Positive electrode for lithium-sulfur battery, lithium-sulfur battery and charge / discharge method thereof

Abstract

To provide a cathode for a lithium sulfur battery, which can concurrently achieve an improvement in rate characteristics and an improvement in charge and discharge cycle characteristics.SOLUTION: According to one embodiment, a cathode for a lithium sulfur battery comprises a cathode current collector, and a sulfur composite material layer overlaid on a surface of the cathode current collector. The sulfur composite material layer includes sulfur and / or a sulfur compound, which are / is a main cathode active material, and a lithium-containing oxide and a nitrogen-containing organic compound. The nitrogen-containing organic compound is a nitrogen-containing hetero ring compound.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode for a lithium-sulfur battery, a lithium-sulfur battery, and a method for charging and discharging the same. [Background technology]

[0002] In recent years, the applications of non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have expanded significantly as power sources for portable devices such as mobile phones, power tools, electric vehicles, etc. As power sources for these devices, non-aqueous electrolyte secondary batteries are required to have even higher energy density, longer life, etc.

[0003] Currently, lithium-sulfur batteries are attracting attention as one of the next-generation batteries due to their high energy density. Such lithium-sulfur batteries are charged and discharged by transferring lithium ions between a positive electrode and a negative electrode. The positive electrode includes, for example, a sulfur composite layer containing a positive electrode active material made of sulfur and / or a sulfur compound, a conductive additive, and a binder, and a positive electrode current collector supporting the sulfur composite layer. The negative electrode includes, for example, lithium metal and a negative electrode current collector supporting the lithium metal. A separator is interposed between the positive electrode and the negative electrode. The electrode plate assembly including the positive electrode, the negative electrode, and the separator is housed in a battery container together with a nonaqueous electrolyte. The nonaqueous electrolyte is generally a nonaqueous electrolytic solution in which an electrolyte such as a lithium salt is dissolved in a nonaqueous solvent.

[0004] However, lithium-sulfur batteries currently lack sufficient charge-discharge cycle performance and suffer from premature endurance. One specific cause of this problem is a phenomenon known as the redox shuttle effect. In this phenomenon, repeated charge-discharge cycles cause lithium polysulfides (e.g., Li2S8, Li2S6), which are intermediate products of reactions at the positive electrode, to dissolve and diffuse into the electrolyte. During charging, the diffused lithium polysulfides undergo electrophoresis and are reduced on the negative electrode, then diffuse further and are oxidized on the positive electrode. This electrochemical reaction continues until the lithium polysulfides become Li2S, causing the battery to self-discharge. To improve the charge-discharge cycle performance of lithium-sulfur batteries, it is necessary to suppress the redox shuttle effect.

[0005] Lithium-sulfur batteries also suffer from the problem of insufficient discharge rate characteristics, particularly low discharge capacity when a high current is applied. One of the reasons for this is that positive electrode active materials made of sulfur and / or sulfur compounds have low ionic and electronic conductivity. To achieve high energy density, a large amount of positive electrode slurry containing the positive electrode active material must be applied to the positive electrode current collector. However, this increases the internal resistance of the sulfur mixture layer, further reducing the discharge capacity per weight of the positive electrode.

[0006] To solve these problems, various research efforts are underway to improve the battery characteristics of lithium-sulfur batteries. Non-Patent Document 1 investigates a method of encapsulating a sulfur / carbon composite material with a nitrogen-containing organic compound, specifically cyclic polyacrylonitrile. Non-Patent Document 2 investigates a sulfur composite layer containing both sulfur and graphene in which some of the carbon atoms are substituted with nitrogen atoms (nitrogen-doped graphene). The cyclic polyacrylonitrile and nitrogen-doped graphene have the role of adsorbing polysulfides and suppressing their elution. In the lithium-sulfur batteries disclosed in Non-Patent Documents 1 and 2, the manufacturing processes of encapsulating a composite material with a nitrogen-containing organic compound and the manufacturing processes of nitrogen-doped graphene are very complicated and expensive.

[0007] In Non-Patent Document 3, graphite / Li4Ti5O 12 The formation of a double layer has been investigated. This double layer traps polysulfides dissolved in the electrolyte and prevents their diffusion to the negative electrode surface. As a result, the redox shuttle effect is suppressed, improving charge-discharge cycle performance. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] Zhi Chang et al., New J. Chem., 2016, 40, p.7680-7686 [Non-patent document 2] Huadong Yuan et al., Energy Storage Materials. 2018, 10, p.1-9 [Non-patent document 3] Jun Ming et al., ACS Nano, 2016, Vol. 10, p. 6037-6044 Summary of the Invention [Problem to be solved by the invention]

[0009] However, in Non-Patent Documents 1 and 2, there are problems in that the ionic conductivity in the sulfur composite layer is insufficient, the internal resistance of the battery is high, and the discharge capacity is reduced. In addition, in the lithium-sulfur battery disclosed in Non-Patent Document 3, there is a problem in that the supply of lithium ions from the non-aqueous electrolyte is hindered by the presence of a double layer, which reduces the ionic conductivity in the sulfur composite layer and increases the internal resistance of the battery.

[0010] For this reason, in these batteries, there has been a demand for reducing the internal resistance of the sulfur mixture layer to suppress a decrease in discharge capacity, and for further improvement in charge-discharge cycle characteristics.

[0011] The present invention solves the above-mentioned problems and provides a positive electrode for a lithium-sulfur battery, a lithium-sulfur battery, and a method for charging and discharging the same, which can simultaneously achieve improved discharge rate characteristics and improved charge-discharge cycle characteristics. [Means for solving the problem]

[0012] In order to solve the above-mentioned problems, according to one embodiment, there is provided a positive electrode for a lithium-sulfur battery, comprising: a positive electrode current collector; and a sulfur composite layer laminated on a surface of the positive electrode current collector, wherein the sulfur composite layer contains sulfur and / or a sulfur compound as a main positive electrode active material, a lithium-containing oxide, and a nitrogen-containing organic compound, and the nitrogen-containing organic compound is a nitrogen-containing heterocyclic compound.

[0013] In order to solve the above problems, according to one embodiment, there is provided a lithium-sulfur battery comprising the above-described positive electrode for a lithium-sulfur battery, a negative electrode capable of absorbing and desorbing lithium ions, and a separator impregnated with a non-aqueous electrolyte solution. In order to solve the above problem, according to one embodiment, the lower limit of the discharge voltage is set to 1.0 V (vs. Li / Li + ) or more 1.5V (vs.Li / Li + ) A method for charging and discharging a lithium-sulfur battery is provided, characterized in that the battery is discharged as follows: [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a positive electrode for a lithium-sulfur battery, a lithium-sulfur battery, and a method for charging and discharging the same, which are capable of simultaneously achieving improved discharge rate characteristics and improved charge-discharge cycle characteristics. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view showing an example of a lithium-sulfur battery according to a first embodiment. [Figure 2] FIG. 4 is a cross-sectional view showing an example of a lithium-sulfur battery according to a second embodiment. [Figure 3] FIG. 10 is a diagram showing charge / discharge curves of Example 20. DETAILED DESCRIPTION OF THE INVENTION

[0016] The positive electrode for a lithium-sulfur battery according to the embodiment will be described in detail below. <Positive electrode> The positive electrode includes a positive electrode current collector and a sulfur composite layer laminated on the surface of the positive electrode current collector. The sulfur composite layer contains sulfur and / or a sulfur compound as a main positive electrode active material, a lithium-containing oxide, and a nitrogen-containing organic compound. The nitrogen-containing organic compound is a nitrogen-containing heterocyclic compound. According to the positive electrode for a lithium-sulfur battery according to this embodiment, it is possible to simultaneously achieve an improvement in discharge rate characteristics and an improvement in charge-discharge cycle characteristics.

[0017] The sulfur mixture layer contains both a lithium-containing oxide and a nitrogen-containing organic compound, and the nitrogen-containing organic compound is a nitrogen-containing heterocyclic compound. This allows lithium polysulfides, which are reaction intermediates on the positive electrode side generated by repeated charge and discharge, to be efficiently captured within the sulfur mixture layer, suppressing the redox shuttle effect and improving the charge and discharge cycle characteristics of the battery. Although both the lithium-containing oxide and the nitrogen-containing organic compound have a high ability to adsorb lithium polysulfides, the inclusion of only one of them does not provide a sufficient effect. The present inventors have found that by including both of these in the sulfur mixture layer, the above-mentioned effects can be significantly achieved due to their synergistic effect.

[0018] In particular, the sulfur mixture layer contains a lithium-containing oxide, which improves ionic conductivity within the sulfur mixture layer and reduces the resistance (charge transfer resistance) at the interface between the positive electrode and the electrolyte, thereby lowering the internal resistance, thereby reducing the internal resistance of a battery incorporating the positive electrode and improving the discharge capacity, particularly when a high current is applied. Furthermore, by including a nitrogen-containing organic compound, the wettability of the sulfur mixture layer with the electrolyte is improved, and the ionic conductivity within the sulfur mixture layer is improved.

[0019] In some embodiments, the positive electrode comprises a positive electrode current collector and a sulfur composite layer formed on one or both surfaces of the positive electrode current collector.

[0020] The positive electrode current collector is not particularly limited, and known or commercially available ones can be used. Examples of the positive electrode current collector include rolled foil, electrolytic foil, metal mesh, porous metal, expanded grid, and punched metal made of aluminum, nickel, copper, or an alloy thereof, or stainless steel. Carbon-coated aluminum foil is preferably used as the positive electrode current collector to improve electrical conductivity between the sulfur mixture layer and adhesion between the sulfur mixture layer and the positive electrode current collector.

[0021] As described above, the sulfur mixture layer contains sulfur and / or a sulfur compound, a lithium-containing oxide, and a nitrogen-containing organic compound, and may further contain a binder and / or a conductive additive.

[0022] Sulfur and / or sulfur compounds are the main positive electrode active material capable of absorbing and releasing lithium ions. Examples of sulfur and / or sulfur compounds include crystalline sulfur, granular sulfur, colloidal sulfur, lithium sulfide, and lithium polysulfide (LiS). x , x=4-8), organic sulfur compounds or carbon-sulfur polymers ((CS x ) n , x=2.5 to 50, n≧2). The sulfur and / or sulfur compound may be a single substance or a mixture of two or more thereof, and the combination and ratio thereof can be selected arbitrarily depending on the purpose. The sulfur and / or sulfur compound is preferably granular sulfur, more preferably colloidal sulfur.

[0023] The conductive additive is not particularly limited as long as it is a material that can improve the electronic conductivity of the sulfur composite layer, and known conductive additives can be used. Examples of the conductive additive include carbon black such as ketjen black and acetylene black, carbon nanotubes, graphene, porous carbon, artificial graphite, natural graphite, activated carbon, and other carbon-based conductive additives. By including a conductive additive, it is possible to improve high-rate characteristics and charge / discharge cycle characteristics and reduce polarization. As the conductive additive, a material having a specific surface area of ​​500 to 2500 m is preferred because of its high effects. 2 g -1 The conductive additive may be one of these alone or a mixture of two or more thereof, and the combination and ratio thereof can be selected arbitrarily depending on the purpose.

[0024] It is preferable to combine part or all of the sulfur and / or sulfur compounds with part or all of a carbon-based conductive additive such as carbon black to form a sulfur-carbon composite material. Methods for preparing the sulfur-carbon composite material include melt impregnation, electrolytic deposition, vapor deposition, immersion, and mechanical mixing, with melt impregnation being preferred. The electronic conductivity of the sulfur and / or sulfur compounds can be improved by preparing the sulfur-carbon composite material. The sulfur-carbon composite material may be, for example, an S / KB composite material prepared by mixing sulfur (S) and Ketjen black (KB) in a weight ratio of 70:30 and heat-treating the mixture for 12 hours. The sulfur and / or sulfur compounds may also be used in combination with the sulfur-carbon composite material. Furthermore, the carbon-based conductive additive to be combined may be the same as the conductive additive.

[0025] The binder is not particularly limited as long as it is a material that binds the materials contained in the sulfur mixture layer together, and known or commercially available binders can be used. Examples of binders include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyacrylic acid (PAA), lithium polyacrylate (PAALi), polytetrafluoroethylene (PTFE), polyvinylpyrrolidone (PVP), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), ethylene-propylene copolymer, styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyethylene glycol (PEG), carboxymethyl cellulose (CMC), polyacrylonitrile (PAN), polyimide (PI), and acrylic resin. The binder may be a single material or a mixture of multiple materials, and the combination and ratio can be selected as desired depending on the purpose. The binder is preferably PVDF, SBR, or CMC.

[0026] The lithium-containing oxide adsorbs lithium polysulfide and supplies lithium ions into the sulfur mixture layer, thereby improving the ionic conductivity in the sulfur mixture layer and reducing the resistance (charge transfer resistance) at the interface between the positive electrode and the electrolyte. Known lithium-containing oxides can be used, for example, Li4Ti5O 12 , LiTiO4, Li2Ti3O7, Li 0.35 La 0.55 TiO 3、 Li 1+x+y Al x (Ti,Ge) 2-x Si y P 3-y O 12 (0≦x≦2, 0≦y≦3) and other lithium-titanium composite oxides, LiCoO2, LiMn2O4, LiNiO2, LiNiCoMnO2, LiNi 0.5 Mn 0.5 O2, LiMPO4 (M = transition metals such as Mn, Fe, Co, Ni), LiNi 0.8 Co 0.15 Al 0.05 O2, LiMn x Fe 1-x PO4(0 <x<1)、LiNi 1-x-y Co x Mn y O2(0 <x,y,z<1)、Li7La3Zr20 12 The lithium-containing oxide may be one of these or a mixture of two or more of these, and the combination and ratio thereof can be selected arbitrarily depending on the purpose.

[0027] The lithium-containing oxide is preferably a lithium-titanium composite oxide, more preferably Li4Ti5O, because it has a high adsorption property for lithium polysulfide, can be expected to be highly effective in capturing lithium polysulfide in the sulfur composite layer, and has high lithium ion conductivity. 12 is.

[0028] The lithium-titanium-based composite oxide in the sulfur mixture layer can efficiently capture lithium polysulfides, which are reaction intermediates on the positive electrode side, within itself through repeated charge and discharge, thereby suppressing the redox shuttle effect and improving the charge and discharge cycle characteristics of the battery. By using a lithium-titanium-based composite oxide as the lithium-containing oxide, the charge and discharge cycle characteristics of a battery incorporating the positive electrode can be further improved.

[0029] The lithium titanium composite oxide preferably has a voltage of 1.0 V (vs. Li / Li + ) greater than 3.0V (vs. Li / Li + ) is a compound that charges and discharges within a potential range less than 1000 kJ / cm. Such compounds include lithium titanium composite oxides, specifically Li4Ti5O 12 , LiTiO4, Li2Ti3O7, etc. In the case of such compounds, the lithium-containing oxide can function as a secondary cathode active material capable of absorbing and releasing lithium. This is because the sulfur and / or sulfur compound, which is the primary cathode active material, is also charged and discharged within this potential range. This increases the discharge capacity of the battery and improves the weight energy density of the battery. The potential range for charging and discharging is preferably greater than 1.2 V and less than 2.8 V, more preferably greater than 1.3 V and less than 2.7 V, more preferably greater than 1.4 V and less than 2.6 V, and even more preferably greater than 1.5 V and less than 2.5 V.

[0030] The total content of the lithium-containing oxide and the nitrogen-containing organic compound is preferably 1% by weight or more and 20% by weight or less relative to the weight of the sulfur composite layer. If the total amount is less than 1% by weight, the adsorption effect of lithium polysulfide may not be obtained, and the charge-discharge cycle characteristics of the battery may be reduced. On the other hand, if the total amount exceeds 20% by weight, the content of the positive electrode active material in the sulfur composite layer may be relatively reduced, and the weight energy density of the battery may be reduced. The total content of the lithium-containing oxide and the nitrogen-containing organic compound is more preferably 4 to 20% by weight, more preferably 5 to 15% by weight, and even more preferably 6 to 12% by weight.

[0031] The content of the lithium-containing oxide is preferably greater than 0 wt % and less than 20 wt % relative to the weight of the sulfur mixture layer. When the sulfur mixture layer contains the lithium-containing oxide in the above content, the high ionic conductivity of the lithium-containing oxide reduces the internal resistance of the sulfur mixture layer, thereby improving the high-rate characteristics in particular. This is because the lithium-containing oxide conducts lithium ions within the sulfur mixture layer during repeated charge and discharge. This reduces the internal resistance of a battery incorporating the positive electrode, thereby improving the high-rate characteristics in particular.

[0032] The content of the lithium-containing oxide is more preferably 5% by weight or more and 10% by weight or less with respect to the weight of the sulfur mixture layer.

[0033] The nitrogen-containing organic compound is a nitrogen-containing heterocyclic compound, a compound containing a nitrogen-containing heterocyclic skeleton. The nitrogen-containing organic compound adsorbs lithium polysulfides therein and suppresses the redox shuttle effect. Furthermore, the wettability of the electrolyte to the sulfur composite layer is improved, thereby improving ionic conductivity within the sulfur composite layer. This improves the discharge rate characteristics and cycle characteristics of a lithium-sulfur battery equipped with the positive electrode.

[0034] The nitrogen-containing heterocyclic skeleton may be, for example, a monocyclic or polycyclic skeleton containing a 4- to 6-membered aromatic heterocyclic skeleton or an aliphatic heterocyclic skeleton. Examples of the nitrogen-containing heterocyclic skeleton include skeletons made of pyridine, pyrrole, pyrimidine, carbazole, benzimidazole, and derivatives thereof.

[0035] Examples of nitrogen-containing heterocyclic compounds include 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, and tetraphenylporphyrin and its derivatives. The structural formulas of these compounds are shown below. n is, for example, a natural number from 3 to 15.

[0036] [ka]

[0037] The nitrogen-containing heterocyclic compound is preferably a material that is not easily dissolved in a non-aqueous electrolyte. The nitrogen-containing heterocyclic compound is preferably a polymer containing a nitrogen-containing heterocyclic skeleton in the repeating structural unit, and examples thereof include the compounds listed above. The nitrogen-containing heterocyclic compound may be, for example, a polymer of a monomer containing a nitrogen-containing heterocyclic skeleton. The nitrogen-containing heterocyclic compound may be a single substance or a mixture of multiple substances, and the combination and ratio thereof can be selected arbitrarily depending on the purpose.

[0038] The nitrogen-containing heterocyclic compound is preferably a compound containing a pyridine ring skeleton, such as cyclic polyacrylonitrile and its derivatives, and poly(N-vinylpyridine) and its derivatives. Compounds containing a pyridine ring skeleton have a high property of adsorbing lithium polysulfides, and are expected to be highly effective in capturing them in the sulfur composite layer. The nitrogen-containing heterocyclic compound is more preferably sulfur-modified polyacrylonitrile. The pyridine ring skeleton can be identified by, for example, X-ray photoelectron spectroscopy.

[0039] Sulfur-modified polyacrylonitrile is a type of derivative of cyclic polyacrylonitrile, and is a cyclic polyacrylonitrile modified with sulfur. Sulfur-modified polyacrylonitrile can be obtained by, for example, mixing sulfur powder with polyacrylonitrile powder and heating in a non-oxidizing atmosphere while preventing sulfur leakage, thereby causing the polyacrylonitrile to undergo a ring-closing reaction and simultaneously reacting sulfur vapor with the polyacrylonitrile. When sulfur-modified polyacrylonitrile is used as a nitrogen-containing heterocyclic compound, the positive electrode active material contains other sulfur or a sulfur compound. The estimated structural formula of sulfur-modified polyacrylonitrile is shown below. Sulfur-modified polyacrylonitrile can be a compound containing the following structure in part or in whole. n is, for example, a natural number of 3 to 15.

[0040] [ka]

[0041] The nitrogen-containing organic compound preferably has a voltage of 1.0 V (vs. Li / Li + ) greater than 3.0V (vs. Li / Li + ) is a compound that is charged and discharged within a potential range less than 1.2V. Examples of such compounds include sulfur-modified polyacrylonitrile and its derivatives. In such compounds, the nitrogen-containing organic compound can function as a secondary positive electrode active material capable of absorbing and releasing lithium. This is because the sulfur and / or sulfur compound that is the primary positive electrode active material is also charged and discharged within this potential range. This increases the discharge capacity of the battery and improves the energy density of the battery. The potential range for charging and discharging is more preferably greater than 1.2V and less than 2.8V, more preferably greater than 1.3V and less than 2.7V, more preferably greater than 1.4V and less than 2.6V, and even more preferably greater than 1.5V and less than 2.5V.

[0042] In the sulfur mixture layer, when the content of the lithium-containing oxide is A and the content of the nitrogen-containing organic compound is B, it is preferable that A≧B. When the sulfur mixture layer contains the lithium-containing oxide in the above content, the high ionic conductivity of the lithium-containing oxide reduces the internal resistance of the sulfur mixture layer, reduces polarization, and improves particularly the high-rate characteristics. Note that the content is calculated in weight terms. The ratio (A:B) of the content of the lithium-containing oxide to the content of the nitrogen-containing organic compound is preferably 5:5 to 9:1, more preferably 6:4 to 9:1, more preferably 7:3 to 9:1, and even more preferably 8:2 to 9:1.

[0043] The content of sulfur and / or sulfur compounds is preferably 50 wt % or more, more preferably 55 to 90 wt %, and even more preferably 55 to 65 wt %, based on the weight of the sulfur mixture layer. If the content of sulfur and / or sulfur compounds is less than 50 wt %, the content of the positive electrode active material in the sulfur mixture layer is low, which is undesirable because it may reduce the weight energy density of the battery.

[0044] The sulfur mixture layer preferably contains 60 to 90 wt% of sulfur, sulfur compounds, and sulfur-carbon composite materials in total, 0 to 25 wt% of conductive additive, 3 to 5 wt% of binder, and 1 to 20 wt% of lithium-containing oxide and nitrogen-containing organic compound in total. The conductive additive is more preferably 4 to 25 wt%. Note that the sulfur mixture layer may contain one or more of sulfur, sulfur compounds, and sulfur-carbon composite materials.

[0045] The sulfur mixture layer preferably contains nitrogen-containing compound particles dispersed within the sulfur mixture layer as particles separate from the positive electrode active material particles. More preferably, the sulfur mixture layer has a structure in which particles of a sulfur-carbon composite material obtained by combining a positive electrode active material with a carbon-based conductive additive, lithium-containing oxide particles, and nitrogen-containing compound particles are dispersed within the sulfur mixture layer as separate particles and bonded to each other. In such a positive electrode, the sulfur mixture layer can be produced by a simple process of dispersing these particles in a solvent to prepare a sulfur mixture layer slurry, applying the sulfur mixture layer slurry to the positive electrode current collector, and then drying. Such a positive electrode can simultaneously achieve improved high-rate characteristics and charge-discharge cycle characteristics.

[0046] The thickness of the sulfur mixture layer may be a general thickness, for example, 20 μm to 300 μm in terms of the thickness of an electrode taken out from a battery with an SOC of 100%.

[0047] In some embodiments, the positive electrode further includes a ceramic layer laminated on a surface of the sulfur mixture layer, in addition to the positive electrode current collector and the sulfur mixture layer. By laminating the ceramic layer on the surface of the sulfur composite layer, lithium polysulfides, which are reaction intermediates on the positive electrode side generated by repeated charge and discharge, can be efficiently captured inside the ceramic layer, suppressing the redox shuttle effect and improving the charge and discharge cycle characteristics of the battery. The presence of another layer, such as a graphite layer, between the sulfur composite layer and the ceramic layer is undesirable because it may hinder the supply of lithium ions from the nonaqueous electrolyte, reducing ionic conductivity in the sulfur composite layer and increasing the internal resistance of the battery.

[0048] The ceramic layer contains ceramic powder as a main component, and may also contain a binder and a conductive additive. The ceramic powder is a compound capable of trapping lithium polysulfide within itself. Examples of ceramic powder include TiO2, TiN, and Li4Ti5O. 12 , TiNb2O7, LiTiO4, Li2Ti3O7, Li 1+x+y Alx (Ti,Ge) 2-x SiyP 3-y O 12 (0≦x≦2, 0≦y≦3), La 0.57 Li 0.29 TiO3 and Li 1+x+y Al x Ti 2-x Si y P 3-y O 12 (0≦x≦1, 0≦y≦1) and other titanium oxides, titanium nitrides, or titanium-based composite oxides, Al2O3, LiCoO2, LiMn2O4, LiNiO2, LiNiCoMnO2, LiNi 0.5 Mn 0.5 O2, LiMPO4 (M = transition metals such as Mn, Fe, Co, Ni), LiNi 0.8 Co 0.15 Al 0.05 O2, Li 0.35 La 0.55 TiO3, LiMn x Fe 1-x PO4(0 <x<1)、LiNi 1-x-y Co x Mn y O2(0 <x,y,z<1)、Li7La3Zr2O 12 etc.

[0049] The ceramic powder is preferably 1.0 V (vs. Li / Li + ) greater than 3.0V (vs. Li / Li + ) and consists of compounds that charge and discharge within a potential range less than By using a compound that charges and discharges in the above potential range in the ceramic powder, the ceramic powder can function as a secondary positive electrode active material that can absorb and release lithium. Sulfur and / or sulfur compounds, which are the main positive electrode active material of the positive electrode, generally have a potential of 2.4 to 1.8 V (vs. Li / Li + ), the ceramic powder can be charged and discharged at the same time. This significantly increases the discharge capacity of the battery and improves the energy density of the battery.

[0050] Because the ceramic powder is made of a compound that charges and discharges within the above potential range and the ceramic layer is laminated on the surface of the sulfur composite layer, sulfur compounds dissolved from the sulfur composite layer during discharge are captured on the ceramic powder and further reduced on the ceramic powder. Discharge can be continued until the sulfur compounds are also sufficiently reduced (discharged) during the reduction of the ceramic powder, improving the sulfur utilization efficiency and improving the discharge capacity of a battery incorporating the positive electrode.

[0051] Examples of compounds that charge and discharge in the above potential range include TiO2, Li4Ti5O 12、 TiNb2O7, LiTiO4, Li2Ti3O7, La 0.57 Li 0.29 TiO3 and Li 1+x+y Al x Ti 2-x Si y P 3-y O 12 (0≦x≦1, 0≦y≦1), and preferably Li4Ti5O 12 is.

[0052] The ceramic powder is preferably 1.8V (vs. Li / Li + ) consists of compounds that undergo reduction reactions at a lower potential than Li4Ti5O 12 is about 1.55V (vs. Li / Li + ) is a compound that discharges at a potential of

[0053] The ceramic layer preferably contains a binder to improve the binding between the ceramic powders. The binder is not particularly limited as long as it is a material that binds the materials contained in the ceramic layer together, and known binders can be used. For example, the same binders as those listed as binders for the sulfur composite layer can be used.

[0054] The ceramic layer preferably contains a conductive additive to improve electronic conductivity. The conductive additive is not particularly limited as long as it is a material that can improve the electronic conductivity of the ceramic layer, and known conductive additives can be used. For example, the same conductive additives as those exemplified as the conductive additives for the sulfur composite layer can be used.

[0055] The content of the ceramic powder is preferably 80% by weight or more and 98% by weight or less, and more preferably 85% by weight or more and 95% by weight or less, based on the weight of the ceramic layer.

[0056] By including the ceramic powder in the ceramic layer at the above content, lithium polysulfide, a reaction intermediate on the positive electrode side that is generated by repeated charge and discharge, can be efficiently captured in the ceramic layer. This suppresses the redox shuttle effect and improves the charge and discharge characteristics of the battery. If the ceramic powder content is less than 80%, the ionic and electronic conductivity decreases and there is a risk that lithium polysulfide cannot be captured inside the ceramic layer, which is undesirable.

[0057] The ceramic layer preferably contains 80 to 98 wt % of ceramic powder, 0 to 10 wt % of a conductive additive, and 1 to 10 wt % of a binder.

[0058] In some embodiments, the ceramic layer further includes a nitrogen-containing heterocyclic compound. The nitrogen-containing heterocyclic compound efficiently adsorbs lithium polysulfides therein. Therefore, by including the nitrogen-containing heterocyclic compound together with the ceramic powder, the redox shuttle effect can be more efficiently suppressed, thereby improving the charge / discharge characteristics of the battery. As the nitrogen-containing heterocyclic compound, for example, the same materials as those listed as materials for the sulfur composite layer can be used. The ceramic layer contains, for example, a titanium-based composite oxide, which is a ceramic powder, and a nitrogen-containing heterocyclic compound in a content ratio of 5:5.

[0059] The thickness of the ceramic layer may be a general thickness, for example, 1 μm to 50 μm in terms of the thickness of an electrode taken out of a battery with an SOC of 100%.

[0060] The positive electrode can be fabricated, for example, by the following method. First, sulfur and / or a sulfur compound, which are the main positive electrode active material described above, a lithium-containing oxide, a nitrogen-containing organic compound, a conductive additive, and a binder are dispersed in a solvent to prepare a sulfur composite layer slurry. Next, the sulfur composite layer slurry is applied to one or both surfaces of a positive electrode current collector and then dried to obtain a sulfur composite layer. This allows the fabrication of a positive electrode comprising a positive electrode current collector and a sulfur composite layer. Examples of solvents used to prepare the sulfur composite layer slurry and the ceramic layer slurry include N-methyl-2-pyrrolidone (NMP) and water. A positive electrode further including a ceramic layer can be produced, for example, as follows: The ceramic material, conductive additive, and binder are dispersed in a solvent to prepare a slurry for the ceramic layer. Next, the slurry for the ceramic layer is applied to the dried sulfur composite layer, and then dried to form the ceramic layer.

[0061] <Lithium-sulfur battery> In some embodiments, a lithium-sulfur battery includes the lithium-sulfur battery positive electrode described above, a negative electrode capable of absorbing and releasing lithium ions, and a separator impregnated with a non-aqueous electrolyte.

[0062] <Negative electrode> In some embodiments, the negative electrode includes a negative electrode active material and may also include a negative electrode current collector.

[0063] The negative electrode current collector is not particularly limited, and known materials can be used. For example, rolled foil, electrolytic foil, etc. made of copper or a copper alloy can be used. Specifically, the material 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 alloys such as aluminum-cadmium alloys can be used. Other examples of the negative electrode current collector include sintered carbon, non-conductive polymers surface-treated with a conductive material, and conductive polymers.

[0064] The negative electrode active material is not particularly limited as long as it is a material capable of absorbing / desorbing lithium ions, and known materials can be used. For example, the negative electrode active material can be selected from metallic materials such as metallic lithium, metallic sodium, lithium-aluminum alloy, lithium-tin alloy, lithium-silicon alloy, sodium-silicon alloy, lithium-antimony alloy, and sodium-antimony alloy; carbon materials such as natural graphite, artificial graphite, carbon black, acetylene black, graphite, activated carbon, carbon fiber, coke, soft carbon, and hard carbon; and oxide materials such as lithium titanate. One or more of the negative electrode active materials can be used. When two or more materials are used, the combination and ratio thereof can be selected as desired depending on the purpose. The negative electrode active material is preferably metallic lithium or a lithium alloy.

[0065] The negative electrode may further contain, together with the negative electrode active material, a conductive additive for facilitating smooth electron transfer within the negative electrode.

[0066] Examples of the conductive additive include carbon-based materials such as carbon black, acetylene black, ketjen black, carbon nanotubes (CNTs), graphene, and reduced graphene oxide, as well as conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole. The conductive additive is preferably contained in an amount of 0 to 20% by mass relative to the total weight of the negative electrode active material layer. Furthermore, if the content of the conductive additive exceeds 20% by mass, the content of the negative electrode active material becomes relatively small, which may result in a decrease in the capacity characteristics of the battery.

[0067] The negative electrode may further include a binder that can serve to make the negative electrode active material into a paste, improve the adhesive strength between the active materials or between the active material and the negative electrode current collector, buffer the expansion and contraction of the active material, etc. Specifically, the same binder as that used in the sulfur mixture layer can be used as the binder.

[0068] In some embodiments, the negative electrode does not include a separate negative electrode current collector and consists solely of a rolled foil of lithium metal or a lithium alloy.

[0069] <Nonaqueous electrolyte> In some embodiments, the non-aqueous electrolyte solution includes an electrolyte comprising a lithium salt and a non-aqueous solvent.

[0070] The electrolyte may be, for example, one or a mixture of two or more selected from the group of lithium salts such as lithium hexafluorophosphate (LiPF), lithium bromide (LiBr), lithium perchlorate (LiClO), lithium bis(oxalatoborate) (LiB(CO)), lithium fluoroborate (LiBF), lithium nitrate (LiNO), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). When a mixture is used, the combination and ratio thereof can be selected as desired depending on the purpose.

[0071] In some embodiments, the non-aqueous solvent contains a cyclic ether and a chain ether as main components. Examples of the cyclic ether include 1,3-dioxolane (DOL). Examples of the chain ether include dimethoxyethane (DME). The non-aqueous solvent may be, for example, a 1:1 volumetric mixture of DOL and DME. When the non-aqueous solvent is a mixture of 1,3-dioxolane and dimethoxyethane, particularly good high-rate performance is obtained.

[0072] A mixture of 1,3-dioxolane and dimethoxyethane, in which lithium polysulfide easily dissolves, has posed a problem in the cycle characteristics of conventional lithium-sulfur batteries. However, this problem can be solved by using the electrode of the present invention, which prevents the elution of lithium polysulfide.

[0073] In some embodiments, the non-aqueous solvent comprises sulfolane (SL), dimethyl sulfoxide (DMSO), dimethyl sulfone, or the like as a major component.

[0074] In some embodiments, the non-aqueous solvent may be ethylene carbonate, ethyl methyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, sulfolane, oxolane, tetraglyme, triglyme, fluoroethylene carbonate, ionic liquids, and the like.Examples of ionic liquids include 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-methyl-3-propylimidazolium bis(trifluoromethanesulfonyl)imide, 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, and 1-ethyl-3-methylimidazolium methanesulfonyl. methylimidazolium methylsulfate, 1-butyl-3-methylimidazolium methanesulfonate, 1,2,3-trimethylimidazolium methylsulfate, methylimidazolium chloride, methylimidazolium hydrogensulfate, 1-ethyl-3-methylimidazolium hydrogensulfate, 1-butyl-3-methylimidazolium hydrogensulfate, 1-ethyl-3-methylimidazolium tetrachloroaluminate, 1-butyl-3-methylimidazolium tetrachloroaluminate, 1-ethyl-3-methylimidazolium acetone tartrate, 1-butyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium ethyl sulfate, 1-butyl-3-methylimidazolium methyl sulfate, 1-ethyl-3-methylimidazolium thiocyanate, 1-butyl-3-methylimidazolium thiocyanate, 1-ethyl-2,3-dimethylimidazolium ethyl sulfate, 1-butylpyridinium bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluoromethanesulfonyl)imide, 1-methyl-1 -propylpyrrolidinium bis(trifluoromethanesulfonyl)imide, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide, tributyldodecylphosphonium bis(trifluoromethanesulfonyl)imide, methyltributylammonium methylsulfate, butyltrimethylammonium bis(trifluoromethanesulfonyl)imide, trimethylhexylammonium bis(trifluoromethanesulfonyl)imide, and the like.The non-aqueous solvent may be one or a mixture of two or more solvents selected from the above group, and when a mixture is used, the combination and ratio thereof may be selected arbitrarily depending on the purpose.

[0075] In some embodiments, when the main electrolyte is different from lithium nitrate, the nonaqueous electrolyte preferably contains a small amount of lithium nitrate as a secondary electrolyte separate from the main electrolyte. This allows the lithium nitrate to be reductively decomposed on the negative electrode to form a coating derived from the lithium nitrate. This coating inhibits the reductive decomposition of lithium polysulfide, inhibits deterioration of the negative electrode, and improves charge-discharge cycle characteristics. The electrolyte may be, for example, LiTFSI at 1 mol dm -3 The non-aqueous solvent may contain lithium nitrate as a secondary electrolyte in an amount of 1% by weight relative to the weight of the non-aqueous solvent.

[0076] <separator> The separator 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 active material, the negative electrode active material, the non-aqueous electrolyte, etc. Examples of organic polymer separators include microporous membranes or nonwoven fabrics made of polyolefin resins such as polyethylene resin and polypropylene resin, nitrocellulose resin, polyimide resin, etc. Examples of inorganic separators include silica glass nonwoven fabric, etc. The separator may be subjected to treatments such as ceramic coating or structure control. One or more of these treatments may be performed, and the combination thereof can be selected as desired depending on the purpose. The microporous membrane or nonwoven fabric may have a single layer or a multilayer structure. It may also be in a sheet or other shape, such as a zigzag fold.

[0077] <Discharge conditions> In some embodiments, the lithium-sulfur battery has a discharge cutoff potential of 1.0 to 1.5 V (vs. Li / Li + Discharging with the discharge cutoff potential in the above potential range means that charging and discharging are performed with the above potential range as the lower limit of the discharge voltage.

[0078] When the discharge cutoff potential is within the above range, for example, the ceramic powder contained in the ceramic layer is Li4Ti5O 12 In this case, it is generally 1.55V (vs. Li / Li + ), it can function as a secondary positive electrode active material that absorbs lithium. In addition, sulfur and / or sulfur compounds, which are the main positive electrode active material of the positive electrode, generally have a voltage of 2.4 to 1.8 V (vs. Li / Li + ) potential range. This results in Li4Ti5O 12 Since the sulfur and / or sulfur compound, which are the main positive electrode active materials of the sulfur mixture layer, are discharged simultaneously, the discharge capacity of the battery can be increased.

[0079] Furthermore, Li4Ti5O 12 At the same time, the sulfur and / or sulfur compounds continue to be discharged, i.e., constant voltage discharge is performed, allowing the sulfur and / or sulfur compounds to react for a long time. This improves the utilization efficiency of sulfur as a positive electrode active material, allows discharge to be continued until the main positive electrode active material is fully discharged, and improves the discharge capacity of a battery incorporating the positive electrode.

[0080] The shape of the lithium-sulfur battery is not particularly limited, and examples thereof include coin type, button type, sheet type, laminated type, cylindrical type, prismatic type, and flat type.

[0081] Hereinafter, a lithium-sulfur battery according to an embodiment will be described with reference to the drawings. FIG. 1 is a cross-sectional view showing an example of a lithium-sulfur battery according to the first embodiment.

[0082] The lithium-sulfur battery 1 includes a positive electrode 2, a negative electrode 3, and a separator 4 disposed between the positive electrode 2 and the negative electrode 3. The positive electrode 2, the negative electrode 3, and the separator 4 are housed in an exterior body (not shown).

[0083] The positive electrode 2 is composed of a positive electrode current collector 21 and a sulfur composite layer 22 provided on the surface thereof facing the separator 4. The negative electrode 3 is composed of a negative electrode current collector 31 and a negative electrode layer 32 provided on the surface thereof facing the separator 4. The separator 4 is impregnated with, for example, a nonaqueous electrolyte solution.

[0084] FIG. 2 is a cross-sectional view showing an example of a lithium-sulfur battery according to the second embodiment. The lithium-sulfur battery according to the second embodiment differs from the first embodiment in that the positive electrode 2 further includes a ceramic layer 23 laminated on the surface of the sulfur mixture layer 22.

[0085] Although the embodiments of the present invention have been specifically described, the present invention is not limited to these embodiments and examples, and various modifications based on the technical concept of the present invention are possible.

[0086] The present invention will be described in more detail below with reference to examples and comparative examples.

[0087] [Examples and Comparative Examples] [Preparation of Positive Electrode of Example 1] Sulfur (S) and Ketjen Black (KB) were mixed in a weight ratio of 70:30. The resulting mixture was then heat-treated at 155°C for 12 hours in an inert gas atmosphere to allow the sulfur to penetrate into the pores of the Ketjen Black, producing a sulfur-carbon composite (S / KB composite).

[0088] Sulfur carbon composite material (S / KB composite material), conductive additives acetylene black (AB) and carbon nanotubes (CNT), binder carboxymethyl cellulose (CMC), and lithium-containing oxide Li4Ti5O 12 The cathode slurry was prepared by adding and dispersing sulfur-modified polyacrylonitrile, a nitrogen-containing organic compound, to ultrapure water, a solvent. A degassing mixer called "Awatori Rentaro" manufactured by Thinky Corporation was used for the mixing.

[0089] Next, the prepared positive electrode slurry was applied to a carbon-coated aluminum foil, which was a positive electrode current collector, using a coater and then dried to form a sulfur mixture layer. The positive electrode slurry was applied in a manner that the sulfur loading in the sulfur mixture layer was 3.5 mg cm -2 The drying was carried out in a vacuum at 60°C overnight. The composition (weight ratio) of the sulfur mixture layer was S / KB composite material: AB: CNT: binder: Li4Ti5O 12 The ratio of sulfur-modified polyacrylonitrile to sulfur-modified polyacrylonitrile was 85:1:2:5:3:4. The sulfur content was 60% by weight based on the weight of the sulfur mixture layer.

[0090] [Preparation of Positive Electrodes in Examples 2 to 5] The positive electrodes of Examples 2, 3, 4, and 5 were fabricated in the same manner as the positive electrode of Example 1, except that the nitrogen-containing organic compound in the sulfur composite layer was changed to nitrogen-containing heterocyclic compounds: cyclic polyacrylonitrile, poly(N-vinyl galvanonitrile), poly(N-vinyl pyridine), and poly(N-vinyl pyrrolidone), respectively.

[0091] [Preparation of Positive Electrodes in Examples 6 and 7] In the positive electrode of Example 6, the lithium-containing oxide in the sulfur mixture layer was Li7La3Zr2O 12 The positive electrode was produced in the same manner as in Example 1, except for the above change. The positive electrode of Example 7 was produced by the same method as that for the positive electrode of Example 1 above.

[0092] [Preparation of Positive Electrode of Comparative Example 1] The positive electrode of Comparative Example 1 was fabricated by the same method as the positive electrode of Example 1, except that the contents of the lithium-containing oxide and the nitrogen-containing organic compound in the sulfur mixture layer were each changed to 0 wt %. The composition (weight ratio) of the sulfur mixture layer was S / KB composite material:AB:CNT:binder=92:1:2:5. The sulfur content was 64 wt % relative to the weight of the sulfur mixture layer.

[0093] [Preparation of Positive Electrodes of Comparative Examples 2 and 3] The positive electrodes of Comparative Examples 2 and 3 were prepared by the same preparation method as the positive electrode of Example 1, except that the content of the lithium-containing oxide in the sulfur composite layer was changed to 7% by weight and 0% by weight, respectively, and the content of the nitrogen-containing organic compound was changed to 0% by weight and 7% by weight, respectively. The composition (weight ratio) of the sulfur composite layer is Li4Ti5O 12 : With respect to the nitrogen-containing organic compound, the ratio is 7:0 in Comparative Example 2 and 0:7 in Comparative Example 3.

[0094] [Preparation of Positive Electrodes of Comparative Examples 4 to 6] The positive electrodes of Comparative Examples 4, 5, and 6 were prepared in the same manner as the positive electrode of Example 1, except that the organic compounds in the sulfur composite layer were changed to polyvinyl alcohol, polyacrylonitrile, and polyethyleneimine, which were not nitrogen-containing heterocyclic compounds, respectively.

[0095] [Preparation of Positive Electrode of Comparative Example 7] The positive electrode of Comparative Example 7 was produced in the same manner as the positive electrode of Example 1, except that the oxide in the sulfur mixture layer was changed to Al2O3, a non-lithium-containing oxide.

[0096] [Preparation of Positive Electrode of Example 8] In the positive electrode of Example 8, the amount of sulfur supported in the sulfur mixture layer was 5.0 mg cm -2 The positive electrode was fabricated in the same manner as in Example 1, except that the coating of the positive electrode slurry was performed so that the positive electrode slurry was applied to the positive electrode. By increasing the amount of sulfur supported compared to Examples 1 to 7, it becomes difficult for a lithium-sulfur battery equipped with the positive electrode to obtain a sufficient discharge capacity, particularly when a high current is applied. Experiments were conducted under these conditions to obtain variations in results depending on the composition (weight ratio) of the lithium-containing oxide and the nitrogen-containing organic compound.

[0097] [Preparation of Positive Electrodes in Examples 9 to 12] The positive electrodes of Examples 9, 10, 11, and 12 were fabricated in the same manner as the positive electrode of Example 8, except that the content of the lithium-containing oxide in the sulfur composite layer was changed to 3.5 wt%, 4 wt%, 5 wt%, and 6 wt%, respectively, and the content of the nitrogen-containing organic compound was changed to 3.5 wt%, 3 wt%, 2 wt%, and 1 wt%, respectively. The composition (weight ratio) of the sulfur composite layer was Li4Ti5O 12 For nitrogen-containing organic compounds, the ratio is 3.5:3.5 in Example 9, 4:3 in Example 10, 5:2 in Example 11, and 6:1 in Example 12.

[0098] [Preparation of Positive Electrode of Example 13] In the positive electrode of Example 13, the lithium-containing oxide in the sulfur mixture layer was Li7La3Zr2O 12 The positive electrode was produced in the same manner as in Example 10, except for the above change.

[0099] [Preparation of Positive Electrodes of Examples 14 to 16] The positive electrodes of Examples 14, 15, and 16 were fabricated in the same manner as the positive electrode of Example 10, except that the nitrogen-containing organic compound in the sulfur composite layer was changed to cyclic polyacrylonitrile, poly(N-vinyl galvanonitrile), and poly(N-vinyl pyridine), respectively.

[0100] [Preparation of Positive Electrodes in Examples 17 to 22] In the positive electrodes of Examples 17 to 22, the content of the lithium-containing oxide in the sulfur mixture layer was changed to 0.2 wt%, 0.4 wt%, 1.3 wt%, 4.3 wt%, 6.4 wt%, and 8.6 wt%, respectively, and the content of the nitrogen-containing organic compound was changed to 0.3 wt%, 0.6 wt%, 1.7 wt%, 5.7 wt%, 8.6 wt%, and 11.4 wt%, respectively, and Li4Ti5O 12 The cathodes were fabricated in the same manner as in Example 1, except that the total content of the sulfur composite layer was changed to 0.5 wt%, 1 wt%, 3 wt%, 10 wt%, 15 wt%, and 20 wt%, respectively. The composition (weight ratio) of the sulfur composite layer was Li4Ti5O 12For nitrogen-containing organic compounds, the ratio is 0.2:0.3 in Example 17, 0.4:0.6 in Example 18, 1.3:1.7 in Example 19, 4.3:5.7 in Example 20, 6.4:8.6 in Example 21, and 8.6:11.4 in Example 22.

[0101] [Preparation of Positive Electrode of Example 23] The positive electrode of Example 23 differs from the positive electrode of Example 8 in that it further includes a ceramic layer laminated on the surface of the sulfur composite layer. The positive electrode of Example 23 was produced by the same production method as the positive electrode of Example 1, except for the following points. The positive electrode slurry was applied so that the amount of sulfur in the sulfur mixture layer was 5.0 mg cm -2 The positive electrode slurry was dried in vacuum at 60° C. for 1 hour after application.

[0102] Next, we used the ceramic powder Li4Ti5O 12 The powder, AB as a conductive additive, and CMC as a binder were added to ultrapure water as a solvent, mixed, and dispersed to prepare a slurry for the ceramic layer. Next, the prepared slurry for the ceramic layer was applied to the dried sulfur composite layer using a coater and dried to form a ceramic layer. The drying was carried out in a vacuum at 60°C overnight. The composition (weight ratio) of the ceramic layer was Li4Ti5O 12 The ratio of AB to binder was 90:5:5. Next, the mixture was pressed and punched into a shape. Through the above steps, a positive electrode of Example 23 including a sulfur mixture layer and a ceramic layer laminated on the surface of the sulfur mixture layer was produced.

[0103] [Preparation of Positive Electrodes in Examples 24 to 26] The positive electrodes of Examples 24, 25, and 26 were prepared by the same preparation method as the positive electrode of Example 23, except that the content of the lithium-containing oxide in the sulfur composite layer was changed to 3.5 wt %, 4 wt %, and 6 wt %, respectively, and the content of the nitrogen-containing organic compound was changed to 3.5 wt %, 3 wt %, and 1 wt %, respectively. The composition (weight ratio) of the sulfur composite layer is Li4Ti5O 12: For nitrogen-containing organic compounds, the ratio is 3.5:3.5 in Example 24, 4:3 in Example 25, and 6:1 in Example 26.

[0104] [Preparation of Positive Electrodes of Examples 27 to 29] The positive electrodes of Examples 27, 28, and 29 were prepared by the same method as the positive electrode of Example 26, except that the content of the ceramic material in the ceramic layer was changed to 45% by weight and the content of the nitrogen-containing heterocyclic compound was changed to 45% by weight. As the nitrogen-containing heterocyclic compound in the ceramic layer, sulfur-modified polyacrylonitrile was used in Example 27, poly(N-vinylcarbazole) in Example 28, and poly(N-vinylpyridine) in Example 29 were used. The composition (weight ratio) of the ceramic layer is Li4Ti5O 12 The ratio of the nitrogen-containing heterocyclic compound:AB:binder was 45:45:5:5.

[0105] [Preparation of Positive Electrode of Comparative Example 8] The positive electrode of Comparative Example 8 was produced in the same manner as the positive electrode of Example 23, except that the contents of the lithium-containing oxide and the nitrogen-containing organic compound in the sulfur mixture layer were each changed to 0 wt %. The composition (weight ratio) of the sulfur mixture layer was S / KB composite material:AB:CNT:binder=92:1:2:5. The sulfur content was 64 wt % with respect to the weight of the sulfur mixture layer.

[0106] [Preparation of Positive Electrodes of Comparative Examples 9 and 10] The positive electrodes of Comparative Examples 9 and 10 were prepared by the same preparation method as the positive electrode of Example 23, except that the content of the lithium-containing oxide in the sulfur composite layer was changed to 7 wt % and 0 wt %, respectively, and the content of the nitrogen-containing organic compound was changed to 0 wt % and 7 wt %, respectively. The composition (weight ratio) of the sulfur composite layer is Li4Ti5O 12 The ratio of nitrogen-containing organic compounds was 7:0 in Comparative Example 9 and 0:7 in Comparative Example 10.

[0107] [Preparation of non-aqueous electrolyte] The non-aqueous electrolytes other than those of Example 7 contained a main electrolyte of lithium bis(trifluoromethane)sulfonimide (LiTFSI) at 1 mol / dm 3 The secondary electrolyte, LiNO3, was mixed at 1% by weight relative to the weight of the non-aqueous solvent. The non-aqueous solvent used was a 1:1 volume mixture of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME). The nonaqueous electrolyte of Example 7 was prepared in the same manner as the nonaqueous electrolyte described above, except that the secondary electrolyte LiNO3 was not added.

[0108] [Test cell assembly] Coin-shaped test cells were fabricated using the positive and negative electrodes of Examples 1 to 29 and Comparative Examples 1 to 10. The negative electrodes were made by depositing lithium metal, the negative electrode active material, on copper foil, the negative electrode current collector. The positive and negative electrodes were arranged in the exterior body with a separator impregnated with an electrolyte interposed between them. The electrode area of ​​each positive electrode was 1.583 cm. 2 A PP separator manufactured by Celgard was used as the separator, and the prepared non-aqueous electrolyte was impregnated into it.

[0109] Table 1 below shows the oxides and organic compounds contained in the sulfur mixture layers of Examples 1 to 7 and Comparative Examples 1 to 7, their contents, the results of the discharge capacity evaluation test described below, and the results of the charge-discharge cycle test described below. The positive electrodes in Table 1 had a sulfur loading of 3.5 mg cm -2 is.

[0110] [Table 1]

[0111] Table 2 below shows the oxides and organic compounds contained in the sulfur mixture layer of Examples 8 to 16, their contents, and the results of the discharge capacity evaluation test described below. The positive electrodes in Table 2 have a sulfur loading of 5.0 mg cm -2 is.

[0112] [Table 2]

[0113] Table 3 below shows the oxide and organic compound materials contained in the sulfur mixture layer of Examples 17 to 22, their contents, the total content of the oxide and organic compound, the proportion of sulfur contained in the sulfur mixture layer, the results of the discharge capacity evaluation test described below, and the results of the charge-discharge cycle test described below. In Table 3, the amount of sulfur supported in the sulfur mixture layer is 3.5 mg cm -2 The thickness of the sulfur mixture layer is different. [Table 3]

[0114] Table 4 below shows the oxides and organic compounds contained in the sulfur mixture layer of Examples 23 to 29 and Comparative Examples 8 to 10, their contents, the materials contained in the ceramic layer, the results of the discharge capacity evaluation test described below, and the results of the charge-discharge cycle test described below. -2 is.

[0115] [Table 4]

[0116] <Discharge capacity evaluation test> As shown in Tables 1 to 4, a discharge capacity evaluation test was carried out on each of the evaluation cells of Examples 1 to 29 and Comparative Examples 1 to 10. The test was carried out in an environment with an atmospheric temperature of 60°C. Each evaluation cell in Table 1 was subjected to constant current discharge at 0.9 mA (0.1 C), 4.5 mA (0.5 C), and 9.0 mA (1.0 C) until the voltage reached 1.0 V. Each evaluation cell in Table 2 was subjected to constant current discharge at 1.3 mA (0.1 C) and 6.5 mA (0.5 C) until the voltage reached 1.0 V. Each evaluation cell in Table 3 was subjected to constant current discharge at 0.9 mA (0.1 C), 4.5 mA (0.5 C), and 9.0 mA (1.0 C) until the voltage reached 1.0 V. Each evaluation cell in Table 4 was subjected to constant current discharge at 1.3 mA (0.1 C) until the voltage reached 1.0 V. The results are shown in Tables 1 to 4. The discharge capacity is shown as a value normalized by the weight of the positive electrode.

[0117] <Charge-discharge cycle test> A charge-discharge cycle test was carried out on each evaluation cell of Examples 1 to 7 and Comparative Examples 1 to 7 in Table 1, Examples 17 to 22 in Table 3, and Examples 23 to 29 and Comparative Examples 8 to 10 in Table 4. The test was carried out in an environment with an ambient temperature of 60°C.

[0118] First, each test cell in Table 1 was subjected to constant current discharge at 0.9 mA (0.1 C) until the voltage reached 1.0 V. Next, each test cell was subjected to constant current charge at the same current density until the voltage reached 3.0 V. This charge / discharge cycle was counted as one cycle, and the charge / discharge cycle was repeated 100 times. The capacity retention rate was then calculated using the following formula. The results are shown in Table 1. Capacity retention rate (%) = Discharge capacity at 100th cycle / Discharge capacity at 1st cycle For each test cell in Table 3, the same tests as in Table 1 were carried out. For each test cell in Table 4, the same test was carried out, except that the constant current charge and constant current discharge were carried out at a current density of 1.3 mA (0.1 C). The results are shown in Tables 1 and 3.

[0119] From the results of Tables 1 to 4, the test cells of Examples 1 to 29 showed high discharge capacities at all current values ​​and good capacity retention rates, particularly because the sulfur composite layer contained both a lithium-containing oxide and a nitrogen-containing organic compound and the nitrogen-containing organic compound was a nitrogen-containing heterocyclic compound.

[0120] As can be seen from the results in Table 1, in the test cells of Comparative Examples 1 to 3, the sulfur composite layer did not contain either a lithium-containing oxide or a nitrogen-containing organic compound, and therefore the discharge capacity, particularly when a high current was applied, was significantly reduced compared to Examples 1 to 7.

[0121] The lithium-containing oxide is Li4Ti5O12 In Example 1, Li7La3Zr2O 12 Due to its high lithium ion conductivity, a better discharge capacity was obtained than in Example 6, which contained Al2O3, a lithium-free oxide. On the other hand, in Comparative Example 7, which contained Al2O3, a discharge capacity at the time of application of a high current was significantly reduced compared to Examples 1 to 7. Furthermore, in Comparative Examples 4 to 6, in which the sulfur mixture layer did not contain a nitrogen-containing heterocyclic compound but contained other polymers, a discharge capacity at the time of application of a high current was significantly reduced compared to Examples 1 to 7. These results show that the high-rate characteristics are improved when the sulfur mixture layer contains a lithium-containing oxide and a nitrogen-containing heterocyclic compound.

[0122] From the results in Table 2, in the test cells of Examples 8 to 16, when the content of the lithium-containing oxide is A and the content of the nitrogen-containing organic compound is B, A≧B was satisfied, and thus better discharge capacity was obtained when a high current was applied (6.5 mA). In the test cells of Examples 13 to 16, the oxide or organic compound material in the sulfur composite layer was changed from that of Examples 8 to 12, but similar results were obtained. Furthermore, by comparing the results of the test cells of Examples 8 to 16, it can be seen that the larger the ratio of the content (A) of the lithium-containing oxide in the sulfur mixture layer, the better the discharge capacity when a high current is applied.

[0123] The results in Table 3 show that all of Examples 17 to 22, in which the total content of the lithium-containing oxide and the nitrogen-containing organic compound contained in the sulfur mixture layer was in the range of 0.5 to 20 wt %, achieved a better capacity retention rate than Comparative Example 1 in Table 1. In Example 17, despite the fact that the total content of the lithium-containing oxide and the nitrogen-containing organic compound was a small amount of 0.5 wt %, a capacity retention rate equivalent to that of Comparative Example 2, in which only the lithium-containing oxide was contained at 7.0 wt % was obtained. Furthermore, in Example 17, a capacity retention rate of about 75% was achieved compared to that of Comparative Example 3, in which only the nitrogen-containing organic compound was contained at 7.0 wt %. Furthermore, in Example 18, in which the total content of the lithium-containing oxide and the nitrogen-containing organic compound was 1 wt %, a capacity retention rate higher than that of Comparative Example 3 was achieved. Therefore, it can be seen that by including both the lithium-containing oxide and the nitrogen-containing organic compound in the sulfur mixture layer, a better capacity retention rate can be achieved than when either the lithium-containing oxide or the nitrogen-containing organic compound is contained at the same content. In addition, it can be seen that Examples 18 to 22, in which the content is 1 wt % or more, have better capacity retention rates. Furthermore, by comparing the results of the test cells of Examples 17 to 22, it can be seen that the lower the proportion of sulfur in the sulfur composite layer, the lower the discharge capacity per positive electrode weight tends to be. To increase the discharge capacity, the lower the amount of sulfur supported per unit area, the thicker the sulfur composite layer must be coated. However, it is presumed that the thicker the sulfur composite layer, the more difficult it is for lithium ions to be conducted into the sulfur composite layer, resulting in a decrease in discharge capacity. Therefore, it is preferable that the total content of the lithium-containing oxide and the nitrogen-containing organic compound be 1 wt % or more and 20 wt % or less with respect to the weight of the sulfur composite layer.

[0124] The results in Table 4 show that in Examples 23 to 29 in which a ceramic layer was laminated on the surface of the sulfur mixture layer, better capacity retention rates were obtained compared to Examples 1 to 7 in Table 1. On the other hand, in the test cells of Comparative Examples 8 to 10, the sulfur mixture layer did not contain either the lithium-containing oxide or the nitrogen-containing organic compound, and therefore the capacity retention rate was significantly lower than in Examples 23 to 29.

[0125] The charge-discharge curves for Example 26 are shown in Figure 3. The ceramic powder contained in the ceramic layer was Li4Ti5O 12 Therefore, in the following, Li4Ti5O 12 layer. Li4Ti5O 12 The charge-discharge curve of the lithium-sulfur battery incorporating the layer was 1.55 V (vs. Li / Li + ) a plateau where the potential becomes constant can be seen. 12 The oxidation-reduction potential of Li4Ti5O is 1.55V. When a charging current is applied due to the battery overvoltage, the charge / discharge curve shifts to the high potential side, and when a discharging current is applied, the charge / discharge curve shifts to the low potential side. Therefore, the potential plateau of the discharge curve at 1.54V and the potential plateau of the charge curve at 1.57V are 12 It can be inferred that this is due to the oxidation-reduction of 1.55V (vs. Li / Li + ) from the discharge curve, it can be seen that the discharge capacity is around 1.54V (vs. Li / Li + ) The discharge capacity per positive electrode weight is 137mAhg -1 On the other hand, the charging curve shows a capacitance component of 1.57V (vs. Li / Li + ) The charging capacity per positive electrode weight is 47mAhg -1 From these results, it can be seen that the discharge capacity is greater than the charge capacity.

[0126] Li4Ti5O in this test 12 The capacitance component of the curve is 47mAhg -1 This corresponds to the charging curve of 1.54 (vs. Li / Li + ) capacity component is Li4Ti5O 12 On the other hand, the discharge curve indicates that the capacity component is derived from the oxidation of Li4Ti5O 12 The capacity component (47mAhg -1 Since a capacity of more than 1000kJ / cm2 was confirmed, it is considered that Li4Ti5O 12 The formation of the layer results in Li4Ti5O 12 In addition to the reduction of Li4Ti5O, it can be inferred that the unreacted sulfur and sulfur compounds reacted simultaneously to produce a capacity component.12 In the positive electrode with the layer introduced, the reduced Li4Ti5O 12 It is thought that the utilization rate of sulfur improved by reacting with unreacted sulfur. + )~3.0V(vs. Li / Li + ), the unreacted sulfur is expected to react in the same way, so a similar effect can be expected. From the above results, it can be seen that in the positive electrode with the ceramic layer, the ceramic powder contained in the ceramic layer can be sufficiently reduced at 1.0 V (vs. Li / Li + ) or more 1.5V (vs.Li / Li + ) or less, the capacity increases significantly.

[0127] As described above, the configuration of the present invention can provide a positive electrode for a lithium-sulfur battery and a lithium-sulfur battery that can simultaneously achieve improved discharge rate characteristics and improved charge-discharge cycle characteristics.

[0128] The present invention may have the following features. According to a first aspect of the present invention, a positive electrode for a lithium-sulfur battery includes a positive electrode current collector and a sulfur composite layer laminated on a surface of the positive electrode current collector, the sulfur composite layer including sulfur and / or a sulfur compound as a main positive electrode active material, a lithium-containing oxide, and a nitrogen-containing organic compound, and the nitrogen-containing organic compound is a nitrogen-containing heterocyclic compound. In a second aspect, in addition to the first aspect, the nitrogen-containing heterocyclic compound may be a compound containing a pyridine ring skeleton. In a third aspect, in addition to the first aspect or the second aspect, when the content of the lithium-containing oxide in the sulfur composite layer is A and the content of the nitrogen-containing organic compound is B, A≧B may be satisfied. In a fourth aspect, in addition to any one of the first to third aspects, the lithium-containing oxide has a voltage of 1.0 V (vs. Li / Li + ) greater than 3.0V (vs. Li / Li +) can be composed of compounds that charge and discharge within a potential range of less than 1000 kJ / cm. In a fifth aspect, in addition to any one of the first to fourth aspects, the lithium-containing oxide may be a lithium-titanium-based composite oxide. In a sixth aspect, in addition to any one of the first to fifth aspects, the nitrogen-containing organic compound has a resistance of 1.0 V (vs. Li / Li + ) greater than 3.0V (vs. Li / Li + ) can be composed of compounds that charge and discharge within a potential range of less than 1000 kJ / cm. In a seventh aspect, in addition to any one of the first to sixth aspects, the nitrogen-containing organic compound may be sulfur-modified polyacrylonitrile or a derivative thereof. According to an eighth aspect, in addition to any one of the first to seventh aspects, the sulfur mixture layer may further include a carbon-based conductive additive. In a ninth aspect, in addition to any one of the first to eighth aspects, a total content of the lithium-containing oxide and the nitrogen-containing organic compound may be 1 wt % or more and 20 wt % or less with respect to a weight of the sulfur composite layer. In a tenth aspect, in addition to any one of the first to ninth aspects, the positive electrode for a lithium-sulfur battery may further include a ceramic layer laminated on the sulfur composite layer, and the ceramic layer may contain ceramic powder as a main component. In an eleventh aspect, in addition to any one of the first to tenth aspects, the ceramic powder may be made of titanium oxide or a titanium-based composite oxide. In a twelfth aspect, in addition to any one of the first to eleventh aspects, the ceramic layer may further contain a nitrogen-containing heterocyclic compound. According to a thirteenth aspect, a lithium-sulfur battery according to one embodiment includes a positive electrode for a lithium-sulfur battery satisfying any one of the first to twelfth aspects, a negative electrode capable of absorbing and desorbing lithium ions, and a separator impregnated with a nonaqueous electrolyte solution. In a fourteenth aspect, in addition to being the lithium-sulfur battery of the thirteenth aspect, the non-aqueous electrolyte solution includes an electrolyte comprising a lithium salt and a non-aqueous solvent, and the non-aqueous solvent may be a mixture of 1,3-dioxolane and dimethoxyethane. In a fifteenth aspect, in addition to being the lithium-sulfur battery of the thirteenth aspect or the fourteenth aspect, the nonaqueous electrolyte may further contain lithium nitrate as a secondary electrolyte separate from a primary electrolyte that is a main component of the electrolyte. In one embodiment of the method for charging and discharging a lithium-sulfur battery, the lower limit of the discharge voltage is set to 1.0 V (vs. Li / Li) for a lithium-sulfur battery that satisfies any one of the thirteenth to fifteenth aspects. + ) or more 1.5V (vs.Li / Li + ) and discharge as follows. [Explanation of symbols]

[0129] 1... Lithium-sulfur battery, 2... Positive electrode, 21... Positive electrode current collector, 22... Sulfur composite layer, 23... Ceramic layer, 3... Negative electrode, 31... Negative electrode current collector, 32... Negative electrode layer, 4... Separator

Claims

1. A positive electrode for a lithium-sulfur battery, comprising: a positive electrode current collector; and a sulfur mixture layer laminated on a surface of the positive electrode current collector, the sulfur mixture layer contains sulfur and / or a sulfur compound as a main positive electrode active material, a lithium-containing oxide, and a nitrogen-containing organic compound, the nitrogen-containing organic compound is a nitrogen-containing heterocyclic compound, A positive electrode for a lithium-sulfur battery, comprising a ceramic layer laminated on the sulfur mixture layer.

2. 2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the nitrogen-containing heterocyclic compound is a compound containing a pyridine ring skeleton.

3. 2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein, in the sulfur mixture layer, when a content of the lithium-containing oxide is A and a content of the nitrogen-containing organic compound is B, A≧B.

4. The lithium-containing oxide has a voltage of 1.0 V (vs. Li / Li ＋ ) greater than 3.0 V (vs. Li / Li ＋ 2. The positive electrode for a lithium-sulfur battery according to claim 1, comprising a compound that is charged and discharged within a potential range of less than 1000 V.

5. 5. The positive electrode for a lithium-sulfur battery according to claim 4, wherein the lithium-containing oxide is a lithium-titanium composite oxide.

6. The nitrogen-containing organic compound has a potential of 1.0 V (vs. Li / Li ＋ ) greater than 3.0 V (vs. Li / Li ＋ 2. The positive electrode for a lithium-sulfur battery according to claim 1, comprising a compound that is charged and discharged within a potential range of less than 1000 V.

7. 7. The positive electrode for a lithium-sulfur battery according to claim 6, wherein the nitrogen-containing organic compound is sulfur-modified polyacrylonitrile or a derivative thereof.

8. The positive electrode for a lithium-sulfur battery according to claim 1 , wherein the sulfur composite layer further comprises a carbon-based conductive additive.

9. 2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein a total content of the lithium-containing oxide and the nitrogen-containing organic compound is 1% by weight or more and 20% by weight or less with respect to a weight of the sulfur mixture layer.

10. A positive electrode for a lithium-sulfur battery as described in claim 1, characterized in that the ceramic layer contains ceramic powder as a main component.

11. 11. The positive electrode for a lithium-sulfur battery according to claim 10, wherein the ceramic powder is made of titanium oxide or a titanium-based composite oxide.

12. The positive electrode for a lithium-sulfur battery according to claim 10, wherein the ceramic layer further contains a nitrogen-containing heterocyclic compound.

13. A lithium-sulfur battery comprising the positive electrode for a lithium-sulfur battery according to claim 1, a negative electrode capable of absorbing and desorbing lithium ions, and a separator impregnated with a non-aqueous electrolyte.

14. the non-aqueous electrolyte solution contains an electrolyte made of a lithium salt and a non-aqueous solvent, 14. The lithium-sulfur battery according to claim 13, wherein the non-aqueous solvent is a mixture of 1,3-dioxolane and dimethoxyethane.

15. 15. The lithium-sulfur battery according to claim 14, wherein the non-aqueous electrolyte further contains lithium nitrate as an electrolyte separate from the electrolyte.

16. A method for charging and discharging the lithium-sulfur battery according to any one of claims 13 to 15, Discharge was performed at 1.0 V (vs. Li / Li ＋ ) or more 1.5V (vs.Li / Li ＋ ) or less as a lower limit of the discharge voltage.

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