Electrolyte for lithium-sulfur batteries and lithium-sulfur batteries

Abstract

To provide an electrolyte for a lithium-sulfur battery and a lithium-sulfur battery having high cycle characteristics.SOLUTION: An electrolyte for a lithium-sulfur battery according to the present invention includes a first component, a second component, and an organic solvent, and when the first component is lithium polysulfide, the second component is lithium bis(fluorosulfonyl)imide, and the concentration of lithium bis(fluorosulfonyl)imide is A, satisfies 0.001 mol / dm3≤A≤0.15 mol / dm3.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an electrolyte for a lithium-sulfur battery and a lithium-sulfur battery.

Background Art

[0002] Conventionally, lithium-ion secondary batteries have been widely used in equipment such as small electronic devices, electric vehicles, and smart grids. On the other hand, for the popularization of electric vehicles and the promotion of the use of natural energy, batteries with a larger energy density are required. However, the increase in the energy density of lithium-ion secondary batteries is reaching its limit, and the study of new materials or battery systems is necessary. Among these, lithium-sulfur batteries are attracting attention as one of the next-generation batteries because high energy density can be expected. A typical lithium-sulfur battery is composed of a positive electrode containing sulfur, a negative electrode containing lithium metal, and a separator containing an organic liquid electrolyte.

[0003] For the commercialization of lithium-sulfur batteries, improvement of cycle characteristics is required. One of the causes of the deterioration of cycle characteristics is the dissolution of lithium polysulfide, which is an intermediate active material, in the electrolyte. Lithium polysulfide is generated on the positive electrode side during charge and discharge and dissolves in the electrolyte. In a lithium-sulfur battery, the capacity of the positive electrode decreases due to this dissolution. As a method for suppressing the dissolution of lithium polysulfide, for example, a method of adding lithium polysulfide to the electrolyte, as shown in non-patent literature, is known. In this method, by previously including lithium polysulfide in the electrolyte, the dissolution of lithium polysulfide dissolved from the positive electrode is suppressed (delayed), and the decrease in the capacity of the positive electrode is suppressed.

[0004] However, in the method described in Non-Patent Document 1, polysulfide ions undergo excessive reduction and decomposition on the lithium anode during charging, promoting the deposition of needle-shaped crystals (dendrites) of lithium metal. It is believed that this process promotes dendrite deposition because an unstable decomposition product film is formed on the lithium metal. Dendrite deposition leads to premature failure of the lithium metal anode, a decrease in Coulomb efficiency, and even short circuits.

[0005] Here, it is known that lithium bis(fluorosulfonyl)imide forms a stable decomposition product film and is a suitable material for overcoming the problems of lithium metal anodes (see, for example, Non-Patent Documents 2 and 3). On the other hand, in lithium-sulfur batteries, it is known that electrolytes containing lithium bis(fluorosulfonyl)imide accelerate the decrease in the capacity of the sulfur cathode and are therefore unsuitable for the sulfur cathode (see, for example, Non-Patent Document 4). [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] M. Agostini et al., Chem. Mater. 2015, 27, 4604 [Non-Patent Document 2] J. Qian et al., Nat. Commu. 2015, 6, 6362 [Non-Patent Document 3] Y. Maeyoshi et al., ACS Appl. Mater. Interfaces. 2019, 11 (29), 25833 [Non-Patent Document 4] R. Cao et al., Adv. Funct. Mater. 2016, 26, 3059 [Overview of the project] [Problems that the invention aims to solve]

[0007] As explained above, lithium polysulfide is a material that can be expected to improve the performance of the sulfur cathode, and lithium bis(fluorosulfonyl)imide is a material that can be expected to improve the performance of the anode containing a lithium alloy or lithium metal. Conversely, lithium polysulfide is a material that promotes the degradation of the anode containing a lithium alloy or lithium metal, and lithium bis(fluorosulfonyl)imide is a material that promotes the degradation of the sulfur cathode. Therefore, using them as materials for lithium-sulfur batteries presented challenges in terms of improving cycle characteristics.

[0008] The present invention has been made in view of the above, and aims to provide an electrolyte for lithium-sulfur batteries and a lithium-sulfur battery having high cycle characteristics. [Means for solving the problem]

[0009] When the effect on battery characteristics was investigated using an electrolyte containing either lithium polysulfide or lithium bis(fluorosulfonyl)imide, sufficient performance could not be obtained. Therefore, when lithium polysulfide and lithium bis(fluorosulfonyl)imide were added simultaneously, and their concentrations were also investigated, it was found that very high performance could be obtained, leading to the completion of the present invention.

[0010] To solve the above-mentioned problems and achieve the objective, the electrolyte for lithium-sulfur batteries according to the present invention comprises, firstly, a first component, a second component, and a solvent, wherein the first component is lithium polysulfide, the second component is lithium bis(fluorosulfonyl)imide, and when the concentration of lithium bis(fluorosulfonyl)imide is A, the electrolyte is 0.001 mol / dm³. 3 ≤A ≤ 0.15 mol / dm 3 It is characterized by satisfying the following conditions.

[0011] Furthermore, in addition to the first aspect described above, the electrolyte for lithium sulfur batteries according to the present invention, as a second aspect, has a concentration of 0.2 mol / dm³ when the concentration of lithium polysulfide is B. 3 ≤B ≤ 2.0 mol / dm 3 It is characterized by satisfying the following conditions.

[0012] In addition, the electrolyte for a lithium-sulfur battery according to the present invention, from a third perspective in addition to the above first perspective and / or second perspective, the concentration A is 0.05 mol / dm 3 ≦A≦0.15 mol / dm 3 and is characterized by satisfying this.

[0013] In addition, the electrolyte for a lithium-sulfur battery according to the present invention, from a fourth perspective in addition to any one of the above first to third perspectives, the concentration B is 0.2 mol / dm 3 ≦B≦0.6 mol / dm 3 and is characterized by satisfying this.

[0014] In addition, the electrolyte for a lithium-sulfur battery according to the present invention, from a fifth perspective in addition to any one of the above first to fourth perspectives, in addition to the first and second components, the electrolyte contains at least one or more lithium salts as a third component, and the concentration of the lithium salt is greater than 0 mol / dm 3 and less than or equal to 1.5 mol / dm 3 and is characterized by this.

[0015] In addition, the electrolyte for a lithium-sulfur battery according to the present invention, from a sixth perspective in addition to any one of the above first to fifth perspectives, the lithium salt contains at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium nitrate, and lithium trifluoromethanesulfonate, and is characterized by this.

[0016] In addition, the electrolyte for a lithium-sulfur battery according to the present invention, from a seventh perspective in addition to any one of the above first to sixth perspectives, the solvent is 1,3-dioxolane and 1,2-dimethoxyethane, and is characterized by this.

[0017] Furthermore, the lithium-sulfur battery according to the present invention is characterized in that, as an eighth aspect, it comprises a negative electrode containing lithium metal or a lithium metal alloy, a positive electrode having sulfur or a sulfur compound as the main component of the positive electrode active material, and an electrolyte for a lithium-sulfur battery according to any one of the first to seventh aspects described above.

[0018] Furthermore, in addition to the eighth aspect described above, the lithium-sulfur battery according to the present invention is characterized in that, as a ninth aspect, the positive electrode contains sulfur-modified polyacrylonitrile and lithium titanate.

[0019] Furthermore, the lithium-sulfur battery according to the present invention is characterized in that, in addition to any one of the eighth to ninth aspects described above, the positive electrode contains a conductive additive, as a tenth aspect.

[0020] Furthermore, the lithium-sulfur battery according to the present invention is characterized in that, in addition to any one of the eighth to tenth aspects described above, the positive electrode contains porous carbon, as an eleventh aspect. [Effects of the Invention]

[0021] According to the present invention, an electrolyte for lithium-sulfur batteries and a lithium-sulfur battery having high cycle characteristics can be obtained. [Brief explanation of the drawing]

[0022] [Figure 1] Figure 1 is a cross-sectional view illustrating the configuration of a lithium-sulfur battery equipped with an electrolyte for a lithium-sulfur battery according to one embodiment of the present invention. [Figure 2] Figure 2 shows scanning electron microscope images of the surface of lithium-sulfur batteries before charging and discharging, according to Example 4, Comparative Example 1, and Comparative Example 3. [Modes for carrying out the invention]

[0023] The embodiments of the present invention will be described below, but the present invention is not limited to the following description. Furthermore, various modifications or improvements can be made to these embodiments, and such modified or improved forms may also be included in the present invention.

[0024] (Embodiment) Figure 1 is a cross-sectional view illustrating the configuration of a lithium-sulfur battery equipped with an electrolyte for a lithium-sulfur battery according to one embodiment of the present invention. The lithium-sulfur battery 1 comprises a positive electrode 2, a negative electrode 3, and a separator 4 disposed between the positive electrode 2 and the negative electrode 3. These positive electrode 2, negative electrode 3, and separator 4 are housed in an outer casing (not shown). The lithium-sulfur battery 1 is formed by the permeation of the electrolyte into the positive electrode 2, negative electrode 3, and separator 4. Note that the lithium-sulfur battery 1 is not limited to the shape shown in Figure 1, and may be coin-shaped, button-shaped, sheet-shaped, stacked, cylindrical, rectangular, flat, etc.

[0025] [Positive electrode] The positive electrode consists of a positive electrode current collector and a positive electrode composite layer. Specifically, the positive electrode 2 consists of a positive electrode current collector 21 and a positive electrode composite layer 22 provided on the surface facing the separator 4.

[0026] <Positive electrode current collector> The positive electrode current collector 21 is not particularly limited, and known or commercially available ones can be used. Examples of the positive electrode current collector 21 include aluminum or an aluminum alloy. Examples of materials for the positive electrode current collector 21 include aluminum foil, carbon-coated aluminum foil, metal mesh such as aluminum, porous metal, expanded metal, perforated metal, etc.

[0027] <Positive electrode composite layer> The positive electrode composite layer 22 contains sulfur and / or sulfur compounds.

[0028] (Sulfur and / or sulfur compounds) Here, for the purpose of obtaining a high energy density, the content of sulfur and / or sulfur compounds is preferably 50% by weight or more, more preferably 55 to 90% by weight, and even more preferably 55 to 65% by weight, relative to the weight of the positive electrode composite layer 22. In this case, if the sulfur and / or sulfur compounds are less than 50% by weight, the content of positive electrode active material in the positive electrode composite layer will be low, which may reduce the energy density of the lithium sulfur battery, so this is undesirable. It is preferable to use a conductive additive in the positive electrode composite layer 22 from the viewpoint of having excellent rate characteristics and cycle characteristics, and furthermore, reducing polarization. Furthermore, if a conductive additive is included, it is more preferable to use a compound in which the sulfur and / or sulfur compounds and the conductive additive are pre-compounded. Hereinafter, the compound in which sulfur and / or sulfur compounds and conductive additives are compounded will be referred to as a composite. The compounding method is not particularly limited, but may be a known method, such as melt impregnation, electrolytic deposition, vapor deposition, immersion, and mechanical milling, more preferably melt impregnation, and even more preferably electrolytic deposition. Furthermore, the positive electrode composite layer 22 may contain a binder to improve its binding properties. In addition, it is preferable to use additives (positive electrode additives) in the positive electrode composite layer 22 to improve rate characteristics and cycle characteristics, and to reduce polarization.

[0029] Known sulfurs and sulfur compounds can be used. Specifically, examples include crystalline sulfur, granular sulfur, colloidal sulfur, lithium sulfide, and sulfur-modified polyacrylonitrile. The positive electrode composite layer 22 may contain only one type of sulfur or two or more types. If it contains two or more types of sulfur, their combination and ratio can be arbitrarily selected according to the purpose. Furthermore, sulfurs and sulfur compounds may be used as a composite, individually, or in a mixture of both. In particular, it is preferable to add a composite and, separately, sulfur-modified polyacrylonitrile. For example, the reason for adding sulfur-modified polyacrylonitrile is that it improves discharge capacity and cycle characteristics.

[0030] (Conductive additive) Known conductive additives can be used. Specific examples include Ketjenblack, carbon nanotubes, graphene, graphene oxide, reduced graphene oxide, acetylene black, and porous carbon. Among these, porous carbon is preferred because it can exhibit high capacity. The conductive additive has a specific surface area of ​​500 to 2500 m². 2 A compound with a concentration of / g is preferred because it exhibits excellent rate characteristics and cycle characteristics, and results in low polarization. The conductive additive may contain only one type or two or more types. If two or more conductive additives are included, their combination and ratio can be arbitrarily selected according to the purpose. Furthermore, the conductive additives may be used as a composite, individually, or in a mixture of both. In particular, it is desirable to add a composite and, separately, individual compounds. For example, the reason for adding individual compounds is that it improves output characteristics.

[0031] (binder) A known binder can be used. Specific examples include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyacrylic acid (PAA), lithium polyacrylate (PAALi), styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyethylene glycol (PEG), carboxymethylcellulose (CMC), polyacrylonitrile (PAN), and polyimide (PI). The binder may contain only one type or two or more types. If two or more binders are included, their combination and ratio can be arbitrarily selected according to the purpose.

[0032] (Positive electrode additive) Specifically, the cathode additives include LiCoO2, LiMn2O4, LiNiO2, LiNiCoMnO2, and LiNi 0.5 Mn 0.5 O2, LiMPO4 (M=Ma, transition metals such as Fe, Co, Ni), LiNi 0.8 Co 0.15 Al 0.05O2, Li 1+x+y Al x (Ti,Ge) 2-x Si y P 3-y O 12 Lithium titanate (Li4Ti5O 12、 LiTiO 4、 Li2Ti3O7, etc. 0.35 La 0.55 TiO3, Li7La3Zr2O 12 Examples include lithium ion-conducting oxides such as cyclic polyacrylonitrile and its derivatives, poly(N-vinylcarbazole) and its derivatives, poly(benzimidazobenzophenanthroline) and its derivatives, poly(N-vinylpyridine) and its derivatives, poly(N-vinylpyrrolidone) and its derivatives, tetraphenylporphyrin and its derivatives, and nitrogen-containing organic compounds. The cathode additive may contain only one type or two or more types. If there are two or more cathode additives, their combination and ratio can be arbitrarily selected according to the purpose. In particular, lithium titanate is preferred. The reason for using lithium titanate is that it has high ionic conductivity and high electronic conductivity. Note that the cathode additive may also act as an active material.

[0033] The positive electrode composite layer 22 can be formed, for example, by dispersing the material in a solvent to form a slurry, applying it to the positive electrode current collector 21, and then drying it to remove the solvent. The positive electrode composite layer 22 may be formed on only one side of the positive electrode current collector 21, or on both sides.

[0034] Examples of solvents for the slurry include N-methyl-2-pyrrolidone (NMP) or water.

[0035] [Negative electrode] As the negative electrode 3, a negative electrode having a negative electrode active material that intercalates and releases lithium is used. As an example, the negative electrode 3 is composed of a negative electrode current collector 31 and a negative electrode composite material layer 32 containing the negative electrode active material, which is provided on the surface facing the separator 4. The negative electrode composite material layer 32 may be formed on only one side of the negative electrode current collector 31 or on both sides.

[0036] The negative electrode current collector 31 can be selected from the group consisting of copper, aluminum, stainless steel, titanium, silver, palladium, nickel, alloys thereof, and combinations thereof. Stainless steel may be surface-treated with carbon, nickel, titanium, or silver, and examples of alloys include aluminum-cadmium alloys. In addition, calcined carbon, non-conductive polymers surface-treated with conductive materials, or conductive polymers can be used as the negative electrode current collector 31.

[0037] The negative electrode active material in the negative electrode composite layer 32 includes, for example, metallic materials such as lithium metal, lithium aluminum alloy, lithium tin alloy, lithium lead alloy, lithium silicon alloy, and other lithium-containing alloys. One or more metallic materials can be used as the negative electrode active material. When two or more metallic materials are used, their combination and ratio can be arbitrarily selected according to the purpose.

[0038] Furthermore, the negative electrode 3 may be configured without a negative electrode current collector 31.

[0039] [Electrolyte] The electrolyte contains a solvent, lithium polysulfide as the first component, and lithium bis(fluorosulfonyl)imide as the second component.

[0040] In the electrolyte, from the perspective of achieving high cycling characteristics, the concentration A of lithium bis(fluorosulfonyl)imide is 0.001 mol / dm³. 3 ≤A ≤ 0.15 mol / dm 3 It is preferable that the following conditions be met, and 0.05 mol / dm 3 ≤A ≤ 0.15 mol / dm 3It is more preferable to satisfy. Also, although the detailed reason is unclear, by including lithium bis(fluorosulfonyl)imide in the above concentration range, the formation of dendrites on the lithium metal negative electrode can be suppressed, and the effects of suppressing early life and the decrease in Coulomb efficiency can be obtained. On the other hand, when A < 0.001 mol / dm 3 the Coulomb efficiency decreases (the irreversible capacity increases). Also, when 0.15 mol / dm 3 < A, the capacity retention rate decreases (accelerating the deterioration of the positive electrode).

[0041] In this specification, the concentration (mol / dm 3 ) of each component means the desired number of moles of each component with respect to 1 dm 3 of the organic solvent.

[0042] As the concentration B of polysulfide lithium, preferably 0.001 mol / dm 3 ≦ B ≦ 2.0 mol / dm 3 is satisfied, more preferably 0.2 mol / dm 3 ≦ B ≦ 2.0 mol / dm 3 is satisfied, and particularly preferably 0.2 mol / dm 3 ≦ B ≦ 0.6 mol / dm 3 is satisfied. When the concentration B of polysulfide lithium is higher than 2.0 mol / dm 3 the viscosity of the electrolyte increases (the ionic conductivity decreases), and there is a risk of a decrease in capacity. Here, the concentration of polysulfide lithium is the total value of that contained in the electrolyte in advance and that eluted from the electrode. Also, the polysulfide lithium here includes Li2S8, Li2S6, Li2S4, and Li2S2, etc. By including polysulfide lithium in the electrolyte, the elution of polysulfide lithium from the electrode can be suppressed. The behavior for elution suppression is theoretically considered to follow the Noyes-Whitney equation and the law of chemical equilibrium.

[0043] In addition to the lithium polysulfide that leaches from the electrodes, it is desirable to add lithium polysulfide to the electrolyte before assembling the lithium-sulfur battery. Adding lithium polysulfide before assembly allows it to function as an active material, resulting in a high discharge capacity. Furthermore, according to the Noyes-Whitney equation and the law of chemical equilibrium, the leaching of lithium polysulfide from the electrodes can be suppressed, thereby preventing a decrease in capacity. In this invention, the degradation of the lithium metal anode caused by the addition of lithium polysulfide before assembly can be suppressed by the addition of lithium bis(fluorosulfonyl)imide. The concentration A of lithium bis(fluorosulfonyl)imide is 0.001 mol / dm³. 3 ≤A ≤ 0.15 mol / dm 3 It is used within the range of 0.15 mol / dm³. In this case, the concentration A is 0.15 mol / dm³. 3 If the limit is exceeded, it will cause degradation of the positive electrode and reduce the cycle performance.

[0044] When preparing lithium polysulfide, it is preferable to synthesize it by mixing sulfur and lithium sulfide in a molar ratio of 7:1 to 3:1. However, there is no problem in using lithium polysulfide synthesized by other methods.

[0045] Furthermore, from the viewpoint of improving ionic conductivity, it is preferable to include a lithium salt as a third component in addition to lithium polysulfide and lithium bis(fluorosulfonyl)imide. Examples of lithium salts include lithium hexafluoride phosphate (LiPF6), lithium perchlorate (LiClO4), lithium bisoxalate borate (LiB(C2O4)), lithium borofluoride (LiBF4), lithium nitrate (LiNO3), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), etc. Preferably, at least one of lithium nitrate, lithium bis(trifluoromethanesulfonyl)imide, and lithium trifluoromethanesulfonate is included. The reason for including these lithium salts is that they can improve the cycle characteristics. There may be only one lithium salt or two or more lithium salts. When there are two or more lithium salts, their combination and ratio can be arbitrarily selected according to the purpose. From the perspective of achieving high capacity, the combined concentration of lithium salts other than lithium polysulfide and lithium bis(fluorosulfonyl)imide should be 0 mol / dm³. 3 Larger 1.5 mol / dm 3 Preferably, it is 0.1 mol / dm³ 3 More than 1.0mol / dm 3 The following is even more preferable:

[0046] Examples of solvents include ethylene carbonate, ethyl methyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, 1,3-dioxolane, 1,2-dimethoxyethane, sulfolane, oxolane, and ionic liquids, with 1,3-dioxolane and 1,2-dimethoxyethane being preferred. The solvent may be one type or two or more types. If there are two or more solvents, their combination and ratio can be arbitrarily selected depending on the purpose.

[0047] As described later, the concentrations of the first, second, and third components in the electrolyte are not calculated from the amount of electrolyte charged, but are measured from the electrolyte extracted from a fully charged lithium-sulfur battery that has completed its initial activation. For example, this can be determined by ion chromatography. More specifically, the type of anion in the electrolyte is identified using ion chromatography, and the anion intensity is measured. The concentration of the anion contained in the electrolyte is determined from this intensity and a pre-prepared calibration curve. Furthermore, even if lithium-sulfur batteries are sold on the market without initial activation, those that undergo initial activation afterward can be considered lithium-sulfur batteries that have completed their initial activation. Whether or not a lithium-sulfur battery has completed its initial activation is determined by the surface shape of the negative electrode when the lithium-sulfur battery is disassembled. If the surface morphology of the lithium metal negative electrode is flat, it is considered a lithium-sulfur battery that has not completed its initial activation.

[0048] Here, the conditions for initial activity are not particularly limited, but for example, an ambient temperature of 0 to 60°C and a current density of 0.01 to 1.0 mA / cm². 2 Discharge the battery until it reaches 1.0-1.7V, then charge it with the same current until it reaches 2.4-3.0V. Repeat this cycle several times.

[0049] <Separator> The separator 4 may be either an organic polymer separator or an inorganic separator, and is made of a material that does not react with the positive electrode 2, the negative electrode 3, or the electrolyte.

[0050] Examples of polymers that make up organic polymer separators include polypropylene, polyolefin, nitrocellulose, and polyimide. Polymer separators may also be treated with ceramic coatings or structural control. The treatment applied may be one type or two or more. If two or more treatments are applied, the combinations and other treatment conditions can be arbitrarily selected according to the purpose.

[0051] Examples of inorganic separators include nonwoven fabrics made of silica glass. Furthermore, inorganic separators that have undergone treatments such as ceramic coating or structural control may also be included. The treatments applied may be one type or two or more. If two or more treatments are applied, the combinations and other treatment conditions can be arbitrarily selected according to the purpose.

[0052] In the embodiment described above, a lithium-sulfur battery comprises a negative electrode containing lithium metal or a lithium metal alloy, a positive electrode whose positive electrode active material is mainly composed of sulfur or a sulfur compound, and an electrolyte containing lithium polysulfide as the first component, lithium bis(fluorosulfonyl)imide as the second component, and an organic solvent, wherein when the concentration of lithium bis(fluorosulfonyl)imide is A, the concentration is 0.001 mol / dm³ 3 ≤A ≤ 0.15 mol / dm 3 This was done to satisfy the above conditions. According to this embodiment, a lithium-sulfur battery with high cycle characteristics can be obtained by satisfying the above conditions. [Examples]

[0053] The present invention will be described in more detail below with reference to examples, but the present invention is not limited in any way by the following examples.

[0054] In this example, lithium polysulfide and lithium bis(fluorosulfonyl)imide were added to the electrolyte, and the concentrations of each and other components were varied. In the comparative example, electrolytes were prepared in which either lithium polysulfide or lithium bis(fluorosulfonyl)imide was added, and also in which the amount of lithium bis(fluorosulfonyl)imide added was outside the range of claim 1.

[0055] [Manufacturing of lithium-sulfur batteries] Sulfur (S) and porous carbon (manufactured by Toyo Tanso Co., Ltd., Knobel®) were mixed in a weight ratio of 70:30. The resulting mixture was heat-treated at 155°C for 12 hours in an inert gas atmosphere to allow the S to penetrate into the pores of the porous carbon. Hereinafter, this will be referred to as the S / porous carbon composite. The materials used are: S / porous carbon composite, acetylene black (AB), carbon nanotubes (CNT), binder, ultrapure water, sulfur-modified polyacrylonitrile (SPAN: manufactured by ADEKA Corporation), and lithium titanate (manufactured by Fujifilm Wako Pure Chemical Industries (manufacturer: Toyoshima Plant), Li4Ti5O 12 The mixture was uniformly kneaded and stirred using a defoaming and stirring device, "Awatori Rentaro" (manufactured by Shinky Co., Ltd.). The resulting cathode mixture slurry was prepared with a sulfur loading of 4.4 mg / cm³. 2 Carbon-coated aluminum foil was coated to achieve the desired result and vacuum-dried overnight at 60°C. The composition of the positive electrode composite was S / porous carbon composite:AB:CNT:binder:lithium titanate:sulfur-modified polyacrylonitrile = 85:1:2:5:6:1 (weight percent). Lithium metal was used as the negative electrode. No negative electrode current collector was used. A PP separator manufactured by Celgard was used as the separator. A lithium-sulfur battery was prepared and presented as an example, with a separator containing an electrolyte made of lithium bis(trifluoromethanesulfonyl)imide (hereinafter referred to as LiTFSI) as a lithium salt and 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME), lithium bis(fluorosulfonyl)imide (LiFSI), or a mixture of lithium polysulfide as a solvent, placed between the positive and negative electrodes. The lithium-sulfur battery was prepared under an inert gas atmosphere. When preparing lithium polysulfide in the electrolyte beforehand, it was synthesized and prepared by mixing sulfur and lithium sulfide in a molar ratio of 7:1. Furthermore, the volume ratio of DOL:DME was 1:1.

[0056] (Example 1) In Example 1, the concentration of LiTFSI in the electrolyte was 1 mol / dm³. 3 The concentration of Li2S8 used as lithium polysulfide is 0.2 mol / dm³ 3 The concentration of LiFSI is 0.01 mol / dm³.3 A lithium-sulfur battery was fabricated using the following method. The composition and physical properties of Example 1 are shown in Table 1. [Table 1]

[0057] (Example 2) In Example 2, the concentration of LiFSI in the electrolyte was set to 0.05 mol / dm³. 3 A lithium-sulfur battery was fabricated in the same manner as in Example 1, except for the change made to [specific component]. The composition and physical properties of Example 2 are shown in Table 1.

[0058] (Example 3) In Example 3, the concentration of LiFSI in the electrolyte was set to 0.1 mol / dm³ 3 A lithium-sulfur battery was fabricated in the same manner as in Example 1, except for the change made to [specific component]. The composition and physical properties of Example 3 are shown in Table 1.

[0059] (Example 4) In Example 4, the concentration of LiFSI in the electrolyte was set to 0.15 mol / dm³. 3 A lithium-sulfur battery was fabricated in the same manner as in Example 1, except for the change made to [specific component]. The composition and physical properties of Example 4 are shown in Table 1.

[0060] (Example 5) In Example 5, a lithium-sulfur battery was prepared in the same manner as in Example 4, except that it did not contain LiTFSI in the electrolyte. The composition and physical properties of Example 5 are shown in Table 1.

[0061] (Example 6) In Example 6, the concentration of LiTFSI in the electrolyte was set to 0.1 mol / dm³. 3 A lithium-sulfur battery was fabricated in the same manner as in Example 4, except for the change made to [specific component]. The composition and physical properties of Example 6 are shown in Table 1.

[0062] (Example 7) In Example 7, the concentration of LiTFSI in the electrolyte was set to 0.5 mol / dm³. 3A lithium-sulfur battery was fabricated in the same manner as in Example 4, except for the change made to [specific component]. The composition and physical properties of Example 7 are shown in Table 1.

[0063] (Example 8) In Example 8, the concentration of LiTFSI in the electrolyte was set to 1.5 mol / dm³. 3 A lithium-sulfur battery was fabricated in the same manner as in Example 4, except for the change made to [specific component]. The composition and physical properties of Example 8 are shown in Table 1.

[0064] (Example 9) In Example 9, the concentration of LiTFSI was set to 2.0 mol / dm³. 3 A lithium-sulfur battery was fabricated in the same manner as in Example 4, except for the change made to [specific component]. The composition and physical properties of Example 9 are shown in Table 1.

[0065] (Example 10) In Example 10, the concentration of lithium polysulfide in the electrolyte was 0.4 mol / dm³ 3 A lithium-sulfur battery was fabricated in the same manner as in Example 7, except for the change made to [specific component]. The composition and physical properties of Example 10 are shown in Table 1.

[0066] (Example 11) In Example 11, the concentration of lithium polysulfide in the electrolyte was 0.6 mol / dm³ 3 A lithium-sulfur battery was fabricated in the same manner as in Example 7, except for the change made to [specific component]. The composition and physical properties of Example 11 are shown in Table 1.

[0067] (Example 12) In Example 12, the concentration of lithium polysulfide in the electrolyte was set to 1.0 mol / dm³. 3 A lithium-sulfur battery was fabricated in the same manner as in Example 7, except for the change made to [specific component]. The composition and physical properties of Example 12 are shown in Table 1.

[0068] (Example 13) In Example 13, the concentration of lithium polysulfide in the electrolyte was set to 2.0 mol / dm³. 3A lithium-sulfur battery was fabricated in the same manner as in Example 7, except for the change made to [specific component]. The composition and physical properties of Example 13 are shown in Table 1.

[0069] (Example 14) In Example 14, a lithium-sulfur battery was prepared in the same manner as in Example 7, except that the lithium salt in the electrolyte was changed to lithium nitrate (hereinafter referred to as LiNO3). The composition and physical properties of Example 14 are shown in Table 1.

[0070] (Example 15) In Example 15, a lithium-sulfur battery was prepared in the same manner as in Example 7, except that the lithium salt in the electrolyte was changed to lithium trifluoromethanesulfonate (LiTFS). The composition and physical properties of Example 15 are shown in Table 1.

[0071] (Example 16) In Example 16, a lithium-sulfur battery was prepared in the same manner as in Example 7, except that the lithium polysulfide contained in the electrolyte was changed to Li2S6. In Example 16, Li2S6 was synthesized and prepared by mixing sulfur and lithium sulfide in a molar ratio of 5:1.

[0072] (Example 17) In Example 17, a lithium-sulfur battery was prepared in the same manner as in Example 7, except that the lithium polysulfide contained in the electrolyte was changed to Li2S4. In Example 17, Li2S4 was synthesized and prepared by mixing sulfur and lithium sulfide in a molar ratio of 3:1. The composition and physical properties of Example 17 are shown in Table 1.

[0073] (Example 18) In Example 18, a lithium-sulfur battery was prepared in the same manner as in Example 7, except that the lithium polysulfide contained in the electrolyte was replaced with Li2S2. In Example 18, Li2S2 was synthesized and prepared by mixing sulfur and lithium sulfide in a molar ratio of 1:1. The composition and physical properties of Example 18 are shown in Table 1.

[0074] (Example 19) In Example 19, a lithium-sulfur battery was prepared in the same manner as in Example 4, except that lithium titanate and sulfur-modified polyacrylonitrile were not added (LTO / SPAN: none), and the composition of the sulfur cathode composite was set to S / porous carbon composite:AB:CNT:binder = 92:1:2:5 (weight percent). The composition and physical properties of Example 19 are shown in Table 1.

[0075] (Example 20) In Example 20, a lithium-sulfur battery was fabricated in the same manner as in Example 4, except that the porous carbon in the S / porous carbon composite was replaced with CNTs. The composition and physical properties of Example 20 are shown in Table 1.

[0076] (Example 21) In Example 21, a lithium-sulfur battery was prepared in the same manner as in Example 7, except that the lithium salt in the electrolyte was changed to lithium perchlorate (LiClO4). The composition and physical properties of Example 21 are shown in Table 1.

[0077] (Example 22) In Example 22, a lithium-sulfur battery was prepared in the same manner as in Example 7, except that the lithium salt in the electrolyte was changed to lithium hexafluoride phosphate (LiPF6). The composition and physical properties of Example 22 are shown in Table 1.

[0078] (Example 23) In Example 23, a lithium-sulfur battery was prepared in the same manner as in Example 7, except that the solvent in the electrolyte was changed to sulfolane (SL). The composition and physical properties of Example 23 are shown in Table 1.

[0079] (Example 24) In Example 24, a lithium-sulfur battery was prepared in the same manner as in Example 7, except that the solvent in the electrolyte was changed to tetraglyme (G4). The composition and physical properties of Example 24 are shown in Table 1.

[0080] (Comparative Example 1) In Comparative Example 1, the electrolyte solution contained lithium polysulfide at a concentration of 0.3 mol / dm³ 3A lithium-sulfur battery was prepared in the same manner as in Example 1, except that it did not contain LiFSI. The composition and physical properties of Comparative Example 1 are shown in Table 1.

[0081] (Comparative Example 2) In Comparative Example 2, a lithium-sulfur battery was prepared in the same manner as in Comparative Example 1, except that the lithium salt in the electrolyte was changed to lithium trifluoromethanesulfonate (LiTFS). The composition and physical properties of Comparative Example 2 are shown in Table 1.

[0082] (Comparative Example 3) In Comparative Example 3, the concentration of LiFSI in the electrolyte was 0.2 mol / dm³. 3 A lithium-sulfur battery was prepared in the same manner as in Example 1, except for the change made to [specific component]. The composition and physical properties of Comparative Example 3 are shown in Table 1.

[0083] [Precipitation morphology of lithium metal] In Example 4 and Comparative Example 1, the batteries were discharged at an ambient temperature of 60°C with a current of 1.1 mA (corresponding to 0.1 C) until the voltage reached 1.7 V, and then charged to 2.6 V with the same current. The lithium-sulfur batteries were then disassembled, the lithium metal negative electrode was removed, washed with DME, and dried overnight at 100°C under vacuum conditions. The surface of the thoroughly dried lithium metal was observed under an inert gas atmosphere using a scanning electron microscope (SEM; JSM-6490A, manufactured by JEOL Ltd.).

[0084] Figure 2 shows scanning electron microscope images of the surface of lithium metal extracted from lithium-sulfur batteries before charging and discharging, and from Example 4 and Comparative Example 1. As shown in Figure 2, Comparative Example 1 has a dendritic deposition morphology, which is highly likely to cause premature failure of the lithium metal anode, a decrease in Coulomb efficiency, and even short circuits. On the other hand, Example 4 has a rounded shape, which can suppress the above-mentioned problems. In addition, Example 4 has a relatively large particle size. A larger particle size reduces the specific surface area of ​​the lithium metal, and thus the contact area with the electrolyte decreases. As a result, Example 4 can suppress side reactions of the electrolyte and can further suppress the degradation of the lithium-sulfur battery.

[0085] [Cycle testing of lithium-sulfur batteries] Table 1 shows the capacity retention rates in the charge-discharge cycles for the examples and comparative examples, respectively. The batteries were discharged at an ambient temperature of 60°C with a current of 1.1 mA until they reached 1.7 V, and then charged to 2.6 V at the same current density. This charge-discharge cycle was repeated 30 times, and the initial discharge capacity normalized by sulfur weight, capacity retention rate, and Coulomb efficiency were compared. The capacity retention rate and Coulomb efficiency were calculated using the following formulas. Note that the Coulomb efficiency was averaged. Capacity retention rate (%) = Discharge capacity of each cycle / Discharge capacity of the first cycle × 100 Coulomb efficiency (%) = Discharge capacity over (n+1) cycles / Charge capacity over the nth cycle ×100

[0086] As shown in Table 1, the electrolyte contains lithium polysulfide as the first component and lithium bis(fluorosulfonyl)imide as the second component, and when the concentration of lithium bis(fluorosulfonyl)imide is A, it is 0.001 mol / dm³ 3 ≤A ≤ 0.15 mol / dm 3 Lithium-sulfur batteries that met the criteria showed high battery performance.

[0087] The results obtained in this embodiment will be discussed below. 1. Effect of lithium bis(fluorosulfonyl)imide concentration Comparing Examples 1-4, there was a tendency for Coulomb efficiency to improve with increasing concentration. However, at 0.15 mol / dm³ 3 When the concentration exceeded this (Comparative Example 3), the volume retention rate decreased. Therefore, the lithium bis(fluorosulfonyl)imide concentration A was 0.05 mol / dm³. 3 ≤A ≤ 0.15 mol / dm 3 It is preferable to use it within that range.

[0088] 2. The effect of the third component 2-1. The necessity of LiTFSI (third component) Comparing the discharge capacities of Example 5 and Example 6, Example 6, which included LiTFSI, showed a higher capacity. This is thought to be due to the improved lithium-ion conductivity of the electrolyte caused by the inclusion of LiTFSI. 2-2. Concentration of LiTFSI (third component) Comparing Example 4 with Examples 7-9, a tendency was observed for the discharge capacity to decrease as the concentration increased. This is due to an increase in viscosity (a decrease in lithium-ion conductivity). Therefore, the concentration of the third component should be 1.5 mol / dm³. 3 Preferably, it is 0.1 mol / dm³ 3 More than 1.0mol / dm 3 The following is even more preferable: 2-3. Types of Influences Comparing the volume retention rates of Examples 7, 14, 15, 21, and 22, Examples 7, 14, and 15 showed higher volume retention rates. Therefore, it is preferable to include at least one of lithium nitrate, lithium bis(trifluoromethanesulfonyl)imide, and lithium trifluoromethanesulfonate as the third component.

[0089] 3. Effect of lithium polysulfide concentration Comparing the discharge capacities of Examples 7, 10-13, a tendency was observed for the discharge capacity to decrease as the concentration of lithium polysulfide increased. Furthermore, the Coulomb efficiency also tended to decrease. A preferred concentration of lithium polysulfide B is 0.001 mol / dm³. 3 ≤B ≤ 2.0 mol / dm 3 Satisfying the following conditions, and more preferably 0.2 mol / dm 3 ≤B ≤ 0.6 mol / dm 3 The above-mentioned effect can be obtained by satisfying the following conditions. Note that the lithium polysulfide concentration B is 2.0 mol / dm³. 3 If the concentration becomes too high, the viscosity of the electrolyte increases (ionic conductivity decreases), which may lead to a decrease in volume.

[0090] 4. Influence of the type of lithium polysulfide Since Examples 7, 16, 17, and 18 showed similar characteristics, it can be concluded that the lithium polysulfide added beforehand can produce similar effects not only with Li2S8, but also with Li2S6, Li2S4, and Li2S2.

[0091] 5. Necessity of lithium titanate and sulfur-modified polyacrylonitrile as cathode additives Comparing the discharge capacities of Example 4 and Example 19, Example 4, which contained lithium titanate and sulfur-modified polyacrylonitrile, showed a higher discharge capacity. Therefore, it can be said that it is preferable to include lithium titanate and sulfur-modified polyacrylonitrile in the positive electrode.

[0092] 6. The necessity of porous carbon Comparing the discharge capacities of Example 4 and Example 20, Example 4, which used porous carbon as the carbon material of the composite, showed a higher discharge capacity, indicating that using porous carbon is preferable.

[0093] 7. Effect of solvent type Comparing the discharge capacities of Examples 7, 23, and 24, Example 4, which used DOL and DME, showed a higher discharge capacity, indicating that using DOL and DME is preferable.

[0094] 8. The necessity of lithium bis(fluorosulfonyl)imide Since Comparative Examples 1 and 2, which did not contain lithium bis(fluorosulfonyl)imide, experienced premature short circuits, it can be inferred that lithium metal dendrites formed due to the absence of lithium bis(fluorosulfonyl)imide. Therefore, it can be said that the inclusion of lithium bis(fluorosulfonyl)imide is necessary. [Explanation of Symbols]

[0095] 1. Rinse-sulfur battery 2 Positive electrode 3 negative electrode 4 Separators 21 Positive electrode current collector 22 Positive electrode composite layer 31 Negative electrode current collector 32 Negative electrode composite layer

Claims

1. It comprises a first component, a second component, and a solvent. The first component is lithium polysulfide, and the second component is lithium bis(fluorosulfonyl)imide. When the concentration of the lithium bis(fluorosulfonyl)imide is A, the concentration is 0.001 mol / dm³. ３ ≦A≦0.15mol / dm ３ Satisfying the conditions, When the concentration of the lithium polysulfide is B, the following conditions are met: 0.2 mol / dm³ ≤ B ≤ 2.0 mol / dm³ An electrolyte for lithium-sulfur batteries characterized by the following features.

2. The aforementioned concentration A is 0.05 mol / dm ３ ≦A≦0.15mol / dm ３ Satisfying The electrolyte for lithium sulfur batteries according to feature 1.

3. The aforementioned concentration B is 0.2 mol / dm ３ ≦B≦0.6mol / dm ３ Satisfying The electrolyte for lithium sulfur batteries according to feature 1.

4. The electrolyte for the lithium-sulfur battery, in addition to the first and second components, contains at least one lithium salt as a third component. The concentration of the lithium salt is 0 mol / dm ３ Larger 1.5 mol / dm ３ The following is: The electrolyte for lithium sulfur batteries according to feature 1.

5. The lithium salt comprises at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium nitrate, and lithium trifluoromethanesulfonate. The electrolyte for lithium sulfur batteries according to feature 4.

6. The solvents are 1,3-dioxolane and 1,2-dimethoxyethane. The electrolyte for lithium sulfur batteries according to feature 1.

7. A negative electrode containing lithium metal or a lithium metal alloy, A positive electrode having sulfur or a sulfur compound as the main component of the positive electrode active material, An electrolyte for a lithium sulfur battery according to any one of claims 1 to 6, A lithium-sulfur battery characterized by having the following features.

8. The positive electrode comprises sulfur-modified polyacrylonitrile and lithium titanate. The lithium sulfur battery according to feature 7.

9. The positive electrode contains a conductive additive. The lithium sulfur battery according to feature 7.

10. The positive electrode contains porous carbon, The lithium sulfur battery according to feature 9.

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