JP6448352B2 - Positive electrode for alkaline metal-sulfur battery and method for producing secondary battery provided with the same - Google Patents

Description

  The technology disclosed in this specification relates to a positive electrode for an alkali metal-sulfur battery and a secondary battery including the same.

  Alkali metal-sulfur secondary batteries have been proposed as one of the next generation high capacity secondary batteries. A theoretical capacity of a positive electrode active material of a general lithium ion secondary battery in which lithium is used as an alkali metal, for example, lithium cobaltate is 274 mAh / g. However, since the reversibility is actually lost due to the collapse of the layered structure, the reversible capacity is about 160 mAh / g. On the other hand, since the theoretical capacity of the sulfur active material is as extremely large as 1675 mAh / g, the alkali metal-sulfur secondary battery is regarded as promising as a high capacity, high energy density secondary battery for the next generation.

  However, in an alkali metal-sulfur secondary battery, since sulfur, which is an insulator, is used for the positive electrode, the electrical resistance at the positive electrode is increased, and a large amount of a conductive material needs to be added to the positive electrode layer. Therefore, it is difficult to increase the amount of sulfur in the positive electrode layer (see Non-Patent Document 1).

  In addition, since battery characteristics vary greatly due to differences in the amount and distribution of sulfur, various methods for effectively and easily introducing sulfur into the positive electrode have been studied. For example, Patent Document 1 describes a method of introducing sulfur into a positive electrode layer by dissolving sulfur in a solvent such as carbon disulfide and impregnating a pretreated positive electrode material. In addition, a method of introducing sublimated sulfur into porous carbon, which is a positive electrode material, under high pressure (see Patent Document 2), or by dispersing and heating sulfur granules and sulfur fine particles on a carbon material. A method of introducing sulfur into the material (see Patent Documents 3 and 4) and the like has been studied. In addition, Patent Document 5 describes a conductive carbon material whose surface is coated with a sulfur film, and Non-Patent Document 2 also describes a method of introducing sulfur into the carbon material.

US Patent Application Publication No. 2011/0052998 US Patent Application Publication No. 2013/0330619 International Publication 2012/070184 JP 2012-238448 A JP 2014-017240 A

Yan Diao et al., J. Elechem. Soc., 159 (11) A1816-A1821 (2012) Jan Bruckner et al., J. Power Sources 268 82-87 (2014)

  However, the methods described in any of the documents have a problem that it takes time and effort to control the amount of sulfur introduced and to carry out the method itself.

  On the other hand, porous carbon that can maintain conductivity even when the amount of sulfur introduced is increased, and a material having a high porosity such as a carbon nanotube (CNT) electrode has been used in the positive electrode layer (Patent Literature). 3-5), it has become possible to maintain conductivity even if the amount of sulfur introduced into the positive electrode increases. However, when the amount of sulfur introduced into the CNT electrode is increased using the methods described in Patent Documents 1 to 5, the number of introduction treatments increases, and it takes a long time for the treatment, or sulfur is unevenly distributed on the CNT surface. Thus, there is a problem that good battery characteristics cannot be obtained.

  In view of the above problems, the present invention provides a method capable of introducing a large amount of sulfur into the positive electrode layer in a short time with good controllability, and a positive electrode for an alkali metal-sulfur battery capable of exhibiting the inherent high capacity of the sulfur active material. The purpose is to provide.

  A positive electrode for an alkali metal-sulfur battery disclosed in the present specification is composed of a plurality of conductive materials, a porous conductive material layer having voids formed between the plurality of conductive materials, and the porous conductive material layer And sulfur held in the gap, and a resin film placed directly on at least a part of the porous conductive material layer.

  Moreover, the lithium-sulfur secondary battery disclosed in the present specification includes the above-described alkali metal-sulfur battery positive electrode, a negative electrode including a material that absorbs and releases lithium ions, and the alkali metal-sulfur battery positive electrode. An electrolytic solution filled with the negative electrode and having lithium ion conductivity, and a separator for insulating between the positive electrode for metal sulfur battery and the negative electrode in the electrolytic solution are provided.

  In addition, the method for producing a positive electrode for an alkali metal-sulfur battery disclosed in the present specification is composed of a plurality of conductive materials, and a porous conductive material layer having voids between the plurality of conductive materials is formed on the upper surface. A step of preparing a current collector, a step of directly placing a resin film containing sulfur on at least a part of the porous conductive material layer, and sulfur contained in the resin film in the porous conductive material layer And a step of inserting.

  According to the method disclosed in the present specification, since a large amount of sulfur can be uniformly introduced into the positive electrode layer in a short time with good controllability, an alkaline metal-sulfur battery positive electrode capable of exhibiting a high capacity inherent in sulfur is obtained. Can be provided.

FIG. 1 is a cross-sectional view illustrating a schematic configuration of an alkali metal-sulfur battery including a positive electrode according to an embodiment disclosed in the present specification. FIG. 2 is a flowchart showing a manufacturing process of the positive electrode for an alkali metal-sulfur battery according to the embodiment. FIG. 3 is a diagram showing a charge / discharge profile of the lithium-sulfur secondary battery according to Example 6.

-Configuration of alkali metal-sulfur secondary battery and positive electrode layer-
FIG. 1 is a cross-sectional view illustrating a schematic configuration of an alkali metal-sulfur secondary battery including a positive electrode according to an embodiment disclosed in the present specification. FIG. 1 shows a lithium-sulfur secondary battery 1 as an example of an alkali metal-sulfur secondary battery. In the present specification, the resin film 5 described later is also described as a part of the positive electrode layer 3 for convenience.

  A lithium-sulfur secondary battery 1 according to this embodiment includes a positive electrode (positive electrode layer) 3, a negative electrode (negative electrode layer) 15 containing a material that absorbs and releases lithium ions, and a positive electrode layer 3 and a negative electrode layer 15. A filled electrolytic solution 17 having lithium ion conductivity and a separator 13 that insulates between the positive electrode layer 3 and the negative electrode layer 15 in the electrolytic solution 17 are provided.

  In addition, the positive electrode layer 3 of the present embodiment includes a current collector 11 having an upper surface (planar) and a plurality of conductive materials 10, and a porous conductive material formed on the upper surface (planar) of the current collector 11. It has a layer 7, sulfur 9 held in the porous conductive material layer 7, and a resin film 5 placed directly on at least a part of the porous conductive material layer 7. In the porous conductive material layer 7, the plurality of conductive materials 10 are arranged so that a large number of voids are formed between the plurality of conductive materials 10. In the present specification, the surface of the current collector 11 that faces the negative electrode layer 15 is referred to as an “upper surface” for convenience.

  The current collector 11 may be a general one made of metal or the like. For example, an aluminum (Al) film as a base film and a conductive material 10 are grown on a metal foil made of nickel (Ni), for example. Any one of an iron (Fe) film, a Ni film, and a cobalt (Co) film may be sequentially formed as the catalyst layer. The shape of the current collector 11 is not particularly limited, but in the example illustrated in FIG. 1, the current collector 11 has a flat upper surface and a lower surface, and has a foil shape having a predetermined planar shape.

  The conductive material 10 may be a material that can hold sulfur 9, for example, porous carbon, but if it is a carbon nanotube (CNT), conductivity is ensured even if a large amount of sulfur 9 is held in the gap between the conductive materials 10. Therefore, when the conductive material 10 is CNT, the plurality of CNTs are oriented perpendicularly to the upper surface (plane) of the current collector 11, so that the CNT and the current collector 11 are directly conducted and are high. Conductivity can be maintained. Conventionally, a positive electrode layer has been generally produced by adding a binder to conductive carbon and sulfur. However, the binder increases the electrical resistance and prevents the ion path at the interface. On the other hand, in the positive electrode layer containing vertically aligned CNT and sulfur, it is possible to introduce sulfur into the gap between CNTs without adding a binder, so that both ion conductivity and conductivity are increased. It becomes possible.

  With this configuration, the sulfur 9 inserted from the resin film 5 into the porous conductive material layer 7 can be easily held in the voids in the CNT when the positive electrode layer 3 described later is manufactured.

In the porous conductive material layer 7, the density of the conductive material (CNT) 10 may be, for example, 1 × 10 ⁸ / cm ² or more and 1 × 10 ¹² / cm ² or less. The length of the CNT may be, for example, 100 μm or more and 1000 μm or less. According to this configuration, since the CNTs can be arranged at an appropriate interval and length, the electrolytic solution 17 containing lithium ions can be easily infiltrated into the porous conductive material layer 7, and the resin film 5 Sulfur 9 to be inserted into the porous conductive material layer 7 can be quickly introduced into the CNT, so that sufficient electrochemical oxidation-reduction reaction proceeds even if the ion path and the conductive path coexist and the amount of the sulfur active material is increased. To do.

  The porous conductive material layer 7 is made of resin components such as polyethylene oxide (PEO), polyvinyl pyrrolidone (PVP), polyvinyl acetate (PVAc) or polytetrafluoroethylene (PTFE), and additives such as dispersants and leveling agents. May be included. In the step of introducing sulfur into the porous conductive material layer 7 to be described later, when the resin film 5 contains a dispersant or additive, the dispersant or additive can also be introduced into the porous conductive material layer 7.

  The resin film 5 may be made of a resin that is insoluble or slightly soluble in the electrolytic solution 17 and that does not chemically react with the in-battery material containing sulfur 9 or a compound such as polysulfide newly generated during charge and discharge. It may be composed of polyethylene oxide (PEO), polyvinyl pyrrolidone (PVP), polyvinyl acetate (PVAc), polytetrafluoroethylene (PTFE), or the like. Among these, if PEO is used as a constituent material of the resin film 5, the sulfur 9 contained in the resin film 5 can be easily and easily inserted into the porous conductive material layer 7 when the positive electrode layer 3 is produced. It is preferable because the ion conductivity of the resin film 5 is not impaired after sulfur is released.

  The film thickness of the resin film 5 is not particularly limited, but may be, for example, about 50 μm or more and 500 μm or less when the positive electrode layer 3 is manufactured. If the film thickness of the resin film 5 is excessively increased, it is difficult for the sulfur 9 to move from the resin film 5 to the porous conductive material layer 7.

When CNT is used as the conductive material 10, the capacity of the sulfur 9 is sufficient if the density as sulfur atoms per unit area in the porous conductive material layer 7 before charging and discharging is 5 mg / cm ² or more. It is more preferable because it can be utilized in the future. Further, in the case where sulfur is contained only in the positive electrode layer 3 in the lithium-sulfur secondary battery 1 before charging, “sulfur contained in the entire lithium-sulfur secondary battery 1 per unit area of the porous conductive material layer 7. And “the total amount of sulfur atoms in the sulfur compound” is preferably 5 mg / cm ² or more.

  Since the CNT layer (porous conductive material layer 7) has sufficient voids, the conductivity of the positive electrode layer 3 can be maintained even when the amount of sulfur 9 introduced is high. When the occupied volume of the entire porous conductive material layer 7 is set to 100 vol%, the positive electrode layer can be used even if the retained amount of sulfur 9 is large as long as the porosity (ratio occupied by the space) of the porous conductive material layer 7 is 60 vol% or more. 3 is preferable because the electrical resistance value of 3 is difficult to increase.

In the lithium-sulfur secondary battery 1, most of the sulfur 9 in the porous conductive material layer 7 is simple sulfur, but part of the sulfur introduced from the resin film 5 is polysulfur anion (Sx ²⁻ ) or the like. It is dissolved in the electrolytic solution 17 in the form of In addition, at least a part of the sulfur 9 retained in the conductive material 10 in the porous conductive material layer 7 is modified with an organic component containing a surface treatment agent such as a surfactant, a polymer pigment, or a silicone resin. It may be. In this case, the concentration of the organic component in the sulfur 9 may be 0.1 wt% or more and 10 wt% or less.

  As described above, the sulfur 9 is modified with an appropriate amount of organic components, so that the dispersibility of the sulfur 9 in the resin film 5 is improved during the production of the positive electrode layer 3, and the sulfur 9 is effectively made porous conductive. It can be inserted into the material layer 7.

  If the mass of the resin film 5 is less than 15% with respect to the mass of sulfur 9 introduced into the porous conductive material layer 7 (= the mass of sulfur 9 contained in the porous conductive material layer 7 before charging and discharging). More preferable. By making the mass of the resin film 5 within this range, when the sulfur 9 is inserted from the resin film 5 into the porous conductive material layer 7, the sulfur 9 that is captured by the resin film 5 and remains in the resin film 5. The amount can be reduced.

  In addition, since a part of sulfur 9 melts into the electrolyte solution 17 from the positive electrode layer 3 after charging / discharging, the mass of the resin film 5 is “sulfur and lithium contained in the entire lithium-sulfur secondary battery 1 after charging / discharging”. If it is less than 15% of the “total amount of sulfur atoms of the sulfur compound”, it can be said to be preferable.

In the lithium-sulfur secondary battery 1 of the present embodiment, as the negative electrode layer 15, a general one can be used as the negative electrode of a lithium ion secondary battery or a lithium sulfur secondary battery. Specifically, as the material of the negative electrode layer 15, for example, an alloy of Li, Li and Al or In, or Si, SiO, Sn, SnO ₂ or hard carbon doped with lithium ions can be used. .

  As the electrolytic solution 17, for example, at least one selected from ether-based electrolytic solutions such as tetrahydrofuran, glyme, diglyme, triglyme and tetraglyme, and ester-based electrolytic solutions such as diethyl carbonate and propylene carbonate, or these A mixture of at least one selected from among them (for example, glyme, diglyme or tetraglyme) with dioxolane for viscosity adjustment can be used.

  As the separator 13, a well-known thing can be used as a separator of a lithium ion secondary battery or a lithium-sulfur secondary battery. For example, the separator 13 is composed of a porous film made of synthetic resin such as polytetrafluoroethylene, polypropylene, or polyethylene, or a porous film made of ceramic, and has a structure in which two or more kinds of these porous films are laminated. You may have. Among these, the porous film made of polyolefin not only has an excellent short-circuit prevention effect, but also improves the safety of the battery by the shutdown effect (the effect of closing the vacancies and closing the current when an excessive current flows). This is preferable.

In the lithium-sulfur secondary battery 1 having the above configuration, when the negative electrode layer 15 is made of metallic lithium, a discharge product from a sulfur active material such as Li ₂ S is charged at the positive electrode according to the following formula (1). Street 8Li ₂ S → 16Li ⁺ + S ₈ + 16e ⁻ (1)
Oxidation reaction occurs, and Li ⁺ is released into the electrolytic solution 17. Li ⁺ via separator 13 to move to the negative electrode, the negative electrode interface Li ⁺ is charged by being oxidized by the following reaction formula (2).

Li ⁺ + e ⁻ → Li (2)
Moreover, at the time of discharge, an electric current can be taken out of the lithium sulfur secondary battery 1 by a reaction reverse to that at the time of charging.

-Manufacturing method of positive electrode layer 3-
FIG. 2 is a flowchart showing manufacturing steps of the positive electrode layer 3 according to this embodiment shown in FIG.

As shown in the figure, when producing the positive electrode layer 3 according to the present embodiment, first, a resin film 5 containing simple sulfur (for example, orthorhombic sulfur S ₈ ) is produced (step S1). Specifically, simple sulfur, a resin having a predetermined molecular weight, and an organic solvent are stirred in a container, and then a light yellow slurry is prepared by ball milling or the like. Next, after applying this slurry to a resin sheet or the like, drying is performed to remove the solvent, and peeling from the resin sheet is performed to form a resin film 5 having a predetermined shape containing sulfur. In addition, the resin film formed at this process is a mixed film (sulfur / resin sheet) of sulfur and resin including sulfur in a state that does not chemically react with the resin, and has a predetermined film thickness.

  In this step, the mass of the resin added when preparing the slurry is not particularly limited, but if the mass of the resin added to the slurry is less than 15% of the mass of sulfur, the porous conductive material layer is formed from the resin film 5 in a later step. This is preferable because sulfur can be efficiently transferred to 7.

  The sulfur used in this step may be unmodified simple sulfur, for example, partially or entirely modified with an organic component in an amount of about 0.1 wt% to 10 wt% with respect to the total sulfur. Sulfur may be used. By using sulfur modified with an organic component, the dispersibility of sulfur in the slurry can be improved, and the resin film 5 containing sulfur uniformly can be easily produced.

  Separately from the production of the resin film 5, the porous conductive material layer 7 is formed on the upper surface of the current collector 11 (step S2). Specifically, a base film having a thickness of 5 to 500 nm and a catalyst layer having a thickness of 0.5 to 20 nm are sequentially formed on the upper surface of the current collector 11 made of, for example, a metal foil. The base film is provided in order to improve the adhesion between the conductive material 10 (for example, CNT) and the current collector 11, and at least one metal selected from Al, Ti, V, Ta, Mo, and W, and the metal It is comprised from the alloy containing this, or the nitride of the metal. The catalyst layer is made of, for example, at least one metal selected from Ni, Fe, or Co, or an alloy containing one or more of these metals.

  Next, a conductive material 10 such as CNT is grown at a predetermined density on the current collector 11 through a catalyst layer by a known chemical vapor deposition (CVD) method or the like to form the porous conductive material layer 7. When growing CNTs in this step, the CNTs can be grown so as to be oriented perpendicular to the upper surface (plane) of the current collector 11. At this time, even if there is some variation in the orientation direction of the CNT, it is allowed.

  Next, sulfur contained in the resin film 5 is introduced into the porous conductive material layer 7 (step S3 in FIG. 2). In this step, the resin film 5 is placed on the porous conductive material layer 7 and pressed and then heated at a temperature at which sulfur becomes liquid, for example, about 115 ° C. or more and 250 ° C. or less. If this heating temperature is around 155 ° C. at which the viscosity of sulfur is the lowest, sulfur can be inserted quickly, which is more preferable.

  Thereby, the sulfur in the resin film 5 is melted and inserted into the voids in the porous conductive material layer 7 by capillary action (sulfur 9 in FIG. 1). This process may be performed in air | atmosphere, may be performed in the atmosphere of inert gas, such as nitrogen and argon, and may be performed under reduced pressure or a vacuum. Furthermore, a pressure operation such as returning from the reduced pressure to the atmosphere during insertion may be performed. By this step, sulfur content is removed from the resin film 5, so that a large number of holes are formed inside the resin film 5.

  After this step, the resin film 5 may be removed, but the positive electrode layer 3 of the present embodiment shown in FIG. 1 can be easily produced by leaving the resin film 5. Even if the resin film 5 remains in the positive electrode layer 3, the performance when the positive electrode layer 3 lithium-sulfur secondary battery 1 is produced is hardly affected. In addition, if the amount of resin in the resin film 5 is small, the resin film 5 is difficult to peel off after the sulfur 9 is introduced into the porous conductive material layer 7. The range of the resin content in the film 5 can be widened.

  Thereafter, the lithium-sulfur secondary battery 1 using the positive electrode layer 3 of the present embodiment can be manufactured by a known method (step S4 in FIG. 2).

  According to the method of the present embodiment, the sulfur content in the resin film 5 and the content ratio of sulfur and resin are adjusted as appropriate, so that sulfur is uniformly distributed in the porous conductive material layer 7 in a short time with good controllability. Can be inserted. Moreover, if the method of this embodiment is used, controllability is better than the method of sprinkling sulfur fine particles directly on the porous conductive material layer 7 or the method of sublimating sulfur and introducing it into the porous conductive material layer 7. The sulfur 9 can be uniformly introduced into the porous conductive material layer 7.

  Further, since a large amount of sulfur can be introduced into the porous conductive material layer 7 by a single sulfur introduction step, the lithium-sulfur secondary battery 1 exhibits a sufficiently high capacity without repeating the introduction operation. A possible amount of sulfur 9 can be introduced into the porous conductive material layer 7. Further, by setting the resin content in the resin film 5 to, for example, 7 wt% or more before introducing sulfur, the resin film 5 can be easily peeled after introducing sulfur.

  The positive electrode layer 3 and the manufacturing method thereof, and the lithium-sulfur secondary battery 1 described above are examples of the embodiment, and the material, shape, slurry preparation procedure, etc. of each component depart from the spirit of the present invention. It can be changed appropriately within the range not to be.

The positive electrode layer for alkali metal-sulfur batteries according to Examples 1 to 5 and Comparative Examples 1 to 4 and the lithium-sulfur secondary battery according to Example 6 were produced as follows.
Example 1
A porous conductive material layer composed of CNTs was formed by growing CNTs having an average length of about 350 μm on a metal foil (current collector) made of Ni having a thickness of 20 μm by a CVD method. The CNTs were oriented in a direction perpendicular to the top surface of the metal foil. The density of CNT in the porous conductive material layer was 1 to 5 × 10 ¹⁰ pieces / cm ² . Hereinafter, a combination of a metal foil and a porous conductive material layer is referred to as a “CNT electrode”.

  Next, 5.0 g of sulfur (trade name: Sulfax PS, manufactured by Tsurumi Chemical Co., Ltd.) modified with 1% organic component, 0.56 g of PEO having a molecular weight of 3 million, and 30 g of zirconia beads having a diameter of 2 mm were weighed. These were put in a polyethylene container and stirred. After adding 20 g of acetonitrile to this polyethylene container and further stirring, ball milling was performed at 90 rpm for 12 hours to prepare a pale yellow viscous slurry. The dispersibility of sulfur in the slurry was good.

Next, the zirconia beads were removed using a mesh, and the slurry was applied onto a polyethylene terephthalate (PET) film coated with a release agent and dried. Thereby, a resin film containing sulfur (sulfur / resin sheet) was produced. The produced resin film had a thickness of about 100 μm and contained 5 mg / cm ² of sulfur. It shape | molded so that the produced CNT electrode might become the same planar shape, and the application | coating surface of the resin film was crimped | bonded to the upper surface of the porous conductive material layer. In this state, sulfur was melted by heating to about 180 ° C., and sulfur was introduced into the voids in the CNTs by capillary action. After the introduction of sulfur, the resin film from which sulfur was lost was left on the porous conductive material layer.

  According to the method of Example 1, almost the entire amount of sulfur could be introduced from the resin film into the porous conductive material layer by one sulfur introduction operation. As a result, the working time could be shortened.

  Moreover, it has confirmed visually that the external appearance of the positive electrode which concerns on Example 1 is favorable.

-Example 2-
A positive electrode for an alkali metal-sulfur battery was prepared in the same manner as in Example 1 except that sulfur not modified with an organic component (trade name: manufactured by Wako Pure Chemical Industries) was used when the slurry was prepared. Although the dispersibility of sulfur in the slurry was inferior to that of Example 1, a positive electrode having a generally good appearance could be formed.

  Moreover, according to the method of Example 2, it was possible to introduce almost the entire amount of sulfur from the resin film into the porous conductive material layer in one sulfur introduction operation. As a result, the working time could be as short as in Example 1. It was confirmed that the appearance of the positive electrode was good.

Example 3
Compared to Example 1, the PEO content in the resin film before introduction of sulfur was halved (5 wt% of the resin film mass). Otherwise, an alkali metal-sulfur battery positive electrode was produced in the same manner as in Example 1. The dispersibility of sulfur in the slurry was as good as in Example 1.

  In addition, according to the method of Example 3, a large part of sulfur could be introduced from the resin film into the porous conductive material layer by one sulfur introduction operation, but the resin film partially after the sulfur introduction. In the collapsed state, it was in close contact with the porous conductive material layer, and the resin film after introduction remained partially uneven on the surface of the CNT electrode. In this example, the working time could be shortened to the same extent as in Example 1, but the appearance of the positive electrode was slightly inferior to that in Example 1 because the resin film was locally broken.

Example 4
When producing the slurry, sulfur not modified with an organic component was used, and the amount of resin in the resin film before introduction of sulfur was 7 wt%. In addition, PTFE was used instead of PEO as the binder resin. At the time of slurry preparation, sulfur containing organic components, PTFE powder, and xylene were kneaded in a mortar and rolled to prepare a resin film. Sulfur was introduced in the same manner as in Example 1. In the method of Example 4, the dispersibility of sulfur in the slurry was good, but was slightly inferior to Example 1.

  Further, according to the method of Example 4, a slight amount of sulfur remained in the resin film after the sulfur introduction work, but most of the sulfur was transferred from the resin film into the porous conductive material layer by one sulfur introduction work. It was possible to introduce. In this example, the working time could be shortened to the same extent as in Example 1, and the appearance of the positive electrode was as good as in Example 1.

-Example 5
In Example 5, a positive electrode for a metal sulfur battery was produced in the same manner as in Example 1 except that the content of PEO in the resin film before introducing sulfur was 30%. The dispersibility of sulfur in the slurry was as good as in Example 1, and a predetermined amount of sulfur could be introduced from the resin film into the porous conductive material layer in one sulfur introduction operation. However, the amount of sulfur that could be introduced into the porous conductive material layer was small compared to the methods of Examples 1 and 2.

-Comparative Example 1-
In Comparative Example 1, unmodified sulfur that had been sieved was sprinkled on the surface of the CNT electrode, and further heated to introduce sulfur into the CNT. However, the maximum amount of sulfur introduced per time is only ² mg / cm ² per unit area of the porous conductive material layer, and the introduction of sulfur is repeated at least three times to introduce 5 mg / cm ^2. Therefore, it was very difficult to control the amount of sulfur introduced. The appearance of the positive electrode was good.

-Comparative Example 2-
In Comparative Example 2, a carbon disulfide solution containing sulfur at a concentration of 8 wt% was prepared, and the CNT electrode was immersed in the solution, and then pulled up and dried to prepare a positive electrode. CNT often peeled from the metal foil and produced a plurality of positive electrodes, but there were only a few positive electrodes that could be used as electrodes. Also, the introduction of sulfur was performed only once for CNT peeling. Moreover, the amount of sulfur that could be introduced into the porous conductive material layer was smaller than in Examples 1-5.

-Comparative Example 3-
In Comparative Example 3, sulfur was heated and sublimated in a sealed container, and the sublimated sulfur was brought into contact with the CNT electrode, and then the introduction of sulfur was attempted by cooling the CNT electrode. However, in order to increase the amount of sulfur introduced to about 5 mg / cm ² , 10 or more introduction treatments were required, and a lot of time was required. Control of the amount introduced was also very difficult.

-Comparative Example 4-
In Comparative Example 4, an attempt was made to pressurize sulfur and mold it into a pellet. However, although it could be molded, it was very brittle and the workability was poor. When the pellet could be placed on the CNT electrode, the sulfur was introduced only once, but the pellet collapsed and sometimes the pellet could not be placed on the CNT electrode. Therefore, it was not possible to judge whether the sulfur introduction property was good or bad.
<Judgment criteria and evaluation results of positive electrode manufacturing method>
In Table 1, the result of having evaluated the method of Examples 1-5 and Comparative Examples 1-4 is shown.

  The item “Amount” shown in Table 1 indicates the resin content in the resin film after drying and before introduction of sulfur. Other items and criteria were as follows.

Number of sulfur introductions: Indicates the number of sulfur introductions required to introduce a predetermined amount of sulfur. However, if repeated introduction work is not possible, the number of introductions is set to one. A case where the number of introductions was 1 was judged as ◯ (good), 2 times as Δ (possible), and 3 times or more as x (impossible). However, when the introduction work could be performed only once and a predetermined amount (5 mg / cm ² ) of sulfur could not be introduced (Comparative Example 2), the determination was x (impossible).

  Workability: The number of introductions and the short time required for the work were evaluated based on Example 1. Specifically, when the time required for introduction is within 30 minutes, ○ (good), when it exceeds 30 minutes, when it is 60 minutes or less, △ (possible), and when it exceeds 60 minutes, × (impossible). . .

  Sulfur introduction property: Judged based on the distribution amount of sulfur introduced into the porous conductive material layer. The resin film after the sulfur has moved, or the trace of the portion where the sulfur has been introduced is almost entirely on the surface of the CNT electrode. Things were judged as x (impossible).

  Dispersibility: Evaluation was made by visually observing the dispersibility of sulfur in the slurry. ○ (good) when the sulfur is uniformly dispersed, △ (possible) when the dispersibility is slightly inferior and some aggregates are seen but no problem occurs when the resin film is produced. The case where the dispersibility of sulfur was not so good as to be difficult to produce was determined as x (impossible).

  Appearance of positive electrode: The result of visual confirmation of the produced positive electrode is shown. When using a resin film, it is judged that the resin film is good if it covers almost the entire surface of the CNT electrode. When the resin film is not used, the judgment is made based on whether there is a trace of sulfur introduction on the entire surface or whether it is not biased. ○ (good) when there is no problem in appearance, or △ (possible if the created positive electrode can function as an electrode, even if there are some problems in appearance such as the resin film is partially broken. ), There was a problem in appearance such as separation of CNT, and when the produced positive electrode could not function as an electrode, it was determined as x (impossible).

  Comprehensive evaluation: If all items are ○ (good) evaluation, it is judged as ◎ (excellent), and if there are 2 or less △ (good) decisions and the rest is ○ (good), ○ (good) Judgment was made. When there was at least one item of x (impossible), or when there were three or more △ (possible) determinations, an x (impossible) determination was made.

  As shown in Table 1, in Examples 1 to 5 using the manufacturing method of the present embodiment, the overall evaluation is all ◯ (possible) or more, the workability is good, and the porous conductive material layer is sufficient. It was confirmed that an amount of sulfur could be introduced.

On the other hand, when sulfur is sprinkled directly on the CNT electrode or when carbon disulfide is used as a solvent, a method of introducing sublimated sulfur introduces a sufficient amount of sulfur into the porous conductive material layer. I couldn't. It was also confirmed that the workability of the methods of Examples 1 to 5 was better than the methods according to Comparative Examples 1, 3, and 4.
<Evaluation of lithium-sulfur secondary battery>
-Example 6
A lithium-sulfur secondary battery using a positive electrode produced in the same manner as in Example 1 was produced. The planar shape of the positive electrode was a circle having a diameter of 14 mm. The amount of sulfur introduced per unit area of the porous conductive material layer was 8.7 mg / cm ² .

A Li—Al alloy foil (Al concentration of 20 vol%) having a diameter of 15 mm and a thickness of 400 μm was used as the negative electrode, and Celgard # 2400 (manufactured by Celgard) was used as the separator. As an electrolytic solution, 100 μL of 1M LiTFSI in which 1,2-dimethoxyethane (DME) / 1,3-dioxolane (DOL) = 90 vol% / 10 vol% was added with 3 wt% LiNO ₃ was used. A CR2032-type coin battery was manufactured using these materials by a known method, and the characteristics of the lithium-sulfur secondary battery were evaluated.

During the measurement, charging / discharging was performed at a constant current of 0.77 mA (0.50 mA / cm ² ), and the cut voltage was set to discharge 1.5V and charge 2.8V.

  FIG. 3 is a diagram showing a charge / discharge profile of the lithium-sulfur secondary battery according to Example 6. The vertical axis represents the measured voltage, and the horizontal axis represents the specific capacity.

  From the results shown in the figure, it was confirmed that if the positive electrode produced by the method according to this embodiment was used, good charge / discharge could be performed with a high electric capacity exceeding 1100 mA / g per sulfur weight.

  As described above, the alkali metal-sulfur battery positive electrode and the manufacturing method thereof disclosed in this specification are applied to alkali metal-sulfur batteries such as lithium-sulfur secondary batteries.

1 Lithium-sulfur secondary battery
3 Positive electrode layer
5 Resin film
7 Porous conductive material layer
9 Sulfur
10 Conductive material
11 Current collector
13 Separator
15 Negative electrode layer
17 Electrolyte

Claims (10)

A porous conductive material layer composed of a plurality of conductive materials and having voids formed between the plurality of conductive materials;
Sulfur retained in the voids of the porous conductive material layer;
A resin film placed directly on at least a part of the porous conductive material layer ,
The positive electrode for an alkali metal-sulfur battery in which the mass of the resin film is less than 15 wt% with respect to the mass of sulfur atoms introduced into the porous conductive material layer .   The positive electrode for an alkali metal-sulfur battery according to claim 1, wherein the resin film is made of polyethylene oxide, polyvinyl pyrrolidone, polyvinyl acetate, or polytetrafluoroethylene and is porous. 3. The positive electrode for an alkali metal-sulfur battery according to claim 1, wherein the density of sulfur atoms per unit area of the porous conductive material layer is 5 mg / cm ² or more. A current collector having a plane on which the porous conductive material layer is formed;
The positive electrode for an alkali metal-sulfur battery according to any one of claims 1 to 3 , wherein the conductive material is a carbon nanotube oriented perpendicular to a plane of the current collector. The current collector is a metal foil;
The density of the carbon nanotube in the porous conductive material layer is 1 × 10 ⁸ pieces / cm ² or more and 1 × 10 ¹² pieces / cm ² or less, and has a length of 100 μm or more and 1000 μm or less. 4. The positive electrode for an alkali metal-sulfur battery according to 4 . 6. The alkali metal-sulfur battery according to claim 4 , wherein when the volume occupied by the porous conductive material layer is 100 vol%, the porosity of the porous conductive material layer is 60 vol% or more. Positive electrode. Wherein the porous conductive material layer, an alkali metal according to any one of claims 1-6, characterized in that it contains at least one binder and additives - positive electrode for sulfur battery. A positive electrode for an alkali metal-sulfur battery according to any one of claims 1 to 7 ,
A negative electrode comprising a material that occludes and releases lithium ions;
An electrolyte having lithium ion conductivity filled between the positive electrode for an alkali metal-sulfur battery and the negative electrode;
A lithium-sulfur secondary battery comprising a separator for insulating between the positive electrode for an alkali metal-sulfur battery and the negative electrode in the electrolytic solution. 9. The lithium battery according to claim 8 , wherein the total amount of sulfur atoms contained in the entire lithium-sulfur secondary battery per unit area of the porous conductive material layer is 5 mg / cm ² or more. Sulfur secondary battery. A step of preparing a current collector composed of a plurality of conductive materials and having a porous conductive material layer formed on the upper surface with a gap between the plurality of conductive materials;
Directly placing a resin film containing sulfur on at least a portion of the porous conductive material layer;
A method for producing a positive electrode for an alkali metal-sulfur battery, comprising a step of inserting sulfur contained in the resin film into the porous conductive material layer.