Positive electrode for secondary battery, method for manufacturing positive electrode for secondary battery, secondary battery

Abstract

Provided are: a positive electrode for secondary cells, that has high positive electrode capacity density per volume and can increase the energy density of secondary cells; a manufacturing method for said positive electrode for secondary cells; and a secondary cell having high energy density. A positive electrode 10 contains polysulfide 16 indicated by Li2Sx (x = 4, 6, 8) in a sulfur amount of 0.70 g / cm3 to 2.0 g / cm3, in a self-supporting sponge structure 14 comprising CNT 12 and having a thickness of at least 5 μm and less than 100 μm. The positive electrode 10 is manufactured by: causing the self-supporting sponge structure 14 comprising CNT 12 and having a thickness of at least 5 μm and less than 100 μm to contain polysulfide 16; and forming the CNT 12 and the polysulfide 16 into a complex. The secondary cell has a structure whereby the positive electrode 10, a negative electrode, and a separator are provided inside a container together with an electrolytic solution.

Description

[Technical Field]

[0001] The present invention relates to a positive electrode for a secondary battery, a method for producing a positive electrode for a secondary battery, and a secondary battery. [Background technology]

[0002] Positive electrodes using sulfur as a positive electrode active material have attracted attention as a means of achieving high-capacity secondary batteries. Sulfur has a theoretical capacity approximately 10 times higher than that of lithium-transition metal composite oxide-based positive electrode active materials, but its poor electrical conductivity is a problem. Furthermore, when sulfur is used as a positive electrode active material, an excess amount of electrolyte, approximately 10 times or more the sulfur content, is required to achieve good secondary battery characteristics, resulting in the failure to obtain high-energy-density secondary batteries. The present inventors have used a highly conductive, self-supporting sponge-like structure of carbon nanotubes (CNTs) as a positive electrode current collector. By incorporating S8 (octasulfur), the low electrical conductivity of sulfur can be compensated for, resulting in a positive electrode that can improve energy density by eliminating the need for a current collector foil. However, even with this positive electrode, an excess amount of electrolyte is required, preventing the production of high-energy-density secondary batteries.

[0003] On the other hand, Li2S, which is a reaction intermediate between S8 in the charged state and Li2S (lithium sulfide) in the discharged state, x Research is also underway into the use of lithium polysulfides (also known as lithium polysulfides, hereafter referred to as polysulfides) represented by the formula (x = 4, 6, 8) as positive electrode materials. x Li2S has high solubility in the electrolyte. x Li2S dissolved in the electrolyte x A method using a solution (also called a catholyte solution) has been studied, but this method also requires an excessive amount of electrolyte, making it impossible to increase the energy density of the secondary battery. x A positive electrode has been proposed in which a positive electrode current collector contains a solid form of ZnO (for example, Non-Patent Documents 1 and 2).

[0004] Non-Patent Document 1 proposes a positive electrode that uses a CNT foam in which CNTs are fixed to a carbonized polyacrylonitrile as a positive electrode current collector, and fabricates the positive electrode by dropping a Li2S4 solution onto the CNT foam and drying it.

[0005] Non-Patent Document 2 proposes a positive electrode that uses a CNF film made of CNF (carbon nanofiber) as a positive electrode current collector, and fabricates the CNF film by dropping a Li2S8 solution onto the CNF film and drying it. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] C. Shen, et al., Journal of Power Sources. 414, 412 (2019). [Non-patent document 2] M. Agostini, et al., Advanced Energy Materials. 8, 1801560 (2018). Summary of the Invention [Problem to be solved by the invention]

[0007] The positive electrode of Non-Patent Document 1 has a positive electrode current collector with a thickness of 100 μm. The positive electrode of Non-Patent Document 2 has a positive electrode current collector with a thickness of 200 to 250 μm. The positive electrodes described in Non-Patent Documents 1 and 2 have thick positive electrode current collectors and small amounts of sulfur per volume, so it is not possible to increase the positive electrode capacity density per volume, and it is not possible to increase the energy density of the secondary battery.

[0008] Therefore, an object of the present invention is to provide a positive electrode for a secondary battery that has a high positive electrode capacity density per volume and can increase the energy density of the secondary battery, a method for manufacturing the positive electrode for a secondary battery, and a high-energy-density secondary battery. [Means for solving the problem]

[0009] The positive electrode for a secondary battery according to the present invention is a self-supporting sponge-like structure made of carbon nanotubes and having a thickness of 5 μm or more and less than 100 μm, in which Li2S x Polysulfides represented by (x=4, 6, 8) are 0.70 g / cm 3 More than 2.0g / cm 3 It is characterized by containing the following amount of sulfur:

[0010] The method for producing a positive electrode for a secondary battery according to the present invention is to deposit LiS in a self-supporting sponge-like structure having a thickness of 5 μm or more and less than 100 μm, which is made of carbon nanotubes. x The carbon nanotube is characterized in that it contains a polysulfide represented by (x=4, 6, 8) and the polysulfide is composited with the carbon nanotube.

[0011] The secondary battery according to the present invention is characterized in that the above-mentioned positive electrode, negative electrode and separator are provided in a container together with an electrolyte solution. [Effects of the Invention]

[0012] According to the present invention, the thickness of the sponge-like structure is 5 μm or more and less than 100 μm, and the Li2S in the sponge-like structure x When the content of sulfur is expressed as the amount of sulfur per volume of the positive electrode, the amount of sulfur is 0.70 g / cm 3 More than 2.0g / cm 3 or less, and a large amount of Li2S is contained in the thin sponge-like structure. x Since the positive electrode contains the compound, the positive electrode capacity density per volume is high and the energy density of the secondary battery can be increased, and a method for manufacturing the positive electrode for the secondary battery and a high-energy-density secondary battery can be provided. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a schematic diagram showing the configuration of a positive electrode for a secondary battery according to a first embodiment. [Figure 2] FIG. 2 is an explanatory diagram illustrating a method for manufacturing a positive electrode for a secondary battery according to the first embodiment. [Figure 3] 2 is a flowchart illustrating a method for manufacturing a positive electrode for a secondary battery according to the first embodiment. [Figure 4] 1A and 1B are schematic diagrams illustrating the configuration of a secondary battery according to a first embodiment during charging and discharging. [Figure 5] FIG. 4 is a schematic diagram showing the configuration of a positive electrode for a secondary battery according to a second embodiment. [Figure 6] FIG. 4 is an explanatory view illustrating a first method, which is an example of a method for producing a positive electrode for a secondary battery according to a second embodiment. [Figure 7] FIG. 10 is an explanatory view illustrating a second method, which is an example of a method for producing a positive electrode for a secondary battery according to the second embodiment. [Figure 8] FIG. 10 is an explanatory view illustrating a third method, which is an example of a method for producing a positive electrode for a secondary battery according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, this embodiment will be described in detail with reference to the drawings.

[0015] 1. First embodiment 1-1. Overall structure In FIG. 1, a positive electrode for a secondary battery (hereinafter referred to as a positive electrode) 10 according to this embodiment is a self-supporting sponge-like structure 14 having a thickness of 5 μm or more and less than 100 μm, which is made of CNTs (carbon nanotubes) 12, and Li2S x Polysulfide 16 represented by (x=4, 6, 8) is 0.70 g / cm 3 More than 2.0g / cm 3 The electrode has a structure containing the following sulfur content: The sulfur content refers to the mass of elemental sulfur contained per unit volume of the electrode.

[0016] The sponge-like structure 14 is a porous, self-supporting film formed by entanglement of a plurality of CNTs 12. The sponge-like structure 14 holds polysulfide 16 and an electrolyte solution (described later). The sponge-like structure 14 functions as a positive electrode current collector for the positive electrode 10.

[0017] The sponge-like structure 14 has a thickness of 5 μm or more and less than 100 μm. If the positive electrode current collector is too thick, the positive electrode capacity density per positive electrode volume (also referred to as volume capacity density) becomes small. If the positive electrode current collector is too thin, the Li2S x Since the amount of carbon that can be retained decreases, the positive electrode capacity density per positive electrode area (also referred to as area capacity density) decreases. It is particularly preferable that the thickness of the sponge-like structure 14 is 10 μm or more and 50 μm or less.

[0018] The polysulfide 16 is supported in a solid state on the CNTs 12 of the sponge-like structure 14 and is composited with the CNTs 12. The polysulfide 16 can have a structure in which it coats the CNTs 12 in an island-like manner, a structure in which it coats almost the entire surface of the CNTs 12, a particulate structure, or any other predetermined structure. The polysulfide 16 functions as a positive electrode active material for the positive electrode 10.

[0019] When the content of polysulfide 16 in the sponge-like structure 14 is expressed as the amount of sulfur per volume of the positive electrode, the amount of sulfur is 0.70 g / cm 3 More than 2.0g / cm 3 If the content of polysulfide 16 in the sponge-like structure 14 is too low, the volumetric capacity density decreases, and the electrode performance cannot be improved. If the content of polysulfide 16 in the sponge-like structure 14 is too high, the voids in the sponge-like structure 14 are filled, resulting in a shortage of electrolyte and a decrease in the diffusibility of lithium ions. From the viewpoint of increasing the volumetric capacity density while facilitating the adjustment of the voids, the content of polysulfide 16 in the sponge-like structure 14 is set to 0.95 g / cm in terms of sulfur. 3 More than 1.6g / cm 3 It is preferable that:

[0020] The length of the CNTs 12 is preferably 1 μm or more. When the length of the CNTs 12 is 1 μm or more, the plurality of CNTs 12 are entangled with each other, and the sponge-like structure 14 is able to stand on its own.

[0021] The CNTs 12 preferably have an average diameter of 20 nm or less. The smaller the diameter of the CNTs 12, the more improved the flexibility, self-supporting ability, and conductivity of the sponge-like structure 14. The average diameter of the CNTs 12 is more preferably 15 nm or less, and particularly preferably 10 nm or less.

[0022] CNT12 has a specific surface area of ​​200m 2 / g or more. 2 / g or more, the number of interfaces with the polysulfides 16 in the sponge-like structure 14 increases, improving electrical connection, and the content of the polysulfides 16 in the sponge-like structure 14 can be increased, thereby improving the volumetric capacitance density and the area capacitance density. 2 / g or more is more preferable, and 400m 2 It is particularly preferable that the specific surface area of ​​the CNTs 12 is 1300 m / g or more. 2 / g or less. If the specific surface area of ​​the CNTs 12 is too large, the CNTs 12 may strongly aggregate with each other, making it difficult to form a composite with the polysulfide 16. The specific surface area of ​​the CNTs 12 is preferably 800 m 2 It is more preferable that the saturation coefficient is 1 / g or less.

[0023] The average number of layers of the CNTs 12 is preferably 1 to 10. The smaller the average number of layers of the CNTs 12, the more flexible the CNTs 12 are, and the more easily the CNTs 12 are entangled with one another, thereby more reliably ensuring the self-supporting properties of the sponge-like structure 14. However, if the average number of layers of the CNTs 12 is too small, the CNTs 12 become too flexible and are more likely to be entangled with one another, making it difficult to disperse the raw material powder of the CNTs 12 and form the sponge-like structure 14. Furthermore, if the average number of layers of the CNTs 12 is too small, the specific surface area of ​​the CNTs 12 becomes too large. The average number of layers of the CNTs 12 is more preferably 1 to 8, and particularly preferably 2 to 5.

[0024] When a secondary battery is constructed using the positive electrode 10, the positive electrode 10 is provided so as to be in contact with a separator. The thickness of the positive electrode 10 may be reversibly changed by charging and discharging the secondary battery. Specifically, the thickness of the positive electrode 10 may decrease during charging and increase during discharging. The positive electrode 10 may be Li2S x is reduced to form Li2S (lithium sulfide), and during charging, Li2S x is oxidized to produce S8 (octasulfur).

[0025] The positive electrode 10 includes the sponge-like structure 14 as a positive electrode current collector, and therefore does not need to include a separate current collector foil. If the positive electrode does not include a current collector foil, the mass and volume of the positive electrode can be reduced, and the positive electrode capacity density (also referred to as mass capacity density) and volume capacity density per positive electrode mass can be increased. Furthermore, the provision of multiple metal comb-shaped electrodes or thin wires is more preferable, as this can reduce the metal mass while ensuring sufficient conductivity. When the positive electrode 10 does not include a current collector foil, the thickness of the positive electrode 10 is approximately the same as the thickness of the sponge-like structure 14.

[0026] 1-2. Manufacturing method A method for manufacturing the positive electrode 10 according to this embodiment will be described.

[0027] As shown in FIG. 2, the positive electrode 10 is formed by disposing Li2S in a self-supporting sponge-like structure 14 made of CNTs 12 and having a thickness of 5 μm or more and less than 100 μm. x It is produced by incorporating polysulfide 16 represented by (x=4, 6, 8) and compositing CNT 12 with polysulfide 16.

[0028] CNT12 can be synthesized by a chemical vapor deposition (CVD) method. Examples of CVD methods include the fluidized bed CVD method described in Japanese Patent No. 5447367, Japanese Patent No. 5862559, D.Y. Kim, H. Sugime, K. Hasegawa, T. Osawa, and S. Noda, Carbon 49(6), 1972-1979 (2011), and Z. Chen, D.Y. Kim, K. Hasegawa, T. Osawa, and S. Noda, Carbon 80, 339-350 (2014). CNT12 may also be synthesized by a floating catalyst CVD method or a substrate-supported catalyst CVD method.

[0029] The sponge-like structure 14 is produced by dispersing CNTs 12 in a dispersion medium to prepare a dispersion liquid and then removing the dispersion medium from the dispersion liquid. Examples of the dispersion medium include water and organic solvents. Examples of organic solvents include ethanol and 2-propanol. The dispersion medium is removed from the dispersion liquid by filtering the dispersion liquid using a filter, for example. During the process of removing the dispersion medium from the dispersion liquid, the CNTs 12 form a network due to van der Waals forces and accumulate on the surface of the filter, forming the sponge-like structure 14. The sponge-like structure 14 is separated from the filter and recovered as a free-standing film. If necessary, the sponge-like structure 14 may be dried using a dryer before or after separation from the filter. Instead of filtering and drying the dispersion liquid, the dispersion liquid may be applied and dried. The thickness of the sponge-like structure 14 and the mass of CNTs per unit volume of the positive electrode 10 (hereinafter referred to as the CNT volume density) can be adjusted by, for example, pressing.

[0030] A specific method for obtaining the positive electrode 10 by compounding the CNTs 12 and the polysulfide 16 will be described.

[0031] As shown in FIG. 3, the positive electrode 10 is formed by dissolving a solution containing polysulfide 16 (hereinafter referred to as Li2S xThe solution of Li2S is supplied to the sponge-like structure 14, and the solution of Li2S is introduced into the sponge-like structure 14. x A supply step S1 in which a solution of Li2S is held; x The sponge-like structure 14 holding the solution is dried, and a drying step S2 in which the polysulfide 16 is supported on the CNTs 12 is then carried out.

[0032] In the supply step S1, first, Li2S x Prepare a solution of Li2S x The solution is prepared by dispersing powders of S8 and Li2S in a solvent. For example, DME (1,2-dimethoxyethane) is used as the solvent. The powders of S8 and Li2S are placed in the solvent, heated to, for example, 45°C, and stirred using a stirrer to obtain the Li2S solution. x Dissolved Li2S x Next, the prepared Li2S solution is x The solution of Li2S is supplied to the sponge-like structure 14. x As a method for supplying the solution to the sponge-like structure 14, x A method of dropping a solution of Li2S onto the sponge-like structure 14 x The sponge-like structure 14 is immersed in the solution of Li2S by a method such as spray coating. x The solution may be applied to the sponge-like structure 14.

[0033] In the drying step S2, Li2S x The sponge-like structure 14 holding the solution is preferably dried at a temperature of −58° C. to 90° C. By setting the drying temperature to −58° C. to 90° C., an appropriate amount of solvent molecules remain in the sponge-like structure 14, and LiS in a state where the solvent molecules are coordinated is obtained. x is held in the sponge-like structure 14. Li2S coordinated with the solvent molecules x It is believed that the electrode performance improves if the drying temperature is too low, the solvent molecules are not sufficiently removed, and if the drying temperature is too high, the Li2S xThis results in a stable crystalline structure, which is thought to inhibit the reaction between sulfur and lithium, making it difficult to improve electrode performance. The drying temperature is more preferably between 0°C and 90°C, and particularly preferably between 20°C and 50°C. The lower limit of the drying temperature (-58°C) is an example assuming the use of DME as the solvent, and is based on the melting point of DME. The lower limit of the drying temperature may be changed as desired depending on the type of solvent.

[0034] The drying time can be set arbitrarily, but is preferably from 1 second to 24 hours, and more preferably from 10 seconds to 10 hours. The drying time is set so that an appropriate amount of solvent molecules remain in the sponge-like structure 14.

[0035] The mass proportion of the solvent molecules remaining in the sponge-like structure 14 can be set arbitrarily, but is preferably 5% by mass or more and 80% by mass or less, and more preferably 20% by mass or more and 80% by mass or less. The mass proportion of the solvent molecules is the value obtained by dividing the mass of the solvent molecules by the mass of the positive electrode 10. The mass of the positive electrode 10 is the sum of the mass of the sponge-like structure 14, the mass of the polysulfide 16, and the mass of the solvent molecules. If the mass proportion of the solvent molecules is too large, it becomes difficult to reduce the E / S ratio in the secondary battery described below. If the mass proportion of the solvent molecules is too small, it becomes difficult to improve the electrode performance. If even a small amount of solvent molecules are present in the sponge-like structure 14, Li2S in a state in which the solvent molecules are coordinated will be formed. x Since the solvent is retained in the sponge-like structure 14, the mass ratio of the solvent molecules may be reduced as long as the desired electrode performance is obtained. The mass ratio of the solvent molecules can be adjusted by adjusting at least one of the drying temperature and the drying time.

[0036] In the drying step S2, Li2S xThe sponge-like structure 14 holding the solution is dried at normal pressure or vacuum. Drying at normal pressure is performed at a pressure almost equal to atmospheric pressure. Vacuum drying (also called reduced pressure drying) is performed at a pressure equal to or less than the vapor pressure of the solvent at 50°C of 28 kPa, preferably equal to or less than the vapor pressure of the solvent at 20°C of 6.6 kPa. Vacuum drying allows for uniform drying without causing uneven drying. S8 has a high vapor pressure and is therefore prone to evaporate or sublimate in a reduced pressure environment, but Li2S x Li2S has a very small vapor pressure and does not evaporate in a reduced pressure environment. x The sponge-like structure 14 holding the solution is dried in a reduced pressure environment to form Li2S x Therefore, the evaporation of Li2S is suppressed, and the polysulfide 16 can be reliably supported on the CNTs 12. x It is preferable to vacuum-dry the sponge-like structure 14 holding the solution. The pressure for vacuum drying described above is an example assuming that DME is used as the solvent, and is based on the vapor pressure of the solvent. The pressure for vacuum drying may be changed as desired depending on the type of solvent.

[0037] The secondary battery according to this embodiment can be manufactured by placing a positive electrode 10, a negative electrode, a separator, and an electrolyte solution in a container. The secondary battery includes a positive electrode 10, a negative electrode, a separator, an electrolyte solution, and a container, and is not particularly limited as long as the positive electrode 10, the negative electrode, and the separator are provided in the container together with the electrolyte solution. The secondary battery may be one in which the positive electrode 10 and the negative electrode expand or contract upon charge and discharge, for example. Below, as an example of an embodiment, a secondary battery in which the positive electrode 10 and the negative electrode expand or contract upon charge and discharge will be described.

[0038] As shown in Fig. 4, the secondary battery 20 (20A, 20B) includes a positive electrode 10 (10A, 10B), a negative electrode 22 (22A, 22B), and a separator 24. The electrolyte and container are not shown in Fig. 4. In the secondary battery 20, the positive electrode 10 is provided on one surface of the separator 24, and the negative electrode 22 is provided on the other surface of the separator 24.

[0039] During charging, secondary battery 20A includes a contracted positive electrode 10A and an expanded negative electrode 22A, which are provided with separator 24 interposed therebetween. During discharging, secondary battery 20B includes an expanded positive electrode 10B and a contracted negative electrode 22B, which are provided with separator 24 interposed therebetween.

[0040] In this embodiment, the area of ​​the surface in contact with the separator 24 does not change substantially, and the thickness of the positive electrode 10 decreases during charging (10A) and increases during discharging (10B), resulting in expansion or contraction. The positive electrode active material is polysulfide 16 (not shown) before the first charge / discharge, becomes S8 (16A) in the charged state, and becomes LiS (16B) in the discharged state.

[0041] The negative electrode 22 may be any of various negative electrodes used in general secondary batteries. In this embodiment, a negative electrode whose thickness changes reversibly with charge and discharge, increasing in thickness during charging (22A) and decreasing in thickness during discharging (22B), is preferred. This is because expansion and contraction of the positive electrode 10 and the negative electrode 22 are offset, allowing for effective use of the space within the secondary battery 20. The negative electrode active material 26 of the negative electrode 22 may be, for example, a carbon material (such as graphite (C)) or lithium titanate, in which lithium ions (Li) are trapped in gaps in the crystal structure. + Active materials that charge and discharge by inserting and desorbing lithium, active materials that charge and discharge by reacting with lithium to form a compound, such as silicon, and lithium metal can be used. During charging and discharging, expansion and contraction of the negative electrode 22 and the positive electrode 10 are offset, and changes in thickness of the secondary battery 20 are suppressed. Therefore, it is desirable to use an active material that changes in volume by reacting with lithium, such as silicon, as the negative electrode active material 26. To enable reversible volume changes, it is preferable to include the negative electrode active material 26 inside the self-supporting sponge-like structure 14 of the CNT 12. For example, when silicon is used as the negative electrode active material 26, the negative electrode active material 26 will react with Li in the charged state. 15 It becomes Si4(26A) and becomes Si(26B) in the discharged state.

[0042] The separator 24 can be made of a microporous polymer film. Examples of the microporous polymer film include polyolefin-based, polyester-based, polyacrylonitrile-based, polyphenylene sulfide-based, polyimide-based, and fluororesin-based microporous membranes and nonwoven fabrics. The separator 24 may also be made of a self-supporting sponge-like structure of insulating fibers. The insulating fibers are boron nitride nanotubes (BNNTs) or organic nanofibers. Examples of organic nanofibers include cellulose nanofibers and chitin nanofibers.

[0043] The electrolyte may be a commonly used electrolyte, such as a non-aqueous electrolyte, an ionic liquid, or a gel electrolyte. The electrolyte may be prepared, for example, by dissolving 1.0 mol / L of LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) and 0.2 mol / L of LiNO3 (lithium nitrate) in a 1:1 volumetric mixture of DOL (1,3-dioxolane) and DME. The E / S ratio, which indicates the ratio of the amount of electrolyte to the amount of sulfur, can be adjusted by adjusting the amount of electrolyte. The smaller the E / S ratio, the smaller the mass of the secondary battery 20, and the higher the energy density of the secondary battery 20. The E / S ratio is preferably 8 or less, and more preferably 5 or less.

[0044] The container may be a metal can made of iron, stainless steel, aluminum, or the like, which is commonly used as a battery can. Alternatively, a laminate sheet of metal and polymer, which is commonly used in laminate batteries, may also be used.

[0045] 1-3. Action and effects The positive electrode 10 according to this embodiment includes a positive electrode current collector made of a self-supporting sponge-like structure 14 of CNTs 12 and Li2S contained in the sponge-like structure 14. x(x=4, 6, 8) and a positive electrode active material made of polysulfide 16. The thickness of the sponge-like structure 14 is 5 μm or more and less than 100 μm. When the content of polysulfide 16 in the sponge-like structure 14 is expressed as the amount of sulfur per volume of the positive electrode, the amount of sulfur is 0.70 g / cm 3 More than 2.0g / cm 3 Therefore, the positive electrode 10 contains a large amount of polysulfide 16 in the thin sponge-like structure 14, and has a large amount of sulfur per volume, so that the positive electrode capacity density per volume (volume capacity density) is high, and the energy density of the secondary battery can be increased.

[0046] The positive electrode 10 has a highly conductive sponge-like structure 14 as a positive electrode current collector, and does not include a current collector foil, so that the mass and volume are small, and the mass capacity density and volume capacity density can be increased.

[0047] The positive electrode 10 can reduce the amount of electrolyte used when constructing the secondary battery 20, so that the mass and volume of the secondary battery 20 can be reduced and the energy density of the secondary battery 20 can be increased.

[0048] The present invention is not limited to the above-described embodiment, and can be modified as appropriate within the scope of the invention.

[0049] 1-4.Example Table 1 summarizes the configurations and electrode performances of the positive electrodes of Examples 1 to 21 and Comparative Examples 1 to 3.

[0050] [Table 1]

[0051] The configuration of the positive electrode will be described. Examples 1 to 21 are positive electrodes fabricated using the manufacturing method for the positive electrode 10 described above. Comparative Example 1 is a positive electrode described in Non-Patent Document 1. In the positive electrode of Comparative Example 1, a CNT foam in which CNTs are fixed by thermal decomposition, i.e., carbonization, of polyacrylonitrile is used as the positive electrode current collector. CNT foam is described in reference

[14] (C. Shen, et al., Electrochimica Acta. 248, 90 (2017)) cited in Non-Patent Document 1. Comparative Examples 2 and 3 are positive electrodes described in Non-Patent Document 2. In the positive electrodes of Comparative Examples 2 and 3, a CNF film made of CNF (carbon nanofiber) is used as the positive electrode current collector. Note that the sponge-like structure made of CNT, the CNT foam, and the CNF film are carbon matrices formed from carbon materials. The term "CNT" in Table 1 refers to the carbon material constituting the carbon matrix, and is not limited to CNT but includes CNF.

[0052] In Table 1, film thickness (a) indicates the thickness of the positive electrode. In Examples 1 to 21, actual measured values ​​were used. In Comparative Example 1, the film thickness (100 μm) of the CNT foam described in Non-Patent Document 1 was used. In Comparative Example 2, the minimum film thickness (200 μm) of the CNF film described in Non-Patent Document 2 was used. In Comparative Example 3, the maximum film thickness (250 μm) of the CNF film described in Non-Patent Document 2 was used.

[0053] The type of sulfur (b) indicates the type of polysulfide supported on the CNTs. Li2S6 was used in Examples 1 to 4 and 7 to 21. Li2S4 was used in Example 5. Li2S8 was used in Example 6. Li2S4 described in Non-Patent Document 1 was used in Comparative Example 1. Li2S8 described in Non-Patent Document 2 was used in Comparative Examples 2 and 3.

[0054] Diameter (c) indicates the diameter of the positive electrode. In Examples 1 to 21, actual measured values ​​were used. In Comparative Example 1, the value is not described in Non-Patent Document 1, so it is indicated as "-". In Comparative Examples 2 and 3, the value of the diameter (10 mm) described in Non-Patent Document 2 was used.

[0055] CNT(d) indicates the mass of the CNT contained in the positive electrode. In Examples 1 to 21, the mass of the CNT used in producing each positive electrode is shown as a numerical value of 0.6 to 4.2 mg. In Comparative Example 1, the mass is not described in Non-Patent Document 1, so it is indicated as "-". In Comparative Examples 2 and 3, the mass is not described in Non-Patent Document 2, so it is indicated as "-".

[0056] Sulfur (e) indicates the mass of sulfur contained in the positive electrode. In Examples 1 to 21, the mass of sulfur used in the production of each positive electrode is listed as a value of 1.1 to 8.4 mg. In Comparative Example 1, since it is not described in Non-Patent Document 1, it is indicated as "-". In Comparative Examples 2 and 3, since it is not described in Non-Patent Document 2, it is indicated as "-".

[0057] The CNT areal density (f) indicates the mass of CNT per positive electrode area. In Examples 1 to 21, it was calculated by dividing CNT (d) (mass of CNT) by the area of ​​the positive electrode. In Comparative Example 1, the density of the CNT foam (125 mg / cm) described in Non-Patent Document 1 was used. 3 ) by the film thickness (100 μm). In Comparative Example 2, the value (1.7 mg / cm 2 In Comparative Example 3, the value (2.2 mg / cm) described in Non-Patent Document 2 was used. 2 ) was used.

[0058] The sulfur areal density (g) indicates the amount of sulfur per positive electrode area. In Examples 1 to 21, it was calculated by dividing sulfur (e) (mass of sulfur) by the area of ​​the positive electrode. In Comparative Example 1, it was calculated by dividing the CNT areal density (f) by the mass proportion of CNTs and multiplying the result by the mass proportion of sulfur. In Comparative Example 1, the mass proportion of sulfur used was the value (84%) described in Non-Patent Document 1, and the mass proportion of CNTs used was the value (16%) calculated from the mass proportion of sulfur. In Comparative Examples 2 and 3, the value (6 mg / cm) described in Non-Patent Document 2 was used. 2 ) was used.

[0059] The CNT volume density (h) was calculated by dividing the CNT surface density (f) by the film thickness (a).

[0060] The sulfur loading mass (i) indicates the mass of sulfur per unit volume of the positive electrode, and was calculated by dividing the sulfur surface density (g) by the film thickness (a).

[0061] The E / S ratio (j) was calculated by adding together the volume of the remaining solvent molecules, which was calculated by dividing the mass of the solvent molecules remaining in the sponge-like structure 14 by the density of the solvent, and the volume of the additional electrolyte, which was calculated by dividing the mass of the additionally added electrolyte by the density of the electrolyte, and then dividing the total volume by the amount of sulfur. In Examples 1 to 21, the E / S ratio was set to 4.0 to 8.0. In Comparative Example 1, the value (4.4 μL / mg) described in Non-Patent Document 1 was used. In Comparative Examples 2 and 3, the amount of electrolyte (10 μL / cm ) described in Non-Patent Document 2 was used. 2 ) to the sulfur content (6 mg / cm 2 ) was calculated by dividing by

[0062] Although not shown in Table 1, the drying conditions were the same for the drying step S2 when producing the positive electrodes of Examples 1 to 8, 11, and 14 to 20, with the drying temperature being 45°C, the drying time being 60 minutes, and drying at normal pressure (normal pressure drying). The other Examples will be explained using Table 2 below, but the drying conditions were different.

[0063] Electrode performance will be described. Test cells were fabricated by placing the positive electrodes of Examples 1 to 21 in a container together with a negative electrode, a separator, and an electrolyte. A 50 μm-thick lithium foil was used as the negative electrode. A polypropylene separator was used as the separator. The electrolyte was prepared by dissolving 1.0 mol / L LiTFSI and 0.2 mol / L LiNO3 in a 1:1 volumetric mixture of DOL and DME. Each fabricated test cell was subjected to a charge-discharge cycle test at a C-rate (Capacity Rate) of 0.1 C or 0.05 C to measure the mass-based capacity (k). The C-rate was defined as 1 C = 1.675 mA / mg, assuming that the theoretical capacity is realized relative to the mass of sulfur. Charge-discharge in Examples 1 to 6, 8 to 13, and 16 to 20 was performed at a C-rate of 0.1 C, while charge-discharge in Examples 7, 14, 15, and 21 was performed at a C-rate of 0.05 C. The area-based capacity (l) was calculated by multiplying the mass-based capacity (k) by the sum of the CNT surface density (f) and the sulfur surface density (g). The area-based capacity (l) was divided by the film thickness (a) to calculate the volume-based capacity (m). The mass-based capacity (k), area-based capacity (l), and volume-based capacity (m) were used to determine the electrode performance. Table 1 shows the maximum mass-based capacity (k) measured in the charge-discharge cycle test, as well as the area-based capacity (l) and volume-based capacity (m) calculated based on this mass-based capacity (k). The mass-based capacity (k) of Comparative Example 1 was calculated based on the mass-based value of sulfur (680 mAh / g) calculated from the discharge profile (Fig. 5b) described in Non-Patent Document 1. sulfur The mass-based capacity (k) of Comparative Example 2 was calculated by multiplying the surface density of CNTs (f) by the surface density of sulfur (g) and dividing the result by the sum of the surface density of CNTs (f) and the surface density of sulfur (g). sulfur The area-based capacity (l) and volume-based capacity (m) of Comparative Examples 1 to 3 were calculated based on the mass-based capacity (k), as in the Examples.

[0064] From Table 1, when comparing the positive electrodes of Examples 1 to 21 with those of Comparative Examples 1 to 3, it was confirmed that in Examples 1 to 21, in which the film thickness was 5 to 62 μm, the sulfur loading mass (i) and the volumetric capacity (m) were larger than in Comparative Examples 1 to 3, in which the film thickness was 100 to 250 μm. Comparing Examples 1 to 21, it was confirmed that as the film thickness was smaller, the volumetric capacity (m) tended to increase while the area-based capacity (l) tended to decrease.

[0065] Table 2 shows the relationship between the drying conditions in the drying step S2 when producing the positive electrodes of Examples 9 to 13, the DME residual rate, and the mass-based capacity.

[0066] [Table 2]

[0067] In Example 13, drying (vacuum drying) was performed for a drying time of 0.17 minutes (10 seconds), at a drying temperature of 20°C (room temperature), and under reduced pressure so that the pressure difference from atmospheric pressure was -0.1 MPa. In Example 9, vacuum drying was performed for a drying time of 10 minutes at a drying temperature of 20°C. In Example 12, vacuum drying was performed for a drying time of 60 minutes at a drying temperature of 20°C. In Example 11, as described above, drying was performed at room pressure (room pressure drying) for a drying time of 60 minutes at a drying temperature of 45°C. In Example 10, drying was performed at room pressure (room pressure drying) for a drying time of 60 minutes at a drying temperature of 90°C. In Table 2, the mass ratio of solvent molecules remaining in the sponge-like structure is shown as "DME residual ratio." Table 2 also shows the mass-based capacities for the first cycle and second cycle measured in the charge-discharge cycle test.

[0068] Table 2 confirms that Examples 9, 11, and 13, in which the drying temperature was 45°C or less, had higher DME residual percentages and mass-based capacities than Example 10, in which the drying temperature was 90°C. Comparing Examples 10 and 11, it was confirmed that Example 11, in which the drying temperature was 45°C, had higher DME residual percentages and mass-based capacities than Example 10, in which the drying temperature was 90°C. It can be seen that the lower the drying temperature, the higher the DME residual percentages and mass-based capacities tend to be. It was confirmed that Example 13, in which vacuum drying was performed for a short time (10 seconds) at room temperature (20°C), had the highest DME residual percentages and mass-based capacities. Comparing Example 11, in which the drying temperature was 45°C, with Examples 9, 12, and 13, in which the drying temperature was 20°C, it was confirmed that Example 11 had a lower mass-based capacity at the first cycle than Examples 9, 12, and 13, but that Example 11 and Examples 9, 12, and 13 had similar mass-based capacities at the second cycle. Comparing Examples 9, 12, and 13, which were subjected to vacuum drying, it was confirmed that Example 13, which had a drying time of 10 seconds, had a higher DME residual rate and mass-based capacity than Example 9, which had a drying time of 10 minutes, and Example 12, which had a drying time of 60 minutes. It can be seen that the shorter the drying time, the higher the DME residual rate and mass-based capacity tend to be.

[0069] 2. Second embodiment 2-1. Overall structure In the first embodiment, polysulfide 16 is contained in sponge-like structure 14 made of CNTs 12, but in the second embodiment, in addition to polysulfide, electrolyte and / or additives are further contained in the sponge-like structure made of CNTs.

[0070] In FIG. 5, a positive electrode for a secondary battery (hereinafter referred to as a positive electrode) 30 according to the second embodiment has a self-supporting sponge-like structure 34 made of CNTs 32 and having a thickness of 5 μm or more and less than 100 μm, and Li2S xIt contains a polysulfide composite 36 composed of a polysulfide represented by (x=4, 6, 8) and an electrolyte and / or an additive. The configurations of the CNTs 32 and the sponge-like structure 34 are the same as those of the CNTs 12 and the sponge-like structure 14 of the first embodiment, and therefore a description thereof will be omitted.

[0071] The polysulfide composite 36 is supported on the CNTs 32 of the sponge-like structure 34 in a solid, semi-solid, or dissolved state in solvent molecules remaining in the sponge-like structure 34, and is composited with the CNTs 32. In other words, the polysulfide, electrolyte, and / or additives that make up the polysulfide composite 36 are supported on the CNTs 32 of the sponge-like structure 34 and are composited with the CNTs 32. The polysulfide composite 36 can have a structure that coats the CNTs 32 in an island-like manner, a structure that coats almost the entire surface of the CNTs 32, a particulate structure, or any other predetermined structure. The polysulfide content in the sponge-like structure 34 is 0.70 g / cm in terms of sulfur, as in the first embodiment. 3 More than 2.0g / cm 3 or less, preferably 0.95 g / cm 3 More than 1.6g / cm 3 The following is the result.

[0072] Examples of electrolytes that can be used to form the polysulfide composite 36 include LiTFSI, lithium bis(perfluoroethanesulfonyl)imide (LiBETI), LiClO4, LiBF4, LiPF6, lithium trifluoromethanesulfonate (Li triflate), lithium bis(oxalato)borate (LiBOB), lithium bis(fluorosulfonyl)imide (LiFSI), LiCF3BO3, Li bromide, and Li iodide. LiNO3, which will be described later as an additive, can also be used as an electrolyte. The content of the electrolyte in the sponge-like structure 34 can be, for example, the desired amount of electrolyte to be contained in the final secondary battery, but is not limited thereto and may be less than the desired amount.

[0073] Examples of additives that constitute the polysulfide composite 36 include LiNO, ammonium bis(trifluorosulfonyl)imide (NHTFSI), and fluoroethylene carbonate (FEC). The electrolyte that constitutes the polysulfide composite 36 can also be used as the additive. The content of the additive in the sponge-like structure 34 can be, for example, a desired amount of additive to be contained in the final secondary battery, but is not limited to this, and may be, for example, less than the desired amount of additive.

[0074] In the positive electrode 30 of the second embodiment, in addition to polysulfide as a positive electrode active material, the electrolyte and / or additives constituting the electrolyte solution of the secondary battery are pre-loaded in the sponge-like structure 34. Therefore, when fabricating a secondary battery, it is possible to inject only the solvent for the electrolyte solution or an electrolyte solution with a low concentration of the electrolyte and / or additives. An electrolyte solution consisting of only a solvent or an electrolyte solution with a low concentration of the electrolyte and / or additives has low viscosity, so it easily permeates the positive electrode 30, the negative electrode, and the separator. This allows the amount of electrolyte solution in the secondary battery to be reduced, thereby reducing the E / S ratio. The smaller the E / S ratio, the smaller the mass of the secondary battery, and the higher the energy density of the secondary battery. Therefore, according to the positive electrode 30 of the second embodiment, by reducing the amount of electrolyte solution in the secondary battery, the E / S ratio and mass of the secondary battery can be reduced, thereby increasing the energy density of the secondary battery.

[0075] 2-2. Manufacturing method A method for manufacturing the positive electrode 30 according to the second embodiment will be described.

[0076] The positive electrode 30 is a self-supporting sponge-like structure 34 made of CNTs 32 and having a thickness of 5 μm or more and less than 100 μm, and Li2S x It is produced by incorporating polysulfide represented by (x=4, 6, 8) with an electrolyte and / or additives, and by compositing CNT32, polysulfide, electrolyte and / or additives.

[0077] An example of a method for compounding CNT32, polysulfide, electrolyte and / or additives will be described below.

[0078] (First method) As shown in Figure 6, polysulfide (LiS x ), an electrolyte, and an additive (hereinafter, the solution containing the polysulfide, the electrolyte, and / or the additive is referred to as "Li2S x The composite solution (hereinafter referred to as the "composite solution") is supplied to the sponge-like structure 34, and Li2S is deposited in the sponge-like structure 34. x a supplying step for holding the composite solution; and Li2S x The sponge-like structure 34 holding the composite solution is dried, and a drying step is performed in which the polysulfide, the electrolyte, and the additive are supported on the CNTs 32 (see FIG. 5), thereby obtaining the positive electrode 30. The drying step in the first method is the same as the drying step S2 (see FIG. 3) in the first embodiment, and therefore a description thereof will be omitted.

[0079] In the supplying step of the first method, Li2S x The composite solution is prepared by dispersing S powder, Li2S powder, electrolyte powder, and additive powder in a solvent. For example, a mixed solution of the electrolyte and additive is prepared by dissolving the electrolyte powder and the additive powder in a solvent, and then S powder and Li2S powder are added to the prepared mixed solution of the electrolyte and additive, and the powder is stirred and dissolved to obtain Li2S. x A composite solution can be prepared: Li2S x The method of supplying the composite solution to the sponge-like structure 34 can be, as in the supplying step S1 in the first embodiment, a method of dropping the solution onto the sponge-like structure 34, or the like. xExamples of solvents that can be used for the composite solution include DME, a mixture of DOL and DME, 1,2-diethoxyethane (DEE), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3), tetraethylene glycol dimethyl ether (G4), hydrofluoroether (HFE), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), 1,3-dimethyl-2-imidazolidinone (DMI), 1-methylimidazole (Melm), ethyl methyl sulfone, sulfolane (SL), dimethyl sulfoxide (DMSO), acetonitrile (AN), tetrahydrofuran (THF), and fluoroethylene carbonate (FEC).

[0080] In the first method, the supplying step is shown in FIG. 6 as a Li2S solution containing polysulfide, an electrolyte, and an additive. x The composite solution was supplied to the sponge-like structure 34, but is not limited to this. x The composite solution may be supplied to the sponge-like structure 34, or Li2S containing polysulfide and additives. x The composite solution may be applied to the sponge-like structure 34 .

[0081] (Second method) In the first method, Li2S x The composite solution was supplied to the sponge-like structure 34, but in the second method, the polysulfide (LiS x ) containing solution (LiS x A solution containing an electrolyte (electrolyte solution), and a solution containing an additive (additive solution) are supplied to the sponge-like structure 34, respectively.

[0082] In the second method, as shown in Figure 7, first, Li2S x The solution was supplied to the sponge-like structure 34, and Li2S was dissolved in the sponge-like structure 34. xA supplying step of supplying the electrolyte solution to the first positive electrode precursor and retaining the electrolyte solution in the first positive electrode precursor, and a drying step of drying the sponge-like structure 34 are performed to produce a first positive electrode precursor. Next, a supplying step of supplying the electrolyte solution to the first positive electrode precursor and retaining the electrolyte solution in the first positive electrode precursor, and a drying step of drying the first positive electrode precursor are performed to produce a second positive electrode precursor. Next, a supplying step of supplying the additive solution to the second positive electrode precursor and retaining the additive solution in the second positive electrode precursor, and a drying step of drying the second positive electrode precursor are performed to produce positive electrode 30. The drying steps in the second method are the same as the drying step S2 (see FIG. 3) in the first embodiment, and therefore will not be described here.

[0083] In the supplying step of the second method, Li2S x The solution is prepared by dispersing S8 powder and Li2S powder in a solvent. The electrolyte solution is prepared by dissolving the electrolyte powder in a solvent. The additive solution is prepared by dissolving the additive powder in a solvent. Li2S x The method of supplying the solution of Li2S, the electrolyte solution, and the additive solution to the sponge-like structure 34 can be, for example, a method of dropping the solution onto the sponge-like structure 34, similar to the supplying step S1 in the first embodiment. The solvents used to prepare each solution may be the same solvent or different solvents. In the second method, x Since the solution of Li2S, the electrolyte solution, and the additive solution are supplied to the sponge-like structure 34, the solvents used to prepare each solution can be independently selected to have high solubility for the polysulfide, the electrolyte, and the additive. xExamples of the solvent for the electrolyte include DME, a mixture of DOL and DME, 1,2-diethoxyethane (DEE), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3), tetraethylene glycol dimethyl ether (G4), hydrofluoroether (HFE), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), 1,3-dimethyl-2-imidazolidinone (DMI), 1-methylimidazole (Melm), ethyl methyl sulfone, sulfolane (SL), dimethyl sulfoxide (DMSO), acetonitrile (AN), tetrahydrofuran (THF), and fluoroethylene carbonate (FEC). x In addition to the above solvents, organic solvents such as alcohols and carbonates (ethanol, ethylene carbonate, etc.) can be used. If the electrolyte is made of a material that does not react with water, water can be used as the solvent in addition to the above organic solvents. As the solvent for the additive, for example, the organic solvents of the electrolyte can be used. If the additive is made of a material that does not react with water, water can be used as the solvent in addition to the above organic solvents.

[0084] In FIG. 7, the sponge-like structure 34 is x The solution, the electrolyte solution, and the additive solution were supplied in this order, but the order in which the solutions are supplied is not particularly limited. x solution, additive solution, electrolyte solution, electrolyte solution, additive solution, Li2S x solution, electrolyte solution, Li2S x solution, additive solution, additive solution, electrolyte solution, Li2S x solution, additive solution, Li2S x The order of the solution and the electrolyte solution may be used. The drying step between each supply step may be omitted as appropriate.

[0085] In Figure 7, Li2S xIn the above example, three types of solutions, namely, the Li2S solution, the electrolyte solution, and the additive solution, are supplied to the sponge-like structure 34, but two types of solutions other than the electrolyte solution or the additive solution may be supplied. The order in which the two types of solutions are supplied to the sponge-like structure 34 is not particularly limited. That is, the order in which the solutions are supplied to the sponge-like structure 34 is as follows: x solution, additive solution, additive solution, Li2S x The solution of Li2S x solution, electrolyte solution, electrolyte solution, Li2S x The drying step between each supply step may be omitted as appropriate.

[0086] If the electrolyte and additive are materials that do not react with water, the electrolyte solution and additive solution may be supplied to the sponge-like structure 34 in an atmospheric environment. By supplying the electrolyte solution and additive solution to the sponge-like structure 34 in an atmospheric environment, the process performed in an argon (Ar) environment can be reduced, and the manufacturing cost and the burden on the environment can be reduced. For example, after supplying the electrolyte solution and additive solution to the sponge-like structure 34 in an atmospheric environment and drying, Li2S x By continuously supplying the solution to the sponge-like structure 34 and drying the sponge-like structure 34 in the drying step in an Ar environment, production costs and environmental loads can be reduced, and the positive electrode 30 can be produced efficiently.

[0087] (Third Method) In the first method, Li2S containing polysulfide, electrolyte and / or additives is x The composite solution is supplied to the sponge-like structure 34, and in the second method, Li2S x The solution, the electrolyte solution, and the additive solution were each supplied to the sponge-like structure 34. In the third method, two types of solutions with different combinations of polysulfide, electrolyte, and additive are each supplied to the sponge-like structure 34.

[0088] In the third method, as shown in FIG. 8, first, polysulfide (Li2S x ) containing solution (LiS xThe solution of Li2S is supplied to the sponge-like structure 34, and the Li2S x A third positive electrode precursor is produced by carrying out a supplying step in which a solution containing an electrolyte and an additive (a solution of an electrolyte and an additive) is supplied to the third positive electrode precursor, and the solution of the electrolyte and the additive is retained in the third positive electrode precursor, and a drying step in which the third positive electrode precursor is dried, thereby producing positive electrode 30. The drying steps in the third method are the same as drying step S2 (see FIG. 3 ) in the first embodiment, and therefore description thereof will be omitted.

[0089] In the supplying step in the third method, Li2S x The solution is prepared by dispersing S8 powder and Li2S powder in a solvent. The electrolyte and additive solution is prepared by dissolving the electrolyte powder and the additive powder in a solvent. Li2S x The method of supplying the solution of Li2S, the electrolyte, and the additive solution to the sponge-like structure 34 can be, for example, a method of dropping the solution onto the sponge-like structure 34, similar to the supplying step S1 in the first embodiment. The solvents used to prepare the respective solutions may be the same solvent or different solvents. In the third method, Li2S x Since the solution of Li2S and the solutions of the electrolyte and additive are supplied to the sponge-like structure 34, the solvents used to prepare each solution can be selected to have high solubility for the polysulfide, electrolyte, and additive. xAs in the second method, examples of the solvent for the electrolyte include DME, a mixture of DOL and DME, 1,2-diethoxyethane (DEE), diethylene glycol dimethyl ether (G2), triethylene glycol dimethyl ether (G3), tetraethylene glycol dimethyl ether (G4), hydrofluoroether (HFE), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMA), 1,3-dimethyl-2-imidazolidinone (DMI), 1-methylimidazole (Melm), ethyl methyl sulfone, sulfolane (SL), dimethyl sulfoxide (DMSO), acetonitrile (AN), tetrahydrofuran (THF), and fluoroethylene carbonate (FEC). Examples of the solvent for the electrolyte and additive include the above-mentioned Li2S. x In addition to the above solvents, organic solvents such as alcohols and carbonates (ethanol, ethylene carbonate, etc.) can be used.

[0090] In FIG. 8, the sponge-like structure 34 is x The solution, the electrolyte and the additive solution were supplied in this order, but the order in which the solutions were supplied is not particularly limited. x In the case where the electrolyte and the additive are materials that do not react with water, water can be used as a solvent in addition to the organic solvents described above. The drying step between each supply step may be omitted as appropriate.

[0091] In Figure 8, Li2S x Two kinds of solutions, a solution of Li2S containing polysulfide and an electrolyte and an additive, were respectively supplied to the sponge-like structure 34. x Two types of solutions, namely, the composite solution and the additive solution, may be supplied to the sponge-like structure 34, respectively, or a Li2S solution containing polysulfide and an additive may be supplied. xThe two types of solutions, the composite solution and the electrolyte solution, may be supplied to the sponge-like structure 34. The order in which the two types of solutions are supplied is not particularly limited. That is, the order in which the two types of solutions are supplied to the sponge-like structure 34 is as follows: x The composite solution, the additive solution, the additive solution, the Li2S solution containing polysulfide and electrolyte were x The composite solution contains Li2S, polysulfides, and additives. x Composite solution, electrolyte solution, electrolyte solution, Li2S containing polysulfide and additives x The drying step between each supply step may be omitted as appropriate.

[0092] If the electrolyte and additive are materials that do not react with water, the electrolyte and additive solutions may be supplied to the sponge-like structure 34 in an atmospheric environment. By supplying the electrolyte and additive solutions to the sponge-like structure 34 in an atmospheric environment, the number of processes performed in an Ar environment can be reduced, thereby reducing manufacturing costs and the burden on the environment. For example, after supplying the electrolyte and additive solutions to the sponge-like structure 34 in an atmospheric environment and drying, Li2S x By continuously supplying the solution to the sponge-like structure 34 and drying the sponge-like structure 34 in the drying step in an Ar environment, production costs and environmental loads can be reduced, and the positive electrode 30 can be produced efficiently.

[0093] The first to third methods according to the second embodiment allow the sponge-like structure 34 to support in advance not only the polysulfide as the positive electrode active material but also the electrolyte and / or additives that constitute the electrolyte solution of the secondary battery. Therefore, when fabricating a secondary battery, it is possible to inject only the solvent for the electrolyte solution or an electrolyte solution with a low concentration of the electrolyte and / or additives. An electrolyte solution consisting of only a solvent or an electrolyte solution with a low concentration of the electrolyte and / or additives has low viscosity, so it easily permeates the positive electrode 30, the negative electrode, and the separator. This allows the amount of electrolyte solution in the secondary battery to be reduced, thereby decreasing the E / S ratio. Therefore, the first to third methods according to the second embodiment allow the amount of electrolyte solution in the secondary battery to be reduced, thereby decreasing the E / S ratio and mass of the secondary battery, and increasing the energy density of the secondary battery.

[0094] The first to third methods described above allow a predetermined amount of electrolyte and / or additive to be supported in advance in the sponge-like structure 34, and therefore the final amount of electrolyte and / or additive can be adjusted by adjusting the amount of electrolyte and / or additive to be contained in the electrolyte solution of the secondary battery. In the manufacturing method of the first embodiment, the amount of electrolyte and / or additive that can be supplied to the secondary battery is limited by the saturated solubility of the electrolyte and / or additive in the solvent of the electrolyte solution, but the first to third methods of the second embodiment described above allow the production of a secondary battery containing electrolyte and / or additive in an amount equal to or greater than the saturated solubility in the solvent of the electrolyte solution.

[0095] The secondary battery according to the second embodiment can be manufactured by placing a positive electrode 30, a negative electrode, a separator, and an electrolyte solution in a container. The secondary battery according to the second embodiment is not particularly limited as long as it includes a positive electrode 30, a negative electrode, a separator, an electrolyte solution, and a container, and has a configuration in which the positive electrode 30, the negative electrode, and the separator are provided in a container together with the electrolyte solution. For example, a secondary battery 20 (see FIG. 4) may be constructed using the positive electrode 30 according to the second embodiment instead of the positive electrode 10 according to the first embodiment.

[0096] The electrolyte used in the secondary battery according to the second embodiment may be different from the electrolyte used in the secondary battery according to the first embodiment. In the first embodiment, for example, an electrolyte prepared by dissolving 1.0 mol / L LiTFSI as the electrolyte and 0.2 mol / L LiNO3 as the additive in a solvent is used. However, in the second embodiment, an electrolyte consisting of only a solvent or an electrolyte prepared by dissolving a smaller amount of electrolyte and / or additive in a solvent than in the first embodiment to contain a low concentration of electrolyte and / or additive may be used. An electrolyte consisting of only a solvent or an electrolyte containing a low concentration of electrolyte and / or additive has low viscosity and easily permeates the positive electrode 30, the negative electrode, and the separator. This reduces the amount of electrolyte in the secondary battery, thereby reducing the E / S ratio. Therefore, according to the secondary battery according to the second embodiment, by reducing the amount of electrolyte, the E / S ratio and mass of the secondary battery can be reduced, thereby increasing the energy density.

[0097] In the secondary battery according to the second embodiment, the amount of electrolyte and / or additive that can be supplied is not limited to the saturated solubility of the electrolyte and / or additive in the solvent of the electrolytic solution. The secondary battery according to the second embodiment can be supplied with an amount of electrolyte and / or additive that is equal to or greater than the saturated solubility in the solvent of the electrolytic solution.

[0098] 2-3. Action and effects The positive electrode 30 according to the second embodiment, like the positive electrode 10 according to the first embodiment, has a self-supporting sponge-like structure 34 made of CNTs 32 and having a thickness of 5 μm or more and less than 100 μm, and Li2S x Polysulfides represented by (x=4, 6, 8) are 0.70 g / cm 3 More than 2.0g / cm 3The positive electrode 30 contains the following sulfur content: 0.01%. Therefore, the positive electrode 30 has the same effects as the positive electrode 10 according to the first embodiment. Specifically, the positive electrode 30 contains a large amount of polysulfide in the thin sponge-like structure 34, resulting in a high sulfur content per volume. This results in a high positive electrode capacity density per volume (volume capacity density), and an increased energy density of the secondary battery. Furthermore, the positive electrode 30 includes the highly conductive sponge-like structure 34 as a positive electrode current collector, and does not include a current collector foil. This reduces the mass and volume of the positive electrode 30, thereby increasing the mass capacity density and volume capacity density. Furthermore, the positive electrode 30 reduces the amount of electrolyte used to construct the secondary battery, thereby reducing the mass and volume of the secondary battery and increasing the energy density of the secondary battery.

[0099] In the positive electrode 30 according to the second embodiment, in addition to polysulfide as the positive electrode active material, at least one of an electrolyte and an additive is pre-loaded in the sponge-like structure 34. Therefore, when fabricating a secondary battery, it is possible to inject only the electrolyte solvent or an electrolyte and / or an electrolyte solution with a low concentration. Because the electrolyte solvent or an electrolyte and / or an electrolyte solution with a low concentration has low viscosity, it easily permeates the positive electrode 30, the negative electrode, and the separator, reducing the amount of electrolyte in the secondary battery and decreasing the E / S ratio. In the positive electrode 30 according to the second embodiment, by reducing the amount of electrolyte in the secondary battery and decreasing the E / S ratio, the mass of the secondary battery can be reduced and the energy density of the secondary battery can be increased.

[0100] When the positive electrode 30 is produced by the second method and the third method, Li2S x Since the solution of Li2S and the solution of the electrolyte and / or additive are supplied to the sponge-like structure 34, it is possible to select a solvent suitable for each solution. For example, water can be used as the solvent for the electrolyte and / or additive, and the burden on the environment can be reduced by not using an organic solvent. x The solvent is Li2S xThe electrolyte solvent may be a solvent with a high saturation solubility of the electrolyte, and the additive solvent may be a solvent with a high saturation solubility of the additive. x By selecting a solvent from the viewpoint of the saturated solubility of each of the electrolyte and additives, and by selecting a solvent from the viewpoint of improving the characteristics of the secondary battery when constructing the secondary battery, good secondary battery characteristics can be obtained.

[0101] In the second and third methods, the electrolyte and / or additive solution is supplied in an atmospheric environment, which reduces the number of processes performed in an Ar environment, thereby reducing the manufacturing cost and the burden on the environment. x By continuously carrying out the steps of supplying and drying the composite solution in an Ar environment, the positive electrode 30 can be produced efficiently.

[0102] 2-4.Example Table 3 summarizes the configurations and electrode performances of the positive electrodes of Examples 22 to 28. (a) to (m) in Table 3 were measured or calculated using the same method as in the first embodiment. The charge-discharge cycle test of the test cells was performed at a C rate of 0.1 C. The C rate was defined as 1 C = 1.672 mA / mg. Table 4 summarizes the preparation conditions for the positive electrodes and test cells of Examples 22 to 28.

[0103] [Table 3]

[0104] [Table 4]

[0105] The methods for producing the positive electrodes of Examples 22 to 28 are described below.

[0106] The positive electrode of Example 22 was fabricated using the method for manufacturing the positive electrode 10 according to the first embodiment. In Example 22, a Li2S8 solution was dropped onto the sponge-like structure 14 in an Ar environment and dried under reduced pressure for 10 minutes. The Li2S8 solution was prepared using DME as a solvent so as to contain Li2S8 equivalent to 0.1 mg / μL of sulfur (S) as the positive electrode active material. In the positive electrode of Example 22, 4.76 mg of Li2S8 was supported on the sponge-like structure 14, and 2.98 mg of undried solvent remained.

[0107] The positive electrode of Example 23 was fabricated using the first method, which is an example of the manufacturing method of the positive electrode 30 according to the second embodiment. In Example 23, a Li2S8 composite solution was dropped onto the sponge-like structure 34 in an Ar environment and dried in the same manner as in Example 22. The Li2S8 composite solution was prepared using DME as a solvent, containing Li2S8 equivalent to 0.1 mg / μL of sulfur (S) as the positive electrode active material, 0.4 M LiTFSI as the electrolyte, and 0.08 M LiNO3 as the additive. M (molar) is synonymous with mol / L. In the positive electrode of Example 23, 4.64 mg of Li2S8, 5.05 mg of LiTFSI, and 0.24 mg of LiNO3 were supported on the sponge-like structure 34, and 5.57 mg of undried solvent remained.

[0108] The positive electrode of Example 24 was produced using the first method, which is an example of the method for producing the positive electrode 30 according to the second embodiment. The production conditions for the positive electrode of Example 24 were the same as those for the positive electrode of Example 23, except that a mixed solution of DME and DOL in a volume ratio of 1:1 was used as the solvent for the Li2S8 composite solution. In the positive electrode of Example 24, 4.40 mg of Li2S8, 4.80 mg of LiTFSI, and 0.23 mg of LiNO3 were supported on the sponge-like structure 34, and 5.54 mg of undried solvent remained.

[0109] The positive electrode of Example 25 was fabricated using the second method, which is an example of the manufacturing method of the positive electrode 30 according to the second embodiment. In Example 25, Li2S8, LiTFSI, and LiNO3 were separately supplied to the sponge-like structure 34. In Example 25, a Li2S8 solution was first dropped onto the sponge-like structure 34 in an Ar environment and dried under reduced pressure for 5 seconds to prepare a first positive electrode precursor. Next, a LiTFSI solution was dropped onto the first positive electrode precursor and a second positive electrode precursor was prepared without drying. Next, a LiNO3 solution was dropped onto the second positive electrode precursor and dried under reduced pressure for 10 minutes. The Li2S8 solution was prepared using DME as a solvent to contain Li2S8 equivalent to 0.1 mg / μL of sulfur (S) as the positive electrode active material. The LiTFSI solution was prepared using a mixed solution of DME and DOL in a volume ratio of 1:1 as a solvent to contain 0.1 g / mL of LiTFSI. The LiNO solution was prepared using a 1:1 volumetric mixture of DME and DOL as a solvent, so as to contain 0.02 g / mL of LiNO. In the positive electrode of Example 25, 4.19 mg of Li2S8, 4.56 mg of LiTFSI, and 0.22 mg of LiNO3 were supported on the sponge-like structure 34, and 1.24 mg of undried solvent remained.

[0110] The positive electrode of Example 26 was fabricated using the second method, which is an example of the manufacturing method of the positive electrode 30 according to the second embodiment. In Example 26, first, a LiTFSI solution was dropped onto the sponge-like structure 34 in an atmospheric environment, and a first positive electrode precursor was fabricated without drying. Next, a LiNO3 solution was dropped onto the first positive electrode precursor in the atmospheric environment, and the precursor was dried under reduced pressure for 10 minutes, to fabricate a second positive electrode precursor. Next, a Li2S8 solution was dropped onto the second positive electrode precursor in an Ar environment, and the precursor was dried under reduced pressure for 10 minutes. The Li2S8 solution, LiTFSI solution, and LiNO3 solution were prepared under the same conditions as the solutions of Example 25. In the positive electrode of Example 26, 4.38 mg of Li2S8, 4.77 mg of LiTFSI, and 0.23 mg of LiNO3 were supported on the sponge-like structure 34, and 4.11 mg of undried solvent remained.

[0111] The positive electrode of Example 27 was fabricated using the first method, which is an example of the manufacturing method of the positive electrode 30 according to the second embodiment. The manufacturing conditions for the positive electrode of Example 27 were the same as those for the positive electrode of Example 23, except that DME was used as the solvent, Li2S8 equivalent to 0.1 mg / μL of sulfur (S) was used as the positive electrode active material, and a Li2S8 composite solution prepared to contain 0.2 M LiNO3 was used as the additive. That is, the Li2S8 composite solution was dropped into a sponge-like structure 34 in an Ar environment and dried. In the positive electrode of Example 27, 4.43 mg of Li2S8 and 0.58 mg of LiNO3 were supported on the sponge-like structure 34, and 4.65 mg of undried solvent remained.

[0112] The positive electrode of Example 28 was fabricated using the second method, which is an example of the manufacturing method for the positive electrode 30 according to the second embodiment. In Example 28, LiNO3 and Li2S8 were separately supplied to the sponge-like structure 34. In Example 28, first, a LiNO3 solution was dropped onto the sponge-like structure 34 in an air environment, followed by drying under reduced pressure for 10 minutes to prepare a first positive electrode precursor. Next, a Li2S8 solution was dropped onto the first positive electrode precursor in an Ar environment, followed by drying under reduced pressure for 10 minutes. The Li2S8 solution and LiNO3 solution were prepared under the same conditions as the Li2S8 solution and LiNO3 solution of Example 26. In the positive electrode of Example 28, 4.33 mg of Li2S8 and 0.68 mg of LiNO3 were supported on the sponge-like structure 34, and 4.03 mg of undried solvent remained.

[0113] Test cells using the positive electrodes of Examples 22 to 28 will be described below.

[0114] The positive electrodes of Examples 22 to 28 were placed in a container together with a negative electrode, a separator, and an electrolyte to prepare test cells. The negative electrode was a 50 μm thick lithium foil. The separator was made of polypropylene. The solvent for the electrolyte was a mixed solution of DME and DOL in a volume ratio of 1:1.

[0115] An electrolyte solution prepared by dissolving 1 mol / L of LiTFSI as an electrolyte and 0.20 mol / L of LiNO3 as an additive in the above solvent was poured into the test cell using the positive electrode of Example 22 on which Li2S8 was supported (see Table 4). From Tables 3 and 4, it was confirmed that Example 22 had a large volumetric capacity and mass capacity.

[0116] In the test cells using the positive electrodes of Examples 23 to 26, in which LiTFSI and LiNO3 were supported in addition to Li2S8, only the above solvent was poured as the electrolyte so that the LiTFSI and LiNO3 concentrations in the test cells were equivalent to the LiTFSI and LiNO3 concentrations in the electrolyte in the test cell using the positive electrode of Example 22. From Tables 3 and 4, it was confirmed that Examples 23 to 26 had large volumetric and mass-based capacities.

[0117] By using the positive electrode of Example 27, it is possible to supply LiNO3 (additive) at a concentration equal to or greater than the saturated solubility in the solvent of the electrolyte in the secondary battery. The positive electrode of Example 27 supports LiNO3 in addition to Li2S8. An electrolyte prepared so that the LiTFSI concentration was equal to the LiTFSI concentration (1 mol / L) of the electrolyte of Example 22 and the LiNO3 concentration was 0.80 mol / L, higher than the LiNO3 concentration (0.20 mol / L) of the electrolyte of Example 22, was injected into the test cell using the positive electrode of Example 27. As a result, in the test cell using the positive electrode of Example 27, the LiNO3 concentration in the test cell was 1.04 mol / L due to the LiNO3 supported on the positive electrode and the LiNO3 contained in the electrolyte. Here, the saturated solubility of LiNO3 in DME is estimated to be approximately 1 mol / L (see, for example, C. Burke et al., Proceedings of the National Academy of Sciences of the United States of America, 112, (2015) 9293). Therefore, the test cell using the positive electrode of Example 27 was supplied with an additive (LiNO3) at a concentration greater than the saturated solubility in the electrolyte solvent. Tables 3 and 4 confirm that Example 27 had large volumetric and mass-based capacities.

[0118] The test cell using the positive electrode of Example 28 does not contain the electrolyte LiTFSI contained in the test cells using the positive electrodes of Examples 22 to 27, and instead uses LiNO3 supported on the positive electrode as the electrolyte. The positive electrode of Example 28 supports LiNO3 in addition to Li2S8. The test cell using the positive electrode of Example 28 was filled with only the above solvent as the electrolyte so that the LiNO3 concentration in the test cell was 0.53 mol / L. Tables 3 and 4 confirm that Example 28 had large volumetric capacity and mass capacity. [Explanation of symbols]

[0119] 10,30 Positive electrode (positive electrode for secondary battery) 12,32 Carbon nanotubes (CNTs) 14,34 Sponge-like structures 16 Polysulfide (Li2S x (x=4,6,8) 20 Secondary battery 22 Negative electrode 24 Separator 36 Polysulfide Compounds

Claims

1. A positive electrode for a secondary battery used in constructing a secondary battery, comprising: A self-supporting sponge-like structure having a thickness of 5 μm or more and less than 100 μm, which is made of carbon nanotubes, is provided. ２ S ｘ (x = 4, 6, 8) polysulfide having a viscosity of 0.70 g / cm ３ 2.0g / cm or more ３ It contains the following sulfur amounts: the sponge-like structure further contains an electrolyte and / or an additive in addition to the polysulfide, the electrolyte is one selected from the group consisting of LiTFSI, lithium bis(perfluoroethanesulfonyl)imide (LiBETI), LiClO 4 , LiBF 4 , LiPF 6 , lithium trifluoromethanesulfonate (Li triflate), lithium bis(oxalato)borate (LiBOB), lithium bis(fluorosulfonyl)imide (LiFSI), LiCF 3 BO 3 , Li bromide, and Li iodide; The additive is one selected from the group consisting of LiNO 3 , ammonium bis(trifluorosulfonyl)imide (NH 4 TFSI), and fluoroethylene carbonate (FEC). Positive electrode for secondary batteries.

2. A secondary battery in which a positive electrode, a negative electrode, and a separator for a secondary battery are provided in a container together with an electrolyte, The positive electrode for the secondary battery comprises a self-supporting sponge-like structure made of carbon nanotubes and having a thickness of 5 μm or more and less than 100 μm, and a polysulfide represented by Li 2 S x (x=4, 6, 8) is contained in an amount of sulfur of 0.70 g / cm 3 or more and 2.0 g / cm 3 or less, and the sponge-like structure further contains an electrolyte and / or an additive in addition to the polysulfide, The electrolyte and / or the additive are contained in an amount greater than the saturated solubility of the solvent of the electrolytic solution. Secondary battery.

3. A self-supporting sponge-like structure made of carbon nanotubes and having a thickness of 5 μm or more and less than 100 μm is provided. ２ S ｘ (x=4, 6, 8) and to combine the carbon nanotubes with the polysulfide, the amount of sulfur in the polysulfide in the sponge-like structure is 0.70 g / cm 3 or more and 2.0 g / cm 3 or less; an electrolyte and / or an additive are contained in the sponge-like structure in addition to the polysulfide, and the carbon nanotubes, the polysulfide, the electrolyte and / or the additive are composited; the electrolyte is one selected from the group consisting of LiTFSI, lithium bis(perfluoroethanesulfonyl)imide (LiBETI), LiClO 4 , LiBF 4 , LiPF 6 , lithium trifluoromethanesulfonate (Li triflate), lithium bis(oxalato)borate (LiBOB), lithium bis(fluorosulfonyl)imide (LiFSI), LiCF 3 BO 3 , Li bromide, and Li iodide; The additive is one selected from the group consisting of LiNO 3 , ammonium bis(trifluorosulfonyl)imide (NH 4 TFSI), and fluoroethylene carbonate (FEC). A method for manufacturing a positive electrode for a secondary battery.

4. A method for producing a positive electrode for a secondary battery, comprising: incorporating a polysulfide represented by Li 2 S x (x = 4, 6, 8) into a self-supporting sponge-like structure having a thickness of 5 μm or more and less than 100 μm, which is made of carbon nanotubes, and combining the carbon nanotubes with the polysulfide, the amount of sulfur in the polysulfide in the sponge-like structure is 0.70 g / cm 3 or more and 2.0 g / cm 3 or less; a supplying step of supplying the solution containing the polysulfide to the sponge-like structure and retaining the solution in the sponge-like structure; a drying step of drying the sponge-like structure holding the solution to cause the polysulfide to be supported on the carbon nanotubes, In the drying step, the sponge-like structure holding the solution is vacuum-dried at a temperature of −58° C. or higher and 20° C. or lower for a period of 10 seconds to 60 minutes. A method for manufacturing a positive electrode for a secondary battery.

5. The method for producing a positive electrode for a secondary battery according to claim 3 , wherein a solution containing the polysulfide, the electrolyte and / or the additive is supplied to the sponge-like structure.

6. The method for producing a positive electrode for a secondary battery according to claim 3 , wherein the solution containing the polysulfide, the solution containing the electrolyte, and / or the solution containing the additive are each supplied to the sponge-like structure.

7. The method for producing a positive electrode for a secondary battery according to claim 6 , wherein the solution containing the electrolyte and / or the solution containing the additive is supplied to the sponge-like structure in an atmospheric environment.

Images