JP2018200827A - Method for manufacturing cathode of lithium sulfur battery, cathode of lithium sulfur battery, and lithium sulfur battery - Google Patents

Abstract

To provide a method for manufacturing a cathode 11 of a high-performance lithium sulfur battery 10.SOLUTION: A method for manufacturing a cathode 11 of a lithium sulfur battery 10 includes: a ground layer formation process for chemically changing a metal on a surface of a collector 11A into a ground layer 11B made of metallic sulfide through sulfurization reaction; and an active material arrangement process for coating a sulfuric active material layer 11C on a surface of the ground layer 11B.SELECTED DRAWING: Figure 2

Description

  Embodiments described herein relate generally to a method for manufacturing a cathode of a lithium-sulfur battery, a cathode of a lithium-sulfur battery, and a lithium-sulfur battery.

  With the widespread use of mobile phone terminals and research and development of electric vehicles, high-capacity secondary batteries have been demanded. As secondary batteries that can be charged and discharged, lithium ion secondary batteries are already widely used.

As a secondary battery having a higher capacity than a lithium ion secondary battery, a lithium-sulfur secondary battery having sulfur as a cathode active material has attracted attention. Sulfur has a theoretical capacity of about 1670 mAh / g, is about 10 times higher in theoretical capacity than LiCoO ₂ (about 140 mAh / g), which is a typical cathode active material of a lithium ion battery, and is low in cost and rich in resources. There is an advantage.

In a lithium-sulfur battery, for example, single sulfur (S ₈ ) is polysulfide anion, S ₈ ^-2 (1), S ₆ ^-2 (2), S ₄ ^2- (3), S ₂ at the cathode during discharge. ^{2- It} reduces sequentially to (4), and finally Li ₂ S is generated. On the other hand, in the anode, lithium in the anode is released as lithium ions, reaches the cathode via the electrolytic solution, and becomes a lithium source for generating Li ₂ S.

Here, lithium polysulfide (Li ₂ S ₈ , Li ₂ S ₆ , Li ₂ S ₄ , Li ₂ S ₂ ) composed of a polysulfide anion and lithium, which is an intermediate product of a sulfur reduction reaction, is an organic solvent. Easily dissolves in the electrolyte of the battery. Since the cathode active material is reduced by elution of lithium polysulfide, the charge / discharge capacity of the battery is lowered.

  Further, during charging, the polysulfide anion eluted in the electrolytic solution is reduced when it reaches the anode surface, is oxidized when it reaches the cathode surface, and a short circuit due to mass transfer occurs in the electrolytic solution. Then, even if charging current is continuously applied, charging / discharging coulomb efficiency is significantly lowered due to a so-called shuttle effect in which charging is not performed.

  That is, in the lithium-sulfur secondary battery, prevention of elution of lithium polysulfide into the electrolytic solution has been a big problem.

  Japanese Unexamined Patent Application Publication No. 2012-109223 discloses a lithium-sulfur secondary battery using an ionic liquid composed of a complex of glyme and a lithium salt as an electrolytic solution. Since a solvated ionic liquid composed of a complex of glyme and a lithium salt has low solubility of lithium polysulfide, a decrease in charge / discharge capacity is prevented.

  On the other hand, in Japanese Patent Application Laid-Open No. 2015-201270, the interface between the cathode and the electrolytic solution is covered with a polymer film that allows lithium ions to pass but not polysulfide ions to prevent a decrease in charge / discharge capacity. A lithium sulfur secondary battery is disclosed.

  Further, there has been a demand for a higher performance lithium-sulfur secondary battery.

JP 2012-109223 A Japanese Patent Laying-Open No. 2015-201270

  Embodiments of the present invention are intended to provide a method for producing a cathode for a high-performance lithium-sulfur battery, a cathode for a high-performance lithium-sulfur battery, and a high-performance lithium-sulfur battery.

  The method for manufacturing a cathode of a lithium-sulfur battery according to the embodiment includes a base layer forming step of chemically changing the metal on the surface of the current collector to a base layer made of a metal sulfide by a sulfidation reaction; And an active material disposing step for coating the system active material layer.

  In another embodiment, the cathode of the lithium-sulfur battery includes a base layer forming step in which the metal on the surface of the current collector is chemically changed to a base layer made of a metal sulfide by a sulfidation reaction. And an active material disposing step for coating the active material layer.

  The cathode of the lithium-sulfur battery according to the embodiment covers a current collector that is a porous body, a base layer made of metal sulfide of the same metal as the current collector, covering the surface of the current collector, and the base layer A sulfur-based active material layer.

  In another embodiment, a lithium-sulfur battery includes a base layer forming step in which a metal on the surface of a current collector is chemically changed to a base layer made of a metal sulfide by a sulfidation reaction, and a sulfur-based active material on the surface of the base layer An anode comprising a cathode manufactured by a method for manufacturing a cathode of a lithium-sulfur battery, an electrolyte, and an anode active material that occludes and desorbs lithium And.

  According to the embodiments of the present invention, it is possible to provide a method for producing a cathode for a high-performance lithium-sulfur battery, a cathode for a high-performance lithium-sulfur battery, and a high-performance lithium-sulfur battery.

It is sectional drawing which shows the structure of the lithium sulfur battery of embodiment. It is a flowchart of the manufacturing method of the cathode material of the lithium sulfur battery of embodiment. It is a microscope picture of a collector (NF). It is a microscope picture of a collector (NF). It is a microscope picture of the electrical power collector (NSF) covered with the base layer. It is a microscope picture of the electrical power collector (NSF) covered with the base layer. It is a microscope picture of the electrical power collector (NSF-S) covered with the active material layer. It is a microscope picture of the electrical power collector (NSF-S) covered with the active material layer. It is an X-ray diffraction pattern. It is a schematic diagram of the manufacturing process of the cathode material of the lithium sulfur battery of embodiment. It is a charge / discharge characteristic of a lithium sulfur battery. It is a rate characteristic of a lithium sulfur battery. It is a cycle characteristic of a lithium sulfur battery. It is a Nyquist plot of a lithium sulfur battery.

<Embodiment>
As shown in FIG. 1, a lithium-sulfur secondary battery (hereinafter referred to as “battery”) 10 of this embodiment includes a cathode (positive electrode) 11, an anode (negative electrode) 12, a separator 13, and an electrolyte solution 14. To do.

  In the coin-type battery 10, a cathode 11 and an anode 12 are stacked via a separator 13 into which an electrolytic solution 14 is injected and sealed in a coin cell case 15. That is, the spring 16 is disposed on the anode 12, and the coin cell case 15 is sealed with the lid 17. A gasket 18 is interposed on the side wall of the coin cell case 15.

The anode 12 is made of metallic lithium which is an anode active material that absorbs and desorbs lithium. The solvent of the electrolytic solution 14 is 50% by weight-DOL (1,3-dioxolane) / 50% by weight-DME (dimethoxyethane), which is also used in a conventional lithium ion battery. In the electrolytic solution 14, 0.2 M-LiNO ₃ and 1.0 M-bis (trifluoromethanesulfonyl) imide lithium (LiTFSA) are dissolved as a lithium salt in a solvent.

  The cathode 11 includes a current collector 11A made of a porous body, a base layer 11B covering the surface of the current collector 11A, and sulfur containing elemental sulfur, which is a sulfur-based cathode active material covering the surface of the base layer 11B. And an active material layer 11C of the system.

The current collector 11A is a porous body having a three-dimensional network structure, and the surface thereof is made of nickel. The underlayer 11B is nickel sulfide (Ni ₃ S ₂ ) formed by nickel sulfide treatment on the surface of the current collector 11A. Note that Ni ₃ S ₂ of the base layer 11B is electrochemically stable in the operating voltage range (1.7 V or more and 3.5 V or less) of the battery 10, is not oxidized or reduced, and is further a polysulfide anion. Adsorb and hold.

  A battery in which the cathode 11 includes a porous current collector 11A having a large surface area, a base layer 11B formed by a metal sulfidation reaction on the surface of the current collector 11A, and an active material layer 11C covering the base layer 11B As will be described later, No. 10 is excellent in initial capacity, rate characteristics and cycle characteristics, has high performance, and has a large capacity (energy density).

<Battery manufacturing method>
A method for manufacturing the battery 10 will be described with reference to FIG.

<Step S10> Current collector manufacturing step It is preferable that the current collector 11A is formed of a porous body having at least a surface of a conductive metal. As the porous body, a foam metal, a metal nonwoven fabric, a metal fiber aggregate, or a metal particle aggregate can be used.

  The foam metal is produced, for example, by coating the surface of the foam resin with a metal layer and then decomposing the foam resin. As the foamed resin, a urethane foam having a high porosity, a high cell diameter uniformity, and excellent thermal decomposability is preferable. When the metal layer is nickel, the surface of the foamed resin is coated with carbon powder or the like and subjected to a conductive treatment, and then nickel is coated by an electroplating method. A metal layer that is difficult to form by electroplating using an aqueous solution, such as a titanium layer, is performed by electroplating using a molten salt bath.

  A metal nonwoven fabric is also produced from a nonwoven fabric like a foam metal. The metal fiber aggregate is produced by compacting or further sintering a fiber having at least a metal surface by compression molding using a roll press or the like. Carbon fibers or carbon nanotubes may be used instead of metal fibers. Similar to the metal fiber aggregate, the metal particle aggregate is made of particles having at least a surface. That is, the current collector 11A may be made of resin or the like, or may be entirely made of metal.

The current collector 11A may be a metal foil (metal layer) having a two-dimensional surface. However, in order to increase the capacity (energy density) of the battery, the current collector 11A is a porous material having a large surface area that can support a large number of cathode active materials. The body is preferred. For example, a current collector made of a metal foil can carry only about 0.3 mg / cm ² of sulfur. On the other hand, as described later, the porous current collector 11A of the embodiment can carry 2.0 mg / cm ² or more of sulfur. For this reason, even if the capacity | capacitance prescribed | regulated by the sulfur carrying amount is the same, the capacity | capacitance (energy density) of the battery 10 is large.

The specific surface area of the current collector 11A is preferably 1500 m ² / ^{m 3} or more, 3000 m ² / ^{m 3} or more is particularly preferable. In order to obtain a large surface area, the current collector 11A preferably has a porosity of 50% or more, has continuous pores, and further contains almost no closed pores.

  The surface area of a flat plate current collector having a two-dimensional surface is proportional to the size in plan view. On the other hand, the surface area of the current collector 11A having a three-dimensional network structure is proportional to the planar view size (outer surface area) and thickness. The surface area of the current collector 11A is preferably at least twice as large as the surface area of the current collector having a two-dimensional surface having the same planar view dimensions, and particularly preferably at least 5 times. From the technical viewpoint, the surface area of the current collector 11A is 100 times or less the surface area of the current collector having a two-dimensional surface having the same planar view size. The surface area of the cathode is substantially the same as the surface area of the current collector.

  The specific surface area is measured by, for example, a capacitance method. The capacitance method is a measurement method that utilizes the fact that the capacitance of a sample is proportional to the surface area. A plurality of metal plates or the like having a known surface area are prepared, and their capacitances are measured. Then, by creating a calibration curve of “capacitance” versus “area”, the surface area of the sample is calculated by the calibration method. On the other hand, the porosity is calculated from the specific gravity and volume of the sample.

  In addition, as will be described later, the current collector 11A of the present embodiment has a metal or alloy whose surface or the whole becomes a sulfide by a sulfidation reaction, for example, nickel, aluminum, titanium, copper, cobalt, iron, silver, or , And their alloys. If the surface is covered with a metal layer that becomes sulfide, another conductor, for example, carbon black, aluminum, copper, gold, or platinum may be present thereunder (inside).

In the present embodiment, Celmet (registered trademark), which is a foam metal made of nickel, is used as the current collector 11A. Celmet has a porosity of 98%, a specific surface area of 8500 m ² / m ³ , a pore diameter of 0.45 mm, and a thickness of 1.4 mm. In order to improve the strength and accommodate the coin cell case 15 of a predetermined size, the cermet was cut, compressed to a thickness of about 0.3 mm, and used as the current collector 11A. The current collector 11A subjected to the compression treatment had a porosity of 85%.

  3A and 3B show photographs of the surface of the current collector 11A, which is a porous body having a three-dimensional network structure. Hereinafter, the current collector 11A such as bubbles made of Ni may be referred to as nickel foam (NF).

<Step S11> Underlayer Formation Step In the underlayer formation step, the metal on the surface of current collector 11A is changed to underlayer 11B made of a metal sulfide by a sulfurization reaction. That is, in the manufacturing method of the present embodiment, the base layer 11B is not physically disposed (coated) with the metal sulfide on the current collector 11A, but the metal of the current collector 11A is caused by a chemical reaction. It is formed by changing to metal sulfide. Further, the base layer 11 </ b> B is formed before the cathode 11 is incorporated into the battery 10.

  In the manufacturing method of the present embodiment, the underlayer forming step is a hydrothermal treatment step that is performed at a temperature higher than 100 ° C. and higher than 1 atm using an aqueous solution containing sulfur. For example, the current collector 11A (3.4 cm square NF) was immersed in a 5 volume% hydrazine aqueous solution in which 6.1 mg of sulfur was dissolved, and subjected to sulfurization treatment at 160 ° C. for 18 hours using an autoclave apparatus. . The current collector (NF) 11A had a gray surface, but became brown in the base layer formation step.

  4A and 4B show photographs of the surface of the current collector (NF) 11A on which the base layer 11B is formed. Hereinafter, the configuration in which the surface of the current collector 11A is covered with the base layer 11B made of Ni sulfide may be referred to as “NSF”.

  When the current collector is a metal foil, the base layer 11B can be coated using, for example, an electron beam evaporation method. However, when the current collector is a porous body, it is not possible to uniformly coat the underlayer to the inner surfaces of the holes of the current collector, particularly the inner surface that cannot be seen from the upper surface, by vapor deposition. For this reason, in the present embodiment, the base layer 11B is formed not by coating the metal sulfide but by the metal sulfidation reaction on the surface of the current collector 11A.

  In this embodiment, a liquid-phase hydrothermal treatment process is used as the sulfidation reaction. However, for example, a gas phase reaction using a gas containing sulfur, for example, hydrogen sulfide may be used. Note that the gas phase reaction is preferably performed while heating or supplying water (water vapor) in order to promote the reaction.

In this embodiment, since the surface of the current collector was nickel, Ni ₃ S ₂ (trinickel disulfide) was formed as the nickel sulfide by the sulfurization reaction. In the nickel sulfidation reaction, NiS, NiS ₂ , Ni ₇ S _6, etc. may be formed, but it is particularly preferable to form the underlayer 11 B made of Ni ₃ S ₂ by examining the sulfidation conditions. .

It should be noted that hydrothermal treatment with an autoclave at 120 ° C. or higher and 300 ° C. or lower during the sulfurization reaction can stably form the base layer 11B made of Ni ₃ S ₂ .

The weight per unit area of the underlayer 11B calculated from the difference in weight before and after the underlayer forming process, that is, the difference between the weight of the current collector (NF) 11A and the weight of NSF is 1.5 mg / cm. ² .

  Here, it was not possible to find a word specifying the structure or characteristics relating to the difference between the underlayer 11B formed by the sulfurization reaction and the underlayer disposed by coating. Furthermore, it has been impossible or impractical to specify by analysis that the underlayer is made of a metal sulfide resulting from a sulfurization reaction. For example, even when the interface structure between the current collector 11A and the base layer 11B is observed using a microscope or the like, the structure related to the difference cannot be clearly identified.

<Step S12> Cathode Active Material Disposition Process The sulfur-based active material layer 11C was disposed on the base layer 11B using a slurry containing an active material slurry and a solvent. That is, 79% by weight of elemental sulfur (S ₈ ) and 29% by weight of carbon black (manufactured by Timcal, SuperP) were mixed, treated at 155 ° C. for 6 hours to be combined, and N-methylpyrrolidone as a solvent. (NMP) was added to prepare a low-viscosity active material slurry. That is, the active material slurry does not contain a resin component.

  First, the active material slurry was disposed on the surface of NSF (current collector 11A whose surface is covered with base layer 11B) (slurry disposing step). That is, when the active material slurry was dropped onto the outer surface of the NSF, the active material slurry was sucked into the NSF that was a porous body. After dropping an appropriate amount of slurry, a drying process was performed at 60 ° C. for 24 hours, and the active material layer 11C was disposed on the surface of the NSF (drying step).

  That is, the low-viscosity active material slurry penetrates into the pores of the porous NSF without any gap due to the interfacial tension. For this reason, it is easy to cover the inner surface of the porous NSF hole, that is, the surface of the base layer 11B with the active material layer 11C.

  Since the drying temperature was set lower than the boiling point (202 ° C.) of N-methylpyrrolidone, which is a solvent, the solvent was not rapidly evaporated, and the dense active material layer 11C was disposed. Of course, in the drying step, the temperature may be gradually increased and finally heated to the boiling point of the solvent or higher. However, it is preferred that the drying temperature be kept below the boiling point until at least 90% by weight of the solvent has evaporated.

Note that the sulfur amount (supported amount) of the sulfur-based active material layer 11C is calculated from the weight measurement before and after the active material layer disposing step. The amount of sulfur in the active material layer 11C disposed on the surface of the NSF, which is a porous body having a large surface area, was 4.0 mg / cm ² , which is 2.0 mg / cm ² or more.

  The sulfur-based cathode active material includes at least one selected from the group consisting of elemental sulfur, sulfides, and organic sulfur compounds that are oxidized / reduced in the operating voltage range of the battery 10. Examples of the sulfide include MSx (M = Li, Ni, Cu, Fe, Ti, 0 <x ≦ 2). Organic sulfur compounds include organic disulfide compounds and carbon sulfide compounds.

  5A and 5B show surface photographs of the current collector 11A in which the base layer 11B is covered with the active material layer 11C. Hereinafter, the current collector 11A that is Ni foam in which the base layer 11B is covered with the active material layer 11C may be referred to as “NSF-S”.

FIG. 6 shows X-ray diffraction results of “NF”, “NSF”, and “NSF-S”. In “NF”, only Ni peaks (44.4 degrees, 51.7 degrees, 76.3 degrees) were confirmed. In “NSF”, a Ni peak and a nickel sulfide (Ni ₃ S ₂ ) peak were confirmed. In “NSF-S”, a sulfur peak was further confirmed.

In addition, in the X-ray spectroscopic analysis (XPS) of “NSF” performed separately, the peak of Ni 2p (256.1 eV and 8737 eV) of nickel sulfide (Ni ₃ S ₂ ) and nickel sulfide (Ni ₃ S ₂ ) S 2p (162.7 eV and 163.9 eV) peaks were confirmed.

  Further, in energy dispersive X-ray analysis (EDX), nickel and sulfur were uniformly present on the surface of “NSF”.

From the above analysis results, the structure shown in FIG. 7 was confirmed. That is, in step S11 (underlayer forming process), a nickel sulfide layer (underlayer) 11B is formed on the surface of the current collector 11A, which is NF (nickel foam), and step S12 (cathode active material disposing process). , The surface of the nickel sulfide layer (Ni ₃ S ₂ ) 11B is covered with the active material layer 11C.

  That is, the cathode 11 (NSF-S) is a porous cathode having a three-dimensional hierarchical structure in which the base layer 11B and the active material layer 11C are stacked on the three-dimensional surface of the current collector 11A. In the NSF of FIG. 7, a part of the current collector 11A: Ni (NF) is exposed on the surface, and a part of the Ni (NF) and the base layer 11B are exposed on the surface even in NSF-S. Actually, for example, the entire surface of the current collector 11A is covered with the base layer 11B and the like.

<Step S13> Battery Production Process As already described, the electrolyte solution 14 was produced by dissolving 1.0M-LiTFSA in DOL / DME containing 0.2M-LiNO ₃ . As the electrolytic solution 14, a solvent containing a lithium salt, which is known as an electrolytic solution for a lithium battery, can be used.

  A lithium metal foil that occludes and desorbs lithium is used for the anode 12. The anode is not limited to lithium, and may have an anode active material capable of occluding and releasing lithium ions, for example, carbon. The separator 13 was Celgard (registered trademark) 2400 (a polypropylene microporous membrane having a thickness of 25 μm) manufactured by Celgard. As the separator, for example, a glass fiber separator, a porous sheet made of a polymer, or a nonwoven fabric may be used.

  In a glove box under an argon atmosphere, an appropriate amount of the electrolyte solution 14 was added to the cathode 11 made of NSF-S, and the electrolyte solution 14 was immersed in the cathode 11 at 60 ° C. for 60 minutes. The cathode 11 and the anode (negative electrode) 12 are laminated via the separator 13, and after the electrolyte solution 14 is injected, the cathode 11 and the anode (negative electrode) 12 are sealed in a 2032 type coin cell case 15 (made of SUS304, thickness 3.2 mm). A spring 16 is disposed on the counter electrode 12. The coin cell case 15 was sealed with a lid 17 from above the spring 16 to produce the lithium-sulfur battery 10 having the structure shown in FIG.

  The battery 10 is not limited to the coin type, and may be various known structures such as a wound type or a laminate type. The battery 10 may have a plurality of unit cells (cathode / electrolyte / anode), or may have a plurality of units composed of a plurality of unit cells.

For comparison, a battery 110 having a cathode in which an active material layer 11C is coated on the surface of a current collector (NF) 11A on which a base layer 11B is not formed and a base layer (Ni ₃ S) are formed in the same manner as the battery 10. ₂ ) A battery 210 having a current collector on which only 11B was formed as a cathode was produced.

<Evaluation of battery characteristics>
The charge / discharge test was performed at a temperature of 30 ° C., a current density of 1 mA / cm ² , and a voltage range of 1.7 V to 3.5 V (vs. Li / Li ⁺ ).

FIG. 8 shows the second charge / discharge result. The battery 210 using the current collector (NF) 11A formed only with the base layer (Ni ₃ S ₂ ) 11B as a cathode could not be charged / discharged. That is, in the operating voltage range of the battery 10 (1.7 V to 3.5 V), Ni ₃ S ₂ was electrochemically stable and was not oxidized or reduced.

Since Ni ₃ S ₂ is reduced at a voltage lower than 1.7 V, it may be used as a positive electrode active material. However, since the operating voltage range of the battery 10 is 1.7 V or more, Ni ₃ S ₂ does not function as a positive electrode active material even when charging / discharging is performed, and as the base layer 11B of the active material layer 11C. Maintain the function of. That is, the base layer 11B is electrochemically stable and does not directly contribute to the charge / discharge reaction.

  On the other hand, the capacity of the battery 110 having the cathode 11 coated with the active material layer 11C without forming the base layer 11B on the current collector (NF) 11A without forming the base layer 11B was 554 mAh / g-S. On the other hand, the capacity of the battery 10 was 664 mAh / g-S.

In addition, the said capacity | capacitance is a capacity | capacitance prescribed | regulated with the amount of sulfur carrying. The capacity (energy density) of the battery 10 is proportional to the amount of sulfur carried, and the amount of sulfur carried is substantially proportional to the surface area of the cathode 11. As already described, the cathode 11 having a porous body and a three-dimensional surface has a large specific surface area. Therefore, the amount of sulfur that can be supported by the cathode 11 is 4 mg / cm ^2, which is 10 times or more that of a flat cathode (supported amount 0.3 mg / cm ² ) having the same planar view size. For this reason, the capacity | capacitance (energy density) of the battery 10 is 10 times or more of the battery which has a flat cathode.

FIG. 9 shows charge / discharge rate characteristics and cycle characteristics. Battery 10 ^at all current densities of ^{1mA / cm 2 ~5mA / cm 2} , a higher capacity than the battery 110. That is, the battery 10 can be charged faster than the battery 110 and has a large output. Further, the capacity after the 150-cycle test is 441 mAh / g-S for the battery 10, which is larger than 309 mAh / g-S for the battery 110.

  FIG. 10 shows the charge / discharge cycle characteristics. The battery 10 had an initial capacity of 655 mAh / g-S, a capacity of 654 mAh / g-S even after 80 cycles, almost no change in capacity, and the Coulomb efficiency maintained at 95% or more. On the other hand, the capacity of the battery 110 decreased to 480 mAh / g-S after 80 cycles with respect to the initial capacity of 518 mAh / g-S, and the coulomb efficiency was 91%.

  FIG. 11 shows a Nyquist plot obtained by measuring the AC impedance of the battery. The Nyquist plot is obtained by plotting the battery impedances Z ′ and Z ″ measured on the complex plane while changing the measurement frequency. The diameter of the semicircle of the Nyquist plot corresponds to the charge transfer resistance RC.

  The resistance RC is (battery 110> battery 10> battery 210). That is, the base layer 11B has an effect of reducing the resistance between the current collector 11A and the active material layer 11C.

This is because the resistivity of Ni ₃ S ₂ constituting the underlayer 11B is as low as 1.8 × 10 ⁻⁵ Ωcm, and the underlayer 11B is between the current collector 11A and the active material layer 11C. This is because the interface resistance is low. That is, since the base layer 11B is formed by the sulfurization reaction on the surface of the current collector 11A, it is completely integrated with the current collector 11A. Further, the active material layer 11C not containing the resin component is adsorbed and held on the surface of the base layer 11B.

Here, the lithium polysulfide solution (1 mM-Li ₂ S ₄ / THF) is yellow. However, when the current collector (NSF) on which the base layer 11B was formed was immersed for 12 hours at 25 ° C., the lithium polysulfide solution became colorless. That is, lithium polysulfide was adsorbed on the base layer 11B made of Ni ₃ S ₂ . On the other hand, even when the current collector (NF) on which the base layer 11B was not formed was immersed in the lithium polysulfide solution, the color hardly changed. That is, the foundation layer 11B made of Ni ₃ S ₂ has a strong adsorption capability for lithium polysulfide.

  From the above results, the reason why the battery 10 including the cathode 11 of the present embodiment is excellent in initial capacity and rate characteristics is that the base layer 11B formed by the sulfurization reaction on the surface of the current collector 11A, This is because it has an active material layer 11C that does not contain a resin component and is disposed in the solvent drying step, and has a low charge transfer resistance RC.

The reason why the battery 10 is excellent in cycle characteristics is that Ni ₃ S ₂ of the base layer 11B of the cathode 11 has an anchor effect of adsorbing lithium polysulfide and is therefore difficult to dissolve in a solvent.

That is, the base layer 11B of the cathode 11 of the lithium-sulfur battery 10 has not only an adhesive improvement effect that prevents the active material layer 11C from peeling from the current collector 11A, but also an electrical resistance reduction effect and lithium polysulfide effect. Has an adsorption effect. In other words, even if the cathode of the embodiment in which the base layer 11B is made of Ni ₃ S ₂ is not a porous body but is flat, the initial capacity, rate characteristics, and cycle characteristics of the battery are excellent.

  As described above, according to the method for manufacturing the cathode of the lithium-sulfur battery of the present embodiment, a cathode of a high-performance lithium-sulfur battery can be provided.

  The battery 10 has a large capacity (energy density) because the cathode 11 is a porous body having a large surface area.

The cathode of the lithium-sulfur battery according to the embodiment having a high energy density (large capacity) includes a step of forming a base layer by a sulfurization reaction from a metal on the surface of the current collector 11A made of a porous body having a large surface area, And a step of coating the active material layer 11C covering the entire surface. For example, a current collector having a three-dimensional network structure whose surface is made of titanium is also made of titanium sulfide (TiS ₂ ) by hydrothermal sulfidation treatment using an aqueous solution containing sulfur and exceeding 100 ° C. and exceeding 1 atm. An underlayer can be formed. In the battery having the underlayer made of TiS _2, operating voltage range TiS ₂ is set to the electrochemically stable range.

<Modification>
Since the lithium-sulfur battery 10A of the modification of the embodiment, the cathode of the lithium-sulfur battery, and the method for manufacturing the cathode of the lithium-sulfur battery (battery etc.) are similar to the battery 10 etc. of the embodiment and have the same effects, Components having the same function are denoted by the same reference numerals and description thereof is omitted.

In the battery 10A of the modification, the electrolytic solution 14A is a solvated ionic liquid having a solubility (25 ° C.) of lithium polysulfide Li ₂ S ₈ of 1000 mM or less as sulfur.

  In the solvated ionic liquid, ether and lithium salt form a complex, and have low volatility, low viscosity, high lithium ion concentration, and high lithium ion conductivity.

  Solvated ionic liquid ethers include tetrahydrofuran (THF), monoglyme (G1: 1,2-dimethoxyethane), diglyme (G2: diethylene glycol dimethyl ether), triglyme (G3: triethylene glycol dimethyl ether), or tetraglyme (G4). : Tetraethylene glycol dimethyl ether) or the like. As the lithium metal salt, lithium bis (trifluoromethanesulfonyl) amide (LiTFSA) or the like is used.

A solvent may be further added to the solvated ionic liquid. Examples of the additive solvent include hydrofluoroethers (HFE) such as HF ₂ CF ₂ CH ₂ C—O—CF ₂ CF ₂ H and F ₃ CH ₂ C—O—CF ₂ CF ₂ H, which are fluorine-based solvents. Is exemplified. The amount of the solvent added is set so that, for example, the ionic conductivity (30 ° C.) is 0.1 mS / cm or more and the viscosity (30 ° C.) is 10 mPa · s or less.

  For example, in the electrolyte solution 14A, tetraglyme (G4) was used as an ether, LiTFSA equimolar to G4 as a metal salt, and hydrofluorether (HFE) 4 times as much as lithium as an additive solvent. That is, the electrolyte solution 14A is represented by [Li (G4)] [TFSA] -4HFE in chemical formula.

To measure the solubility of lithium polysulfide in the electrolytic solution, first, the electrolytic solution to which an excess amount of lithium polysulfide Li ₂ S ₈ was added was centrifuged to collect a saturated lithium polysulfide solution. Li ₂ S ₈ is known to have the highest solubility in organic solvents among lithium polysulfides. And the lithium polysulfide in this saturated solution was oxidized to sulfur, and the solubility as sulfur was calculated from the visible absorption spectrum of sulfur.

The solubility of lithium polysulphides _Li 2 _{S 8} of electrolyte 14A (25 ° C.) was 61.7mM as sulfur. In the [Li (G4)] / [TFSA] electrolyte solution, when Li / G4 = 1.5, the solubility was 5.7 mM. On the other hand, the solubility of Li / G4 = 0.8 was 1659 mM, and that of Li / G4 = 0.25 was 6046 mM. That is, the solubility of lithium polysulfide also increases or decreases depending on the composition of the solvated ionic liquid. The electrolytic solution 14A is selected from solvated ionic liquids having a lithium polysulfide solubility (25 ° C.) of 1000 mM or less as sulfur.

  Battery 10A has better cycle characteristics than battery 10 because electrolyte solution 14A has a low solubility of lithium polysulfide.

  The present invention is not limited to the above-described embodiments and the like, and various modifications, combinations, and applications are possible without departing from the spirit of the invention.

10, 10A ... Lithium sulfur battery 11 ... Cathode 11A ... Current collector 11B ... Underlayer 11C ... Sulfur-based active material layer 12 ... Anode 13 ... Separator 14, 14A .... Electrolyte 14A ... Electrolyte 15 ... Coin cell case 16 ... Spring 17 ... Lid 18 ... Gasket

Claims (16)

A base layer forming step in which the metal on the surface of the current collector is chemically changed to a base layer made of metal sulfide by a sulfurization reaction;
An active material disposing step of coating a surface of the underlayer with a sulfur-based active material layer. A method for producing a cathode of a lithium-sulfur battery, comprising:   The method for producing a cathode of a lithium-sulfur battery according to claim 1, wherein the underlayer forming step is a hydrothermal treatment step using an aqueous solution containing sulfur and exceeding 100 ° C and exceeding 1 atm.   The method for producing a cathode of a lithium-sulfur battery according to claim 1 or 2, wherein the current collector is a porous body.   The active material disposing step includes a slurry disposing step of disposing a slurry containing a sulfur-based cathode active material and a solvent on the underlayer, and a drying step of evaporating the solvent. Item 4. A method for producing a cathode of a lithium-sulfur battery according to Item 3.   The method for producing a cathode of a lithium-sulfur battery according to claim 4, wherein the sulfur-based active material layer does not contain a resin component.   The method for producing a cathode of a lithium-sulfur battery according to any one of claims 1 to 5, wherein the metal sulfide is electrochemically stable in an operating voltage range.   The method for producing a cathode of a lithium-sulfur battery according to claim 6, wherein the operating voltage range is 1.7 V or more and 3.5 V or less. The metal on the surface of the current collector is Ni,
The metal sulfide, Ni ₃ cathodes of the method for manufacturing a lithium sulfur battery claimed in any one of claims 7, characterized in that the S _2.   A cathode for a lithium-sulfur battery manufactured by the method for manufacturing a cathode for a lithium-sulfur battery according to any one of claims 1 to 8.   A current collector that is a porous body; a base layer made of a metal sulfide of the same metal as the current collector that covers a surface of the current collector; and a sulfur-based active material layer that covers the base layer. A cathode of a lithium-sulfur battery characterized by that.   The cathode of a lithium sulfur battery according to claim 10, wherein the sulfur-based active material layer does not contain a resin component.   The cathode of a lithium-sulfur battery according to claim 10 or 11, wherein the metal sulfide is electrochemically stable in an operating voltage range.   The cathode of the lithium sulfur battery according to claim 12, wherein the operating voltage range is 1.7V or more and 3.5V or less. The metal on the surface of the current collector is Ni,
14. The cathode of a lithium sulfur battery according to claim 10, wherein the metal sulfide is Ni ₃ S ₂ .   A lithium-sulfur battery comprising: the cathode of the lithium-sulfur battery according to any one of claims 9 to 14; an electrolyte; and an anode containing an anode active material that absorbs and desorbs lithium. battery. The lithium-sulfur battery according to claim 15, wherein the electrolyte is a solvated ionic liquid having a solubility (25 ° C.) of lithium polysulfide Li ₂ S ₈ of 1000 mM or less as sulfur.