JP6065450B2 - Positive electrode and secondary battery containing sulfur composite - Google Patents

Description

  The present invention relates to a sulfur composite whose sulfur surface is coated with a metal layer or a metal sulfide layer, and a positive electrode and a secondary battery including the sulfur composite.

In recent years, with the rapid spread of portable electronic devices and electric vehicles, secondary batteries that can be repeatedly charged and discharged with high capacity are required and are actively developed. Among them, lithium batteries are particularly attracting attention because they are expected to be lightweight and have high output. Currently, as lithium batteries, LiCoO ₂ , LiMn ₂ O ₄ , LiFePO ₄ or the like is used for the positive electrode, and carbon, lithium, or the like is used for the negative electrode. When the negative electrode is carbon, the capacity is 300 to 370 Ah / kg, and when lithium is 3830 Ah / kg, the capacity of the positive electrode LiCoO ₂ or LiMn ₂ O ₄ is about 110 to 140 Ah / kg, LiFePO ₄ Since the capacity is about 150 to 170 Ah / kg, development of a high capacity positive electrode material is desired.

On the other hand, sulfur is attracting attention as a positive electrode material having a high energy density. Assuming that sulfur reacts completely with lithium up to Li ₂ S, it has a theoretical energy density of 2600 Wh / kg and a theoretically high capacity of 1672 Ah / kg. Furthermore, sulfur has the advantage of being inexpensive because it has low toxicity and is rich in resources.

  However, since sulfur is poor in reactivity and is an insulator, in order to be used for a positive electrode material, it is necessary to increase the activity of sulfur and further impart conductivity. Therefore, developments have been made to compensate for these problems. For example, as a secondary battery using metallic sodium as a negative electrode material and sulfur as a positive electrode material, a proposal has been made to increase the operating temperature to 300 ° C. or higher in order to increase the activity of sulfur. Further, as a sodium / sulfur battery for improving the operation at such a high temperature, a mixture comprising 50 to 70% by weight of sulfur, 15 to 30% by weight of carbon as a conductive agent, and 15 to 20% by weight of polyethylene oxide. A sodium / sulfur battery operating at room temperature has been proposed by using sodium as a positive electrode, using sodium, sodium-containing carbon or sodium oxide as a negative electrode, and using a glymid solution containing a sodium salt as an electrolyte. Reference 1).

  Also, instead of simply mixing sulfur and carbon, a method of mechanically attaching carbon particles to the surface of the sulfur particles was proposed, and a lithium / sulfur battery using this sulfur particle as the positive electrode and lithium as the negative electrode was proposed. (Patent Document 2).

  However, a battery that operates at a high temperature of 300 ° C. has problems such as an increase in size of the device and lack of stability. Further, in order to operate the battery at room temperature, even if sulfur and carbon are mixed or carbon is mechanically attached to sulfur, conductivity and reactivity are not sufficiently imparted to sulfur. Furthermore, in the above method using sulfur and carbon, it is necessary to add a large amount of carbon for imparting conductivity and reactivity to sulfur, and the content of sulfur acting as an active material in the positive electrode is reduced. There was a point.

  In order to improve these problems, the present inventors have developed a composite in which fibrous sulfur is coated with a conductive polymer, and have proposed to use this as a positive electrode material (Patent Document 3).

Special table 2007-522633 Japanese Patent Laid-Open No. 2006-92985 JP 2011-222389 A

  Since sulfur can theoretically obtain a high electric capacity, development as a positive electrode material is desired. However, a positive electrode material that has a high capacity and uses sulfur at a normal temperature for a long period of time has not yet been developed. In the conventional technique using sulfur and carbon, sufficient conductivity and reactivity cannot be imparted to sulfur which is an insulator and has low reactivity. Moreover, elution of the sulfur compound from the positive electrode during charging / discharging into the electrolyte solution could not be prevented. Further, since the capacity of the battery increases as the amount of the active material contained in the positive electrode increases, a positive electrode containing a high amount of sulfur has been desired. However, in order to use insulating sulfur, a large amount of conductive agent such as carbon is used. It was necessary to mix, and the content of sulfur in the positive electrode could not be increased.

  The problem of the present invention is to solve these problems, and as a positive electrode material is a highly conductive and reactive sulfur material, which can prevent elution of sulfur compounds that occur during charging and discharging into the electrolyte, Another object of the present invention is to provide a sulfur material capable of increasing the sulfur content in the positive electrode, and a positive electrode and a secondary battery that use the sulfur material and have a high capacity and a small capacity decrease due to charge / discharge.

  In order to find a method using sulfur as a positive electrode material that operates stably at room temperature for a long period of time with a high capacity, the present inventors first use a mixture of sulfur and carbon, which are conventionally known as a positive electrode material. A method, a method of mechanically attaching carbon particles to the surface of sulfur particles, and the like were examined. However, in these methods, although the conductivity is improved to some extent, the conductivity and reactivity are still not sufficient. Furthermore, since the output decreased during use, the cause was investigated, and it was found that the sulfur compound produced during the sulfur reduction reaction elutes into the electrolyte. In the method of simply mixing sulfur and carbon, the sulfur surface cannot be modified, and even if carbon particles are attached to the surface of the sulfur particles, there is a gap between the adhering carbon particles and the sulfur compound. It was found that elution into the electrolyte could not be prevented. Therefore, paying attention to covering the surface of sulfur, we started studying the materials and coating methods used for coating. And the composite body which coat | covered sulfur with the conductive polymer was developed by polymerizing a monomer on the surface of sulfur (patent document 3), and the coating layer of the sulfur surface is an electrolyte solution of a sulfur compound. It has been found to affect elution into the inside. Therefore, as a result of further investigation, the present inventors can impart high conductivity and reactivity to sulfur compared to the conventional case by covering the surface of sulfur with a metal layer or metal sulfide layer, Furthermore, it has been found that the effect of preventing the elution of sulfur compounds into the electrolyte can be improved as compared with coating with a conductive polymer. Moreover, it discovered that the mobility of an electron further improved and the activity of sulfur can be improved by making the shape of sulfur into a fibrous form. Based on the above findings, the present invention has been completed.

  That is, the present invention includes (1) a sulfur composite characterized in that the sulfur surface is coated with a metal layer or a metal sulfide layer, and (2) a sulfur composite according to the above (1) having a fiber shape, (3) The sulfur composite described in (1) or (2) above, wherein the metal layer is a bismuth layer, and (4) the sulfur described in (1) or (2) above, wherein the metal sulfide layer is a copper sulfide layer. A positive electrode comprising a composite, (5) a positive electrode active material layer containing the sulfur composite according to any one of (1) to (4), and a current collector; (6) above (5 And a positive electrode, an electrolyte, and a negative electrode.

  According to the present invention, it is possible to provide a sulfur composite that can be used as a positive electrode material, has high conductivity and reactivity, and suppresses elution of a sulfur compound that occurs during charge and discharge into an electrolytic solution. Moreover, the sulfur composite which can raise content of sulfur in a positive electrode can be provided. Furthermore, the positive electrode and secondary battery which use the said sulfur composite_body | complex and with few capacity | capacitance fall by charging / discharging can be provided.

Cyclic voltammetry in the case where the sulfur composite of the present invention in which the sulfur surface of Example 1 was coated with a copper sulfide layer was used for an electrode and in the case where sulfur whose surface was not coated in Comparative Example 1 was used for an electrode It is a graph which shows.

  The sulfur composite of the present invention is characterized in that the sulfur surface is coated with a metal layer or a metal sulfide layer. The sulfur composite of the present invention is a composite of sulfur and a metal layer and / or metal sulfide layer coated on the sulfur surface. The metal layer is not particularly limited. For example, cobalt, nickel, copper, ruthenium, rhodium, palladium, silver, indium, tin, antimony, platinum, gold, lead, bismuth, vanadium, chromium, manganese And a layer made of one or more metals selected from iron, zinc, molybdenum, tungsten, rhenium and the like, and alloys thereof. Among these, a bismuth layer can be preferably exemplified. Bismuth has high conductivity and stability. When a sulfur-bismuth composite is used for the positive electrode, the conductivity and reactivity are further improved, and the resulting sulfur compound is highly effective in preventing elution. As described above, the sulfur composite of the present invention may be coated with one type of metal or alloy, or may be coated with a plurality of types of metal or alloy.

  The metal sulfide layer is not particularly limited. For example, cobalt, nickel, copper, ruthenium, rhodium, palladium, silver, indium, tin, antimony, platinum, gold, lead, bismuth, vanadium, chromium List one or more metals selected from manganese, iron, zinc, molybdenum, tungsten, rhenium, etc., and one or more metal sulfide layers selected from sulfides of these alloys. Can do. Among these, a copper sulfide layer can be preferably exemplified. Copper sulfide has high stability and conductivity as a metal sulfide, and when the sulfur-copper sulfide composite is used for the positive electrode, the conductivity and reactivity are improved, and the elution prevention effect of the generated sulfur compound is high. As described above, the sulfur composite of the present invention may be coated with one type of metal sulfide or may be coated with a plurality of types of metal sulfides.

Thus, in the sulfur composite of the present invention, the sulfur surface is coated with the metal layer and / or the metal sulfide layer, so that the conductivity and reactivity are high, and the electrolyte solution of the sulfur compound generated during charge / discharge Since elution into the inside is suppressed, it can be suitably used as a positive electrode material. Since the coated metal layer and metal sulfide layer have high conductivity, the mobility of electrons during charge / discharge can be improved, and the reactivity of sulfur can be increased. The reaction efficiency of the sulfur redox reaction can be improved. When sulfur is used for the positive electrode and lithium is used for the negative electrode, lithium ions react with sulfur on the sulfur surface of the positive electrode during the discharge reaction, and the sulfur is reduced to form sulfur compounds such as Li ₂ S ₄ , Li ₂ S ₂ and Li ₂ S. Produces. During the charging reaction, lithium ions are released from these sulfur compounds. Here, Li ₂ S is difficult to dissolve in the electrolytic solution, but polysulfides such as Li ₂ S ₄ and Li ₂ S ₂ are soluble in the electrolytic solution and are eluted into the electrolytic solution. The eluted polysulfide reaches the negative electrode, and generates slightly soluble Li ₂ S at the negative electrode to inhibit the charge / discharge reaction between the positive electrode and the negative electrode. . On the other hand, in the sulfur composite of the present invention, the surface of sulfur is covered with a metal layer or metal sulfide layer, so that lithium ions can reach the sulfur surface through the metal layer or metal sulfide layer. It can pass through the metal layer and the metal sulfide layer and exit into the electrolyte solution. However, sulfur compounds such as Li ₂ S ₄ , Li ₂ S ₂ , Li ₂ S, S ₄ ²⁻ , Polysulfide ions such as S ₂ ²⁻ cannot easily pass through a metal layer or a metal sulfide layer. For this reason, the elution into the electrolyte of the sulfur compound to produce | generate can be prevented, without inhibiting reaction with lithium and sulfur in a positive electrode.

  The thickness of the metal layer or metal sulfide layer in the sulfur composite of the present invention is not particularly limited, but is preferably 0.01 to 5 μm, more preferably 0.1 to 3 μm. Such metal layers and metal sulfide layers may have different thicknesses depending on locations, but preferably have a uniform thickness. When the thickness of the metal layer or metal sulfide layer is 0.01 to 5 μm, the effect of imparting conductivity or reactivity to sulfur and the effect of preventing the sulfur compound from eluting into the electrolyte are further increased. In the conventional method using carbon for imparting conductivity to sulfur, the content of sulfur in the positive electrode active material layer is usually 50 to 60% by mass, and the carbon particles are mechanically attached to the surface of the sulfur particles. Even if it adheres, it is about 70% by mass, but the content can be increased by using the sulfur composite of the present invention. This is because by forming a metal layer or metal sulfide layer with the above thickness on the surface of sulfur, it is possible to impart sufficient conductivity and reactivity to sulfur, so the amount of carbon (conductive agent) is reduced. It is because it can reduce, and the compounding ratio of sulfur can be raised to 80 mass% or more.

  In the sulfur composite of the present invention, the entire sulfur surface may be covered with a metal layer and / or a metal sulfide layer, or a part of the surface may be covered with a metal layer and / or a metal sulfide layer. However, it is preferable that the surface of sulfur is coated with a metal layer and / or a metal sulfide layer to such an extent that the electrochemical activity is not impaired and the effect of preventing the elution of the sulfur compound into the electrolyte is not impaired. .

  Although it does not specifically limit as a shape of the sulfur composite_body | complex of this invention, For example, spherical shape, plate shape, columnar shape, indefinite shape, fibrous shape etc. can be mentioned. In the case of a particulate sulfur composite such as a spherical shape, a plate shape, a columnar shape, an irregular shape, the size is not particularly limited, but the diameter of the primary particle or the length of the longest side is preferably 500 μm or less, Furthermore, 300 micrometers or less are preferable. Since it is the surface of sulfur or the vicinity of the surface that mainly contributes to the reaction at the electrode, when the size of the sulfur composite particles is in the above range, the surface area per unit mass or unit volume increases, This is because the area of the surface portion contributing to the reaction increases. Further, the lower limit of the diameter of the primary particles or the length of the longest side is not particularly limited, but is preferably 5 μm or more, and more preferably 10 μm or more from the viewpoint of workability when used for the positive electrode. From the above, the diameter or the length of the longest side of the primary particles of the particulate sulfur composite such as spherical shape, plate shape, columnar shape, and irregular shape is preferably 5 to 500 μm, and more preferably 10 to 300 μm.

  The shape of the sulfur composite of the present invention is preferably a fiber shape. When a fiber-shaped sulfur composite is used, the surface area per unit mass or unit volume increases, resulting in an increase in the area of the surface portion that contributes to the reaction at the electrode, and the sulfur activity is increased. The reaction efficiency is improved. In addition, the amount of metal and metal sulfide supported increases, so the conductivity is improved. In addition, in the particulate sulfur composite, the movement of electrons may be restricted at the interface where the sulfur composite particles are in contact with each other, but in the fiber-shaped sulfur composite, a metal layer is formed on the surface of one fiber. In addition, since the metal sulfide layer is continuously formed, the influence of this is small and the electron mobility is further improved.

  The fibrous shape is not particularly limited, but the ratio (L / d) between the length (L) and the diameter (d) of the fiber is preferably 50 or more, more preferably 200 or more, and still more preferably 500 or more. Also, the fiber diameter is not particularly limited, but in the case of a fiber, the smaller the diameter, the larger the surface area per unit mass or unit volume, and the mobility of sulfur electrons, which is an insulator and is less reactive And the effect of improving activity increases. From this viewpoint, the diameter of the fiber is preferably 60 μm or less, and more preferably 50 μm or less. Further, the lower limit of the fiber diameter is not particularly limited as long as it can be practically used, but from the viewpoint of increasing the strength of the fiber, the fiber diameter is preferably 5 μm or more, and more preferably 10 μm or more. From the above, it is desirable that the fiber has a diameter of 5 to 60 μm, preferably 10 to 50 μm, and more preferably 20 to 40 μm. Further, the length of the fiber is not particularly limited, but when used as a nonwoven mat, the length of the fiber is preferably 1 to 40 cm from the viewpoint that the nonwoven can be easily formed into a mat. 5-30 cm is more preferable, and 5-20 cm is still more preferable.

  Examples of the method for producing the sulfur composite of the present invention include the following methods. First, sulfur particles and fibers are prepared. And when coat | covering with a metal, the electroless-plating bath containing the metal to coat | cover is prepared, and sulfur particle | grains and a fiber are immersed in this electroless-plating bath for predetermined time. By performing such electroless plating, a metal can be deposited on the surface of sulfur to form a metal coating layer. For electroless plating, a commonly used method can be used, and a catalyst such as palladium and a reducing agent such as formaldehyde can be used as necessary. According to electroless plating, a uniform and void-free metal layer can be coated on the surface of sulfur of various shapes regardless of the shape of sulfur, and a strong metal layer can be coated.

  In the case of coating with a metal sulfide, a metal sulfide coating layer can be formed by contacting and coating a metal on the surface of sulfur and sulfiding the metal. For example, a metal sulfide coating layer can be formed by depositing a metal on the surface of sulfur particles or fibers by electroless plating and sulfiding the deposited metal at the surface portion. According to this method using electroless plating, regardless of the shape of the sulfur, it is possible to coat the surface of various shapes of sulfur with a uniform and void-free metal sulfide layer, and to form a strong metal sulfide layer. Can be coated. Instead of electroless plating, the surface of sulfur can be coated with a metal or metal sulfide by using a film forming method by a gas phase such as sputtering or vapor deposition. In addition, the sulfur composite of the present invention is not only in a form in which a metal layer or a metal sulfide layer is subsequently coated on the sulfur surface, but also the sulfur surface itself is modified to form a metal layer or a metal sulfide layer. Forms are also included.

  The sulfur fiber used for production of the sulfur composite of the present invention can be produced, for example, by the following method. The melting point of sulfur is 112.8 ° C. (α sulfur) to 119.6 ° C. (γ sulfur), but the viscosity increases up to 195 ° C. and decreases again at higher temperatures. Therefore, sulfur fibers can be produced by melting sulfur at about 180 to 250 ° C., preferably about 190 to 220 ° C., and extruding it from a nozzle.

  As another method, a syringe with a needle (needle) is prepared, sulfur is put into the syringe and heated, and a voltage is applied between the needle (needle) and a collector (current collector). When the voltage exceeds the threshold value, the charge repulsive force overcomes the surface tension of the molten sulfur to generate a charged jet. In the electric field, the jet stretches to form very thin fibers and deposits on the collector. In this way, sulfur fibers having a diameter of nano to micron can be produced (melt electrospinning method). In this case, the diameter of the sulfur fiber can be determined by the viscosity of the molten sulfur, the applied voltage, and the distance between the needle and the collector. When the distance between the needle and the collector is excessively short, a discharge occurs between the needle and the collector, and when it is excessively long, the electrostatic attractive force that pulls the fiber may be small. preferable. The applied voltage is preferably 8 kV to 20 kV because the electrostatic attractive force is small when the voltage is excessively low and discharge may occur between the needle and the collector when the voltage is excessively high. According to this method, the apparatus is simple and can be miniaturized in a short time, and sulfur fibers having a small diameter can be obtained. By using the sulfur fiber thus obtained and forming a metal layer or metal sulfide layer on the surface of the sulfur fiber by the above method, the fiber-shaped sulfur composite of the present invention is obtained.

  The positive electrode of the present invention is a positive electrode provided with a positive electrode active material layer on the surface of a current collector, and the sulfur composite of the present invention is contained in the positive electrode active material layer. Although it does not specifically limit as a collector used for the positive electrode of this invention, For example, aluminum foil, stainless steel foil, carbon, etc. can be mentioned. The positive electrode active material layer of the positive electrode of the present invention contains the sulfur composite of the present invention and, if necessary, a binder and a conductive additive. The binder is not particularly limited, and examples thereof include polyvinylidene fluoride, polytetrafluoroethylene, polyimide, polyamide, polyacrylonitrile, porethylene, and polypropylene. Although it does not specifically limit as a conductive support agent, For example, carbon black, acetylene black, ketjen black, graphite, carbon fiber etc. can be mentioned. The content of the sulfur composite of the present invention in the positive electrode active material layer is not particularly limited, but it exhibits the characteristics of the sulfur composite of the present invention, and is a secondary battery that has a high capacity and a small capacity decrease due to charge / discharge. From the viewpoint of obtaining a battery, 50% by mass or more is preferable, 70% by mass or more is more preferable, and 80% by mass or more is further preferable. The positive electrode active material layer may contain a positive electrode active material other than the sulfur composite of the present invention.

  Examples of the method for producing the positive electrode of the present invention include the following methods. The sulfur composite of the present invention is mixed with a solvent, the binder, the conductive auxiliary agent, and the like as necessary. This mixture is applied onto a current collector and dried to form a positive electrode active material layer on the current collector. Although it does not specifically limit as a solvent, For example, N-methyl-2- pyrrolidone, N, N- dimethylformaldehyde, xylene, toluene etc. can be mentioned. In the case of a fiber-shaped sulfur composite, the fiber length can be increased to form a mat. In this case, the mat-shaped sulfur composite is placed on the current collector and fixed with a binder, and the positive electrode active material Layers can also be formed.

The secondary battery of the present invention includes an electrolyte and a negative electrode in addition to the positive electrode of the present invention. In addition, a separator or the like may be provided. As the electrolyte, but are not particularly limited, examples thereof _{include LiPF 6, LiBF 4, LiAsF 6} , LiI, and LiTFSI like. The electrolyte can be used by dissolving in a non-aqueous solvent such as propylene carbonate, ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dioxolane, dimethoxyethane, and the like. Although it does not specifically limit as a negative electrode, For example, the substance which occludes and discharges lithium, such as substances containing lithium, such as metallic lithium and a lithium alloy, and graphite can be mentioned. In the secondary battery of the present invention, for example, when metal lithium is used for the negative electrode, lithium ions reach the sulfur surface through the metal layer or metal sulfide layer of the sulfur composite of the present invention and react with sulfur. Further, during this sulfur reduction reaction, a sulfur compound such as lithium polysulfide soluble in the electrolytic solution is generated, but elution into the electrolytic solution is suppressed by the metal layer or metal sulfide layer. Therefore, it is possible to obtain a secondary battery with little capacity reduction due to repeated charge and discharge. Furthermore, since electrons move through a metal layer or metal sulfide layer with high conductivity and exchange electrons with the current collector, a high-capacity secondary battery can be obtained. The secondary battery of the present invention can be produced by combining the positive electrode of the present invention, the electrolyte and the negative electrode, and the shape of the secondary battery is not particularly limited, but is a coin type, a stacked type, a cylinder Examples include molds.

  EXAMPLES Hereinafter, although an Example of this invention is given and this invention is demonstrated concretely, the technical scope of this invention is not limited to these illustrations.

(Production of fibrous sulfur)
Silicon cord heater (1.5 m) connected to a transformer [TYPE S-130-10 made by YAMABISHI Corporation] to a 3 ml glass syringe [made by MITSUBA Corporation] equipped with a stainless needle (inner diameter 0.7 mm) [mutual chemistry Glass Manufacturing Co., Ltd. SKH-0151] was wound. A stainless steel plate (9 × 9 cm) was used as a collecting plate. A high voltage power supply [Matsusada Precision Co., Ltd.] was connected so that the stainless needle was positively charged and the collector plate was negatively charged when voltage was applied. The temperature of the code heater was measured and controlled by a contact temperature sensor [THI-303F manufactured by TASCO JAPAN Co., Ltd.] and a digital meter relay with sensor power supply [TAT-806A manufactured by TASCO JAPAN CO., LTD.]. The distance between the tip of the stainless needle and the collecting plate was 6 cm. After the sulfur powder was added to the glass syringe, the glass syringe was heated to 200 ° C. with a silicon cord heater. Thereafter, a voltage of 15 KV was applied to the stainless needle and the collecting plate, and melted electrospinning of sulfur was performed on the collecting plate to produce fibrous sulfur used in Example 1.

(Surface coating)
Electroless plating containing 0.05 mol / L of copper sulfate, 10 mL / L of formaldehyde, 1.25 mol / L of sodium hydroxide, and 0.10 mol / L of EDTA (ethylenediaminetetraacetic acid), in the form of fibrous sulfur produced. Immerse in the bath for 2 hours. Copper was first deposited on the sulfur surface, but this copper changed to copper sulfide and finally deposited on the sulfur surface as copper sulfide. Thus, a sulfur composite having a sulfur surface coated with a copper sulfide layer was obtained. When the obtained sulfur composite was observed with a scanning electron microscope (SEM), the length was 5 to 10 cm, and the diameter was 30 to 40 μm. Moreover, when the cross section of the sulfur composite was observed with a scanning electron microscope (SEM) and the thickness of the copper sulfide layer was measured, it was 1 μm to 2 μm. Further, the copper sulfide layer was uniformly coated on almost the entire surface.

(Measurement of discharge capacity)
The obtained sulfur composite is placed on a glassy carbon, and the obtained sulfur composite is covered with a separator [Celgard "Celgard (registered trademark) 2500"] to fix the sulfur composite on the current collector. As a positive electrode active material layer, a positive electrode was produced. This positive electrode was incorporated into a tripolar cell to produce a secondary battery. Metal lithium was used for the negative electrode, and 1 mol LiTFSI (lithium bis (trifluoromethanesulfonyl) imide) / DOX (dioxolane): DME (dimethoxyethane) (volume ratio 1: 1) was used for the electrolyte. Using the secondary battery thus produced, the discharge capacity at 0.05 C was measured. The results are shown in Table 1.

The same fiber shape sulfur used in Example 1, ethylenediaminetetraacetic acid 0.10 mol / L, trisodium citrate dihydrate 0.17 mol / L, nitrilotriacetic acid 0.10 mol / L, It was immersed in an electroless plating bath containing 0.06 mol / L of bismuth trichloride and 0.03 mol / L of a stannous chloride 20% hydrochloric acid solution for 2 hours. Bismuth was deposited on the sulfur surface, and a sulfur composite in which the sulfur surface was coated with a bismuth layer was obtained. When the obtained sulfur composite was observed with a scanning electron microscope (SEM), the length was 5 to 10 cm, and the diameter was 30 to 40 μm. Moreover, when the cross section of the sulfur composite was observed with a scanning electron microscope (SEM) and the thickness of the bismuth layer was measured, it was 1 μm to 2 μm. Further, the bismuth layer was uniformly coated on almost the entire surface. Using the obtained sulfur composite, a positive electrode and a secondary battery were produced in the same manner as in Example 1, and the discharge capacity was measured. The measurement results are shown in Table 1.
[Comparative Example 1]

  Using the same fiber-shaped sulfur as used in Example 1 without coating, a positive electrode and a secondary battery were produced in the same manner as in Example 1, and the discharge capacity was measured. The measurement results are shown in Table 1.

(Cyclic voltammetry (CV) measurement)
Using the sulfur composite of Example 1 and the sulfur of Comparative Example 1, CV measurement was performed to investigate the electrochemical behavior of the electrode. For the measurement, an electrochemical system [HZ-5000 manufactured by Hokuto Denko Co., Ltd.] was used. The potential scanning range is 1.8 V to 3.2 V for all the electrodes, the scanning speed is 0.5 mVs ^−1, and the cell temperature is constant at 30 ° C. in a thermostatic bath [EYELA MG-2300 manufactured by Tokyo Science Instruments Co., Ltd.]. The measurement was carried out. All these operations were performed in a glove box under an argon atmosphere. The electrolyte is 1 mol LiTFSI (lithium bis (trifluoromethanesulfonyl) imide) / DOX (dioxolane): DME (dimethoxyethane) (volume ratio 1: 1). The measurement results are shown in FIG.

Example 1 using a sulfur composite having a sulfur surface coated with a copper sulfide layer and Example 2 using a sulfur composite having a sulfur surface coated with a bismuth layer were compared using sulfur with no coating on the surface. A higher discharge capacity was obtained than in Example 1. Thereby, it turned out that a highly conductive and reactive sulfur material is obtained as a positive electrode material by covering the sulfur surface with a copper sulfide layer or a bismuth layer. Moreover, from the result of the cyclic voltammetry (CV) measurement, the oxidation current value of the sulfur composite of Example 1 increased from that of Comparative Example 1. From this, it can be seen that by covering the sulfur surface with a copper sulfide layer, the lithium polysulfide produced during the sulfur reduction reaction is reoxidized, and the elution of sulfur compounds into the electrolyte is suppressed. It was.

Claims (4)

A sulfur composite having enhanced conductivity and reactivity with lithium compared to uncoated sulfur, wherein the sulfur surface is coated with a bismuth layer and is in the form of a fiber . A sulfur composite having enhanced conductivity and reactivity with lithium compared to uncoated sulfur, wherein the sulfur surface is coated with a copper sulfide layer and is in the form of a fiber . Cathode, wherein the cathode active material layer that contains the sulfur composite according to claim 1 or 2, further comprising a current collector. A secondary battery comprising the positive electrode according to claim 3 , an electrolyte, and a negative electrode.

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