possible, lithium ions are conducted, and the structure does not break even if volume expansion occurs due to the change from sulfur to lithium sulfur compound during discharge. Is required. Moreover, when returning from a lithium sulfur compound to sulfur at the time of charge, it is also necessary to have a high adsorption power so that sulfur does not desorb from the structure.

In the negative electrode, an active material replacing lithium metal having a theoretical capacity comparable to the theoretical capacity of 1675 mAh / g of the sulfur active material is required. Silicon is attractive as a negative electrode because it has a theoretical capacity of about 3500 mAh / g. However, when Li compound (Li _4.4 Si) is produced, the volume increases by about 4 times, so the volume expansion is very large and cycle deterioration occurs. Is big.
A problem as a secondary battery having a high energy density is charging time when applied to an electric vehicle. Therefore, a high input density is required to allow a large current to flow in a short period of time and complete charging more quickly. It is also important that the power density at which a large current flows instantaneously is large. Furthermore, although the lithium ion battery has a problem that it cannot be charged at 0 ° C. or lower, the high energy density battery has the same problem.
Accordingly, an object of the present invention is to provide a positive electrode used for a lithium-sulfur secondary battery having a high energy density and good cycle characteristics.

The above problem is solved by the following invention 1).
1) A plurality of mesopores are continuously arranged, and some or all of the adjacent mesopores communicate with each other through micropores, and contain sulfur as an active material in and around the pores. A positive electrode used for a lithium-sulfur secondary battery comprising at least porous carbon and a binder.

  ADVANTAGE OF THE INVENTION According to this invention, the positive electrode used for a lithium sulfur secondary battery with a high energy density and favorable cycling characteristics can be provided. Furthermore, it is possible to provide a secondary battery that can be charged even at a low temperature and a lithium sulfur secondary battery with high safety.

The figure which shows typically the structure of the porous carbon in this invention. (B) is an enlarged view of a portion surrounded by a dotted line in (A). The figure which shows typically the structure of the porous carbon used for the comparative example 1. FIG. FIG. 6 shows a TEM (transmission electron microscope) image of porous carbon in Example 5. The figure which shows the TEM image after making the said porous carbon carry | support sulfur. The figure which shows the cycle frequency dependence of the discharge capacity of the cell of Example 5. FIG. The figure which shows the cycle frequency dependence of the discharge capacity of the cell of Example 13. FIG.

Hereinafter, the present invention 1) will be described in detail, but the following 2) to 5) are also included in the embodiment of the present invention, and these will also be described together.
2) The positive electrode used for the lithium-sulfur secondary battery according to 1), wherein the porous carbon phase is amorphous.
3) The positive electrode used for the lithium-sulfur secondary battery according to 1) or 2), further comprising a vapor-grown carbon fiber.
4) The lithium sulfur secondary battery according to any one of 1) to 3), wherein the porous carbon has a specific surface area of 1000 to 2000 m ² / g and a pore volume of 3 to 4 cm ³ / g. The positive electrode used.
5) A lithium-sulfur secondary battery having the positive electrode according to any one of 1) to 4) and a negative electrode using silicon monoxide (SiO) as an active material.

The lithium-sulfur secondary battery in the present invention refers to a secondary battery using lithium ions and containing sulfur as a positive electrode active material, and is not limited to a combination battery using lithium metal (foil) as a negative electrode. Absent.
It is known that Li ₂ Sx (x = 2, 4, 6, 8), which is an intermediate product of a compound of sulfur and lithium, becomes polysulfide ions and elutes into the electrolyte when it comes into contact with the organic electrolyte. . In the present invention, porous carbon having a plurality of pores for supporting sulfur is used as the solution. Although the size of multiple pores is not the same, mesopores that can confine sulfur and mitigate volume changes that occur during the reversible reaction of sulfur and lithium-sulfur compounds, and micropores that allow lithium ions to conduct ions are mainly used. It is. Here, the mesopores are those having a size of 2 to 50 nm, and the micropores are those having a size of less than 2 nm. The preferred size of mesopores is 5-30 nm on average. Sulfur particles are supported in both mesopores and micropores. In some cases, the sulfur particles are supported between the mesopores and the micropores without any gap, and there are also some gaps where the sulfur particles are not supported. It is preferable that the mesopores carry sulfur particles in a state where there is a gap so that the volume change can be reduced. There may be holes larger than mesopores, but the ratio is preferably very small.

FIG. 1 schematically shows the structure of a porous carbon according to the present invention. (B) is an enlarged view of a portion surrounded by a dotted line in (A). White circle portions in (A) are mesopores, white linear portions connecting the white circle portions in (A), and a plurality of white linear portions in (B) are micropores. It is characteristic that micropores exist between mesopores.
It is necessary that some or all of the adjacent meso holes communicate with each other through the micro holes, and it is preferable that as many meso holes as possible communicate with each other. In addition, the number of micropores between a pair of adjacent mesopores may be one or more, but in consideration of the production method and structure of porous carbon, the number is generally plural. Further, microscopically, the micropores are not uniformly formed as a whole, and it is difficult to accurately measure the proportion of mesopores in the porous carbon and the number of micropores. It is also difficult to actually control the process. Therefore, whether or not it is suitable for the positive electrode of the present invention is determined with reference to the specific surface area and pore volume of porous carbon described later.

The specific surface area of the porous carbon is preferably 1000 m ² / g or more, more preferably 1200 m ² / g or more. If it is 1000 m ² / g or more, sufficient pores for trapping sulfur can be secured, and the deterioration of cycle characteristics is small. However, if the specific surface area exceeds 2500 m ² / g, the micropores occupy most, and the volume change cannot be relaxed, and the cycle characteristics deteriorate. Preferably it is 2000 m < ² > / g or less.
Further, the pore volume serving as an index of the depth of the pore is preferably 2 cm ³ / g or more, more preferably 3 cm ³ / g or more. If it is 2 cm ³ / g or more, it becomes easy to confine sulfur. On the other hand, porous carbon exceeding 4 cm ³ / g is difficult to produce. Therefore, a preferable pore volume is 2 to 4 cm ³ / g, and more preferably 3 to 4 cm ³ / g.

It is also important to increase the adsorptivity of sulfur to carbon. Since sulfur has a very low electrical conductivity, it is necessary to increase the adsorption power with carbon and improve the conductivity. It is preferable that the electrical conductivity of the porous carbon itself is higher.
There are several methods for producing porous carbon having a large specific surface area, a large pore volume, and a high electrical conductivity, but a method that is relatively easy to produce and low cost is preferred.
For example, there is a method in which phenol, formalin and a small amount of hydrochloric acid are converted into a resin and used as a carbon precursor. Silica (SiO ₂ ) is used as a template for the pores. The size is preferably a commercially available average particle diameter of 5 to 50 nm. Moreover, you may use MgO as another material. Since MgO can use dilute sulfuric acid that is safer than hydrofluoric acid as a reagent used in the subsequent template removal process, it is preferable in view of mass production, but it is difficult to use it in a large amount like hydrofluoric acid.
Although it is possible to use sodium hydroxide, it takes a long time to dissolve. The silica colloidal nanoparticles are centrifuged and then dried to obtain nanoparticles. Phenol and formalin are mixed at a predetermined weight ratio, and after adding a small amount of hydrochloric acid, the solution is allowed to stand while leaving or evacuating to allow the solution to penetrate between the particles. Next, heat treatment is performed at 500 ° C. for 1 hour in an argon gas atmosphere to remove decomposition products. Next, it is carbonized by heat treatment in an argon gas atmosphere in the range of 800 ° C. to 1200 ° C. for about 5 hours. Next, the template is removed using hydrofluoric acid to form porous carbon.

  In order to support sulfur on the porous carbon, a method of infiltrating the pores in a dissolved state of sulfur while applying a temperature of about 150 ° C. using the low melting point (about 120 ° C.) of sulfur is used. Thereby, sulfur can be efficiently confined in the pores and adsorbed on the entire carbon surface. In addition, since the porous carbon produced by this method is in the form of plate-like particles, it can be applied thinner when coated on the electrode current collector after the slurry is produced. Since the conductivity is difficult to decrease, not only the energy density but also the output density can be improved. Moreover, although porous carbon is fundamentally an amorphous structure, a graphite phase (crystal phase) may exist locally. As the volume of the graphite phase increases, the pore volume decreases.

Vapor-grown carbon fiber (VGCF) is a carbon material used as a conductive aid for graphite negative electrode active material of lithium ion batteries. VGCF has a fiber diameter of 150 nm, a length of 10 μm or less, and has a low resistivity of the order of 10 ⁻⁴ Ω · cm, so it is effective to add it to the sulfur positive electrode. That is, the sulfur positive electrode does not have the same electrode configuration as that of the lithium ion battery such as sulfur, a conductive additive, and a binder, but uses porous carbon for confining sulfur, and VGCF is used to improve the conductivity between these carbons. It is effective. Although carbon nanotubes (CNT) having a smaller diameter are also effective, VGCF is more effective even when added in a small amount because the cost is high and the size of the porous carbon particles is on the order of microns.

On the other hand, as the negative electrode material, an active material of 1000 mAh / g or more suitable for the high energy density of sulfur is preferably used, and silicon (Si) and silicon monoxide (SiO) can be used in addition to lithium metal. Silicon is problematic because its volume increases approximately four times when producing alloys with lithium. When silicon with a particle size of micron size is used, cracks occur during repeated expansion and contraction, fine particles progress, and cycle deterioration occurs due to peeling from the conductive additive. It is important to solve this problem It is.
On the other hand, SiO is preferable as an active material having a small volume expansion and a large energy density. Although graphite or hard carbon can be used, when SiO is used, a stainless steel foil that hardly causes lithium dendrite precipitation at a low temperature can be used. In the conventional copper foil, the foil is deformed by expansion.

In order to operate as a secondary battery using a sulfur positive electrode and a SiO negative electrode, lithium is pre-doped with SiO in advance. After pre-doping, the positive electrode becomes 6.6S, and the negative electrode becomes charged with a change of 17.2Li + 4SiO → 3 (Li _4.4 Si) + Li ₄ SiO ₄ . After the discharge, the positive electrode and the negative electrode are in the following states, respectively.
Negative electrode: 3 (Li _4.4 Si) + Li ₄ SiO ₄ → 3Si + Li ₄ SiO ₄ + 13.2Li ⁺ +
26.4e ⁻
Positive electrode: 13.2Li ⁺ + 26.4e ⁻ + 6.6S → 6.6Li ₂ S

The specific configuration and operation of the present invention will be described below.
As the positive electrode current collector, Al, Ni, or stainless steel can be used. Forms include foil, mesh, and foam having pores. Al foil is preferred because it is easy to process and inexpensive.
As a method of supporting the positive electrode mixture on the positive electrode current collector, a method of pressure forming the positive electrode mixture on the positive electrode current collector, the positive electrode mixture was applied in a paste form and dried on the positive electrode current collector Later, there is a method of fixing by pressing or the like. In the case of forming a paste, a slurry made of a positive electrode active material, a conductive material, a binder, and an organic solvent is prepared. Examples of the organic solvent include amide solvents such as N-methyl-2-pyrrolidone (NMP). Further, water may be used as a solvent. Examples of the binder when slurried using an organic solvent include polyether polymer compounds such as polyvinylidene difluoride (PVDF), polyethylene oxide (PEO), and polypropylene oxide (PPO). For an aqueous solvent, a binder in which styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) are mixed can be used.

  As the separator, a porous film, a nonwoven fabric, a woven fabric, or the like made of a polyolefin resin such as polyethylene or polypropylene, a fluororesin, a nitrogen-containing aromatic polymer, or the like can be used. Moreover, it is good also as a laminated separator which laminated | stacked two or more layers which consist of different materials. The thickness is preferably reduced because the volume energy density of the battery is increased and the internal resistance is reduced, and is usually about 10 to 200 μm, preferably about 10 to 30 μm.

Examples of the negative electrode current collector include Cu, Ni, and stainless steel, and Cu that is difficult to form an alloy with lithium and is easy to process into a thin film is preferable.
The method of supporting the negative electrode mixture on the negative electrode current collector is the same as in the case of the positive electrode. The method of pressure forming the negative electrode mixture on the negative electrode current collector; There is a method of fixing by applying, drying and then pressing.

As the electrolyte of the organic electrolyte used, LiClO ₄ , LiPF ₆ , LiSbF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiC (SO ₂ CF ₃ ) ₃ , Li Examples thereof include lithium salts such as ₂ B ₁₀ Cl ₁₀ , a lower aliphatic carboxylic acid lithium salt, and LiAlCl ₄ . Among these, LiN (SO ₂ CF ₃ ) ₂ is preferable. A mixture of two or more of these may be used.
Examples of the organic solvent for the electrolyte include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, 1,3-dioxolane, ethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether. A mixture of two or more kinds may be used. Preferred is triethylene glycol dimethyl ether or a mixed solvent of tetraethylene glycol dimethyl ether and 1,3-dioxolane.
The concentration of the electrolytic solution is 1M or higher and 3M at the highest.

When the sulfur positive electrode in the present invention is used, the effect of suppressing the dissolution of polysulfide ions into the organic electrolyte is high, but it is considered difficult to eliminate the 100% dissolution. Therefore, polysulfide ions are generated, move toward the negative electrode, and an overcharge phenomenon may occur. Therefore, it is preferable to add LiNO ₃ so as not to cause this phenomenon. The amount, for _{example, 1M LiN (SO 2 CF 3} ) to _2, 0.1-0.5 M is preferred.

As another electrolytic solution, an ionic liquid or a mixture of an organic electrolytic solution and an ionic liquid may be used. A so-called gel type in which an organic electrolyte solution is held in a polymer can also be used. The ionic liquid is non-volatile compared to the organic electrolyte, and can be expected to improve safety. The ionic liquid is preferably in a liquid state from a low temperature to a high temperature.
Among the ionic liquids, cations include organic nitrogen (ammonium), organic phosphorus (phosphonium), and organic sulfur (sulfonium). There are many imidazolium cation systems, and AlCl ₄ ⁻ , NO ₂ ⁻ , NO ₃ ⁻ , I ⁻ , BF ₄ ⁻ , PF ₆ ⁻ , AsF ₆ ⁻ , SbF ₆ ⁻ , NbF ₆ ⁻ , TaF ₆ ⁻ , F ( HF) _2.3 ⁻ , p-CH ₃ PhSO ₃ ⁻ , CH ₃ CO ₂ ⁻ , CF ₃ CO ₂ ⁻ , CH ₃ SO ₃ ⁻ , CF ₃ SO ₃ ⁻ , (CF ₃ SO ₂ ) ₃ C ⁻ , C ^{_{3 F 7 CO 2 -, C}} 4 F 9 SO 3 -, (CF 3 SO 2) 2 N -, (C 2 F 5 SO 2) 2 N -, (CF 3 SO 2) (CF 3 CO) N - , (CN) ₂ N ^— An anion represented by a combination is used.

In addition to the above, a solid electrolyte that does not elute polysulfide ions and is considered to have the highest safety may be used. As the solid electrolyte, for example, a polymer electrolyte such as a polyethylene oxide polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain can be used. Also, sulfides such as Li ₂ S—SiS ₂ , Li ₂ S—GeS ₂ , Li ₂ S—P ₂ S ₅ , Li ₂ S—B ₂ S ₃ , Li ₂ S—P ₂ S ₅ —GeS ₂ . An oxide-based solid electrolyte such as Li-P—O—N, Li—La—Ti—O, Li—La—Zr—O, or Li—Al—Ge—P—O can also be used.

You may make a negative electrode contain a binder and a conductive support agent.
A thermoplastic resin can be used as the binder, and specific examples include PVDF, thermoplastic polyimide, carboxymethyl cellulose, polyethylene, and polypropylene. The mixed binder of CMC and SBR has high adhesiveness against deformation and is suitable for the negative electrode. Moreover, you may use thermosetting resins, such as a polyimide.
Examples of the conductive aid include carbon black and / or VGCF.
When sulfur is used as the positive electrode and silicon is used as the negative electrode, it is necessary to pre-dope lithium into the silicon negative electrode in advance. Examples of the pre-doping method include a method in which a silicon negative electrode and a lithium foil are short-circuited via a separator, an electrolyte is injected, and then left standing.

  EXAMPLES Hereinafter, although an Example and a comparative example are shown and the positive electrode and lithium sulfur secondary battery of this invention are demonstrated in more detail, this invention is not limited by these Examples.

<Preparation of porous carbon used for positive electrodes of Examples 1 to 10>
Each silica nanoparticle with an average particle size of 5 nm, 12.5 nm, 22.5 nm, 45 nm, and 85 nm is obtained from a silica colloid dispersion (manufactured by Nissan Chemical Co., Snowtex), and an ultracentrifuge (manufactured by Hitachi Koki Co., Ltd.). Used and separated at 30000 rpm for 8 hours.
Subsequently, phenol (manufactured by Wako Pure Chemical Industries, Ltd.), formalin (manufactured by Wako Pure Chemical Industries, Ltd.), and each of the above silica nanoparticles were mixed at a weight ratio of 0.8: 0.2: 1.0. Specifically, phenol and formalin were sufficiently mixed, 0.1 mL of hydrochloric acid was added, each silica nanoparticle was added to the mixed solution, and the mixture was allowed to stand for 12 hours. After the liquid sufficiently permeated between the particles, the liquid on the upper surface not containing the particles was removed, and the rest was transferred to a Teflon (registered trademark) container and heated at 130 ° C. for 22 hours to be resinized. The obtained resin was put into an alumina crucible and heat-treated at 500 ° C. for 1 hour in an argon atmosphere using an electric furnace to remove organic decomposition products. Furthermore, carbon containing silica nanoparticles was obtained by heat treatment at 800 ° C., 900 ° C., 1000 ° C., 1100 ° C., 1300 ° C. (heating rate 5 ° C./min) for 5 hours for carbonization. Next, the carbon was put in a Teflon (registered trademark) container and left in a hydrofluoric acid solution (48 wt%, manufactured by Wako Pure Chemical Industries, Ltd.) for 3 hours to elute the nanoparticles to obtain porous carbon.
About each obtained porous carbon, the specific surface area and pore volume were measured by BET method using Tristar II3020 (made by Shimadzu Corporation).

In each column of Examples 1 to 10 in Table 1, the average particle diameter and heat treatment temperature of the silica nanoparticles used for the production of the porous carbon, the specific surface area of the obtained porous carbon, the pore volume and the phase (amorphous, graphite) are shown. Shown together.
When the heat treatment temperature exceeds 1300 ° C., the graphite phase (crystal phase) grows and the micropores begin to disappear, so the pore volume becomes small. If it is 1100 degrees C or less, it will become an amorphous phase.
In FIG. 3, the TEM (transmission electron microscope) image of the porous carbon in Example 5 is shown.
FIG. 4 shows a TEM image after sulfur is supported on the porous carbon. It turns out that sulfur is carry | supported uniformly by the pore of porous carbon. However, there is no clear regular arrangement structure in the pores. From this result, it can be seen that the regular arrangement structure is not important, and the specific surface area of the pore volume, mesopores, and micropores affects the characteristics.

(Examples 1 to 10)
A half cell for evaluation was produced using the porous carbon according to Example 5 described above.
Porous carbon and sulfur powder (manufactured by High Purity Chemical Co., Ltd.) are mixed at a weight ratio of 30:70 and heat treated in the atmosphere at 155 ° C. for 6 hours to support sulfur in and around the pores of the porous carbon. It was. Ideally, the sulfur should be supported only in the pores of the porous carbon, but it is difficult to control the sulfur so that it is not in the pores, and it is clear from Fig. 4 whether sulfur is attached. Not. Thereafter, 5% by weight of a conductive assistant (supercal 65 made by TIMCAL) and 5% by weight of a binder (Kureha KF polymer) were added, and (sulfur / porous carbon): conductive assistant: binder = 90: 5: 5. A slurry by weight ratio was prepared. The binder was previously dissolved in an NMP solvent. The viscosity of the slurry was adjusted by adding an appropriate amount of NMP. Next, the slurry was coated on an aluminum foil having a thickness of 20 μm at a thickness of 200 μm (applicator setting value), and then dried in the air at 80 ° C. for 3 hours. The thickness after drying was 100 μm including the aluminum foil. This was punched to a size of 16 mm in diameter to obtain a positive electrode. The weight per unit area of sulfur was 3.7 mg / cm ² .
As the separator, a glass fiber filter (Advantest GA-55) and a polytetrafluoroethylene membrane filter (Millipore JGWP-25) were used.
The electrolyte uses LiTFSI (lithium trifluorosulfonimide: manufactured by Kishida Chemical Co., Ltd.) as the electrolyte, and DOL (1,3-dioxolane, manufactured by Wako Pure Chemical Industries, Ltd.) and TEGDME (triethylene glycol dimethyl ether, Kishida Chemical Co., Ltd.) as the solvent. A mixed solvent having a volume ratio of 50:50 (manufactured by Kogyo Co., Ltd.) and 0.25 M of LiNO ₃ (manufactured by Kishida Chemical Co., Ltd.) was further added to obtain a 1 M electrolytic solution. The amount of the electrolytic solution was 50 μL.
As the negative electrode, a 16 mm diameter lithium foil (Honjo Chemical Co., Ltd., thickness: 0.1 mm) was used.
As the cell, a CR2032-type coin cell was used.
A cell of Example 5 was produced as described above.
Moreover, the cell of each Example was produced like Example 5 except having changed the porous carbon into the porous carbon shown in each column of Examples 1-4 of Table 1 and 6-10.

About each cell of the said Examples 1-10, the capacity | capacitance measurement was performed by the cut-off voltage 1.8V and 2.6V using the charge / discharge measuring apparatus (made by Hokuto Denko). The current value was 0.8 mA. This corresponds to a charge / discharge rate of 0.2C.
Table 1 shows the discharge capacity after the first time and after 100 times (cycle). The numerical value is the discharge capacity per sulfur weight. Coulomb efficiency after 100 times was 95% or more.
As can be seen from Table 1, when the specific surface area was 1000 m ² / g or more and the pore volume was 2 cm ³ / g or more, the discharge capacity after 100 times became 75% or more of the initial discharge capacity.
Therefore, it can be seen that a specific surface area of 1000 m ² / g or more and a pore volume of 2 cm ³ / g or more are preferable to maintain a volume ratio of 75% or more after 100 times.

Furthermore, Table 1 shows the discharge capacity after 100 times in the case of a charge / discharge rate of 2 C (8 mA).
At any rate, the ratio (charge capacity / discharge capacity) was 99% from 2 times to 100 times later.
FIG. 5 shows the cycle number dependence of the discharge capacity of the cell of Example 5.

<Preparation of porous carbon used for positive electrode of Comparative Example 1>
Silica nanoparticles having a particle size of 40 nm (Aerosil OX 50, manufactured by Nippon Aerosil Co., Ltd.) were used as a template, and methanesulfonic acid was adsorbed on the surface of the particles. Next, tetrafurfuryloxysilane (cationic polymer synthesized by adding 0.3% by weight of KOH to a mixture of 0.4 mol of tetraethoxysilane and 1.6 mol of furfuryl alcohol) and methanesulfonic acid monomer Was added to form a twin polymer. The proportion of methanesulfonic acid monomer was 35% by weight. Then, while flowing argon, the inside of the electric furnace was put into an argon atmosphere, heat-treated at 800 ° C. for 5 hours, and carbonized to obtain carbon containing silica nanoparticles.
Next, carbon containing the silica nanoparticles was immersed in a hydrofluoric acid solution to dissolve the silica nanoparticles, thereby obtaining porous carbon. When the specific surface area and pore volume of this porous carbon were examined with a BET specific surface area / pore distribution measuring device (TriStarII3020 manufactured by micrometrics), it had 12 nm mesopores and a plurality of micropores less than 2 nm. The porous carbon had the structure shown in FIG. That is, it was a structure in which hollow-type spherical shell-structured carbon was separately formed and aggregated. The specific surface area was 1290 m ² / g and the pore volume was 1.90 cm ³ / g.

(Comparative Example 1)
Except for the use of the porous carbon, a half cell was prepared in the same manner as in the example, and charge / discharge measurement was performed. The results are shown in Table 1.
As can be seen from Table 1, the discharge capacity was as low as 400 mAh / g at 0.2 C for the first time and was 1200 mAh / g for the second time, but after 100 times it was 670 mAh / g, which was about half of the second time. It was. In the case of 2C, the number after 100 times was about 50% of the example.
The ratio (charge capacity / discharge capacity) after 100 times was 95%.

Examples 11 to 12 VGCF-added positive electrode A VGCF-added positive electrode was produced using the same porous carbon as in Example 5, and a coin cell was produced using this. VGCF (VGCF-H manufactured by Showa Denko KK, fiber diameter 150 nm, length 6 μm) was used as the conductive additive, and the addition amount was 5 wt% (Example 11) and 10 wt% (Example 12). In Example 12 of 10% by weight, the amount of sulfur and porous carbon was 85% by weight. Other manufacturing conditions are the same as in the previous embodiment.
Table 2 shows the charge / discharge measurement results of the coin cells.
As can be seen from Table 2, in the case of 0.2C, the discharge capacity after 100 times increased slightly compared to the maximum value in Table 1 (960 mAh / g of Example 5), and in the case of 2C, The discharge capacity after 100 times was clearly increased compared to the maximum value in Table 1 (605 mAh / g in Example 4).
The (charge capacity / discharge capacity) ratio was 99% from the second time to 100th time.

(Example 13) ... full cell production Silicon monoxide (SiO, manufactured by Osaka Titanium Technologies, Inc., particle size 45 μm, purity 3 Nup), conductive aid Ketjen Black (made by Lion, EC300J), and polyimide binder (I. ST Bond, Dream Bond) was mixed at a weight ratio of 85: 5: 10 to obtain a slurry. This was coated on a copper foil, dried at 100 ° C. for 1 hour, and then punched out to a diameter of 16 mm. The amount of the active material was 4.5 mg / cm ² . Thereafter, a lithium foil was placed on the electrode, the same electrolytic solution as in Example 1 was added, and lithium was doped for 24 hours to form a negative electrode. The sulfur positive electrode produced under the conditions of Example 12 was used. Both the positive electrode and the negative electrode were vacuum dried at 150 ° C. for 4 hours. A polypropylene microporous membrane (Cellguard 2500) was used as a separator, and a coin cell was produced using the same electrolytic solution as in Example 1.

A constant current of 0.8 mA was passed through the coin cell, and charge / discharge measurement was performed at a cut-off voltage of 1V and 2.6V. The discharge capacity was 1360 mA / g for the first time, 742 mAh / g after 500 times, and the capacity retention rate was 55%. FIG. 6 shows the cycle number dependency of the discharge capacity.
Subsequently, when charge / discharge measurement was performed at −10 ° C., the charge capacity was 550 mAh / g for the first time and 350 mAh / g after 100 times, and the charge capacity was low, but charging could be confirmed.
Furthermore, when a laminate type battery having a size of 5 cm × 2 cm was produced and a nail test was performed, ignition and smoke were not generated. This is presumably because the sulfur active material and the SiO active material itself of the negative electrode are low in electrical conductivity, so that even if a short circuit occurs, a large current is difficult to flow.

JP 2010-95390 A

Z. W. Seh, W .; L, Judy J. et al. Cha, G .; Zheng, Y. et al. Yang, M .; T.A. McDowell, Po-Chun Hsu and Y.M. Cui. , Nature Communications, Vol. 4, no. 1331 (2013) Falko Bottger-Hiller et al., Angew. Chem. Int. Ed. 2013, 52, 6088-6091

Claims (5)

  A plurality of mesopores are continuously arranged, a part or all of adjacent mesopores communicate with each other through micropores, and a porous material containing sulfur as an active material in and around the pores A positive electrode used for a lithium-sulfur secondary battery, comprising at least carbon and a binder.   The positive electrode used for a lithium-sulfur secondary battery according to claim 1, wherein the porous carbon phase is amorphous.   Furthermore, the vapor growth carbon fiber was added, The positive electrode used for the lithium sulfur secondary battery of Claim 1 or 2 characterized by the above-mentioned. 4. The positive electrode used for a lithium-sulfur secondary battery according to claim 1, wherein the porous carbon has a specific surface area of 1000 to 2000 m ² / g and a pore volume of 3 to 4 cm ³ / g. .   A lithium-sulfur secondary battery comprising the positive electrode according to claim 1 and a negative electrode using silicon monoxide (SiO) as an active material.