JP2023066785A - Positive electrode for lithium sulfur battery and lithium sulfur battery - Google Patents

Abstract

To provide a positive electrode for a lithium sulfur battery capable of simultaneously achieving an increase in discharge capacity and an improvement in charge and discharge cycle characteristics.SOLUTION: The positive electrode for a lithium sulfur battery includes a sulfur mixture layer including sulfur and/or sulfur compounds, a binder, and a lithium-containing oxide and a ceramic layer including ceramic powder and a binder. The sulfur mixture layer is laminated on the surface of a current collector. The ceramic layer is laminated on the surface of the sulfur mixture layer. The lithium-containing oxide content is between 1 weight % or more and 20 weight % or less of the weight of the sulfur mixture layer.SELECTED DRAWING: Figure 1

Description

The present invention relates to positive electrodes for lithium-sulfur batteries and lithium-sulfur batteries.

In recent years, the use of non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries has greatly expanded as power sources for mobile devices such as mobile phones, power tools, electric vehicles and the like. Non-aqueous electrolyte secondary batteries are required to have higher energy density, longer life, etc. as power sources for these.

Lithium-sulfur batteries are currently attracting attention as one of the next-generation batteries because of their high energy density. Such a lithium-sulfur battery charges and discharges by moving lithium ions between the positive electrode and the negative electrode. The positive electrode includes, for example, a sulfur composite material layer containing a positive electrode active material made of sulfur and/or a sulfur compound, a conductive aid, and a binder, and a positive current collector that supports the sulfur composite layer. The negative electrode includes, for example, lithium metal and a negative electrode current collector that supports the lithium metal. A separator is interposed between the positive electrode and the negative electrode. An electrode plate group including a positive electrode, a negative electrode, and a separator is housed in a battery container together with a non-aqueous electrolyte. As the non-aqueous electrolyte, a non-aqueous electrolytic solution in which an electrolyte such as a lithium salt is dissolved in a non-aqueous solvent is generally used.

However, at present, the lithium-sulfur battery cannot obtain sufficient charge-discharge cycle characteristics and has a problem of reaching early life. One specific cause of this is a phenomenon called a redox shuttle effect. In such a phenomenon, lithium polysulfide (Li ₂ S ₈ , Li ₂ S _{6 ,} etc.), which is a reaction intermediate product on the positive electrode side, dissolves and diffuses into the electrolytic solution due to repeated charging and discharging. The diffused lithium polysulfide electrophoreses during charging, is reduced on the negative electrode, further diffuses and is oxidized on the positive electrode side. This electrochemical reaction proceeds until the lithium polysulfide becomes Li ₂ S, causing self-discharge of the battery. In order to improve the charge-discharge cycle characteristics of lithium-sulfur batteries, it is necessary to suppress the redox shuttle effect.

In order to solve such problems, as described in Non-Patent Document 1, the formation of a graphite/Li ₄ Ti ₅ O ₁₂ double layer on the surface of the sulfur compound layer of a lithium-sulfur battery has been investigated. there is This double layer has the role of trapping the polysulfide dissolved in the electrolytic solution inside and suppressing diffusion to the negative electrode surface. As a result, the redox shuttle effect can be suppressed and the charge/discharge cycle characteristics can be improved.

Jun Ming et al., ACS Nano, 2016, Vol. 10, p. 6037-6044

However, in the lithium-sulfur battery disclosed in Non-Patent Document 1, the presence of the double layer hinders the supply of lithium ions from the non-aqueous electrolyte, and the ionic conductivity in the sulfur compound layer decreases, resulting in a battery failure. There was a problem that the internal resistance increased. This increase in internal resistance contributes to a decrease in discharge capacity when a high current is applied. Therefore, in this battery, it is required to reduce the internal resistance of the sulfur compound layer to suppress the decrease in discharge capacity and to further improve the charge-discharge cycle characteristics.

The present invention solves the above problems, and provides a positive electrode for a lithium-sulfur battery and a lithium-sulfur battery that can simultaneously achieve an increase in discharge capacity and an improvement in charge-discharge cycle characteristics, especially when a high current is applied. be.

In order to solve the above problems, according to one embodiment, a positive electrode current collector, a sulfur compound layer containing sulfur and/or a sulfur compound, a binder and a lithium-containing oxide, and ceramics containing ceramic powder and a binder A positive electrode for a lithium-sulfur battery, wherein the sulfur composite layer is laminated on the surface of the current collector, and the ceramic layer is laminated on the surface of the sulfur composite layer, wherein the lithium-containing oxide is 1% by weight or more and 20% by weight or less with respect to the weight of the sulfur compound layer.

In order to solve the above problems, according to one embodiment, there is provided a lithium-sulfur battery comprising a positive electrode for a lithium-sulfur battery, a negative electrode, and a separator impregnated with a non-aqueous electrolyte. .

According to the present invention, it is possible to provide a positive electrode for a lithium-sulfur battery and a lithium-sulfur battery that can simultaneously achieve an increase in discharge capacity especially when a high current is applied and an improvement in charge-discharge cycle characteristics.

1 is a cross-sectional view showing an example of a lithium-sulfur battery according to an embodiment; FIG.

Hereinafter, positive electrodes for lithium-sulfur batteries according to embodiments will be described in detail.

<Positive electrode>
The positive electrode includes a positive electrode current collector, a sulfur compound layer containing sulfur and/or a sulfur compound, a binder and a lithium-containing oxide, and a ceramic layer containing ceramic powder and a binder. The sulfur composite material layer is laminated on the surface of the positive electrode current collector, and the ceramic layer is laminated on the surface of the sulfur composite material layer. The content of the lithium-containing oxide is 1% by weight or more and 20% by weight or less with respect to the weight of the sulfur compound layer.
According to the positive electrode for a lithium-sulfur battery according to such an embodiment, it is possible to simultaneously achieve an increase in discharge capacity and an improvement in charge-discharge cycle characteristics.

By stacking the ceramic layer on the surface of the sulfur compound layer, lithium polysulfide, which is an intermediate reaction product on the positive electrode side generated by repeated charging and discharging, can be efficiently trapped inside the ceramic layer. It is possible to suppress the gas shuttle effect and improve the charge-discharge cycle characteristics of the battery.

Further, the sulfur composite layer contains both sulfur and/or a sulfur compound, a binder and a lithium-containing oxide, and the content of the lithium-containing oxide is defined within the above range, thereby improving the ionic conductivity in the sulfur composite layer. can be improved and the internal resistance can be lowered. This is because the lithium-containing oxide conducts lithium ions into the sulfur compound layer during repeated charging and discharging. As a result, the internal resistance of the battery in which the positive electrode is incorporated can be reduced, and the discharge capacity can be improved particularly when a high current is applied.

In some embodiments, the positive electrode includes a positive electrode current collector, a sulfur composite material layer formed on one or both surfaces of the positive electrode current collector, and a ceramic layer laminated on the surface of the sulfur composite material layer. I have.

The positive electrode current collector is not particularly limited, and a known or commercially available one can be used. For example, rolled foils, electrolytic foils, metal meshes, metal porous bodies, expanded metals, punching metals, etc. made of aluminum or aluminum alloys can be used. It is preferable to use carbon-coated aluminum foil for the positive electrode current collector in order to improve the electrical conductivity between the sulfur mixture layer and the adhesion between the mixture layer and the current collector. .

The sulfur composite layer contains sulfur and/or a sulfur compound, a binder and a lithium-containing oxide, as described above. It may contain a conductive aid.

Sulfur and/or sulfur compounds are positive electrode active materials capable of intercalating and deintercalating lithium ions. Sulfur and/or sulfur compounds include, for example, crystalline sulfur, granular sulfur, colloidal sulfur, lithium sulfide, lithium polysulfide (Li ₂ S _x , x=4-8), organic sulfur compounds or carbon-sulfur polymers ( (C ₂ S _x ) _n , x=2.5 to 50, n≧2) and the like. Sulfur and/or sulfur compounds may be a single substance or a mixture thereof, and the combination and ratio thereof can be arbitrarily selected according to the purpose. The sulfur and/or sulfur compounds are preferably particulate sulfur, more preferably colloidal sulfur.

The conductive aid is not particularly limited as long as it is a material capable of improving the electron conductivity of the sulfur compound layer, and known materials can be used. Examples of conductive aids include carbon black such as ketjen black and acetylene black, carbon nanotubes, graphene, artificial graphite, natural graphite, and carbon-based conductive aids such as activated carbon. The conductive aid may be a single substance or a mixture of these, and the combination and ratio thereof can be arbitrarily selected according to the purpose.

Part or all of the sulfur and/or sulfur compound is preferably compounded with part or all of a carbon-based conductive aid such as carbon black to form a sulfur-carbon composite material. Examples of the method for producing the sulfur-carbon composite material include a melt impregnation method, an electrolytic deposition method, a vapor deposition method, an immersion method, a mechanical mixing method and the like. A sulfur-carbon composite material can improve the electronic conductivity of sulfur and/or sulfur compounds. The sulfur-carbon composite material may be, for example, an S/KB composite material produced by mixing sulfur (S) and ketjen black (KB) at a weight ratio of 70:30 and heat-treating the mixture for 12 hours. Moreover, sulfur and/or a sulfur compound and a sulfur-carbon composite material may be used in combination. Furthermore, the compounded carbon-based conductive aid may be the same as the conductive aid.

Furthermore, it is preferable that part or all of the sulfur and/or sulfur compound contain sulfur-modified polyacrylonitrile or a derivative thereof. Sulfur-modified polyacrylonitrile, like ceramic powder, has the property of adsorbing lithium polysulfide and can suppress the redox shuttle phenomenon.

The binder is not particularly limited as long as it is a material that binds the materials contained in the sulfur compound layer together, and any known or commercially available binder can be used. Examples of binders include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-propylene hexafluoride 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), acrylic resin and the like. The binder may be a single substance or a mixture of these, and the combination and ratio thereof can be arbitrarily selected according to the purpose. The binders are preferably PVDF, SBR and CMC.

The lithium-containing oxide supplies lithium ions into the sulfur composite layer to improve ion conductivity within the sulfur composite layer. Lithium-containing oxides include, for example, Li ₄ Ti ₅ O ₁₂ , LiTiO ₄ , Li ₂ Ti ₃ O ₇ , Li _0.35 La _0.55 TiO _{3 ,} Li _1+x+y Al _x (Ti, Ge) _2-x Si _Lithium -titanium composite oxides such as _{yP3- yO12} (0≤x≤2, _0≤y≤3 ), _LiCoO2 , _LiMn2O4 , _LiNiO2 , _LiNiCoMnO2 , _{LiNi0.5Mn0 . 5O 2} , LiMPO ₄ (M=transition metal such as Ma, Fe, Co, Ni), LiNi _0.8 Co _0.15 Al _0.05 O ₂ , LiMn _x Fe _1-x PO ₄ (0<x< 1), LiNi _1-xy Co _x Mn _y O ₂ (0<x, y, z<1), Li ₇ La ₃ Zr ₂ O ₁₂ and the like. The lithium-containing oxide may be a single substance or a mixture of these, and the combination and ratio thereof can be arbitrarily selected according to the purpose.

Since the lithium-containing oxide has high lithium ion conductivity, it is preferably a lithium-titanium composite oxide, more preferably Li ₄ Ti ₅ O ₁₂ .

Like the ceramic layer, the lithium-titanium-based composite oxide in the sulfur compound layer can efficiently trap lithium polysulfide, which is an intermediate product of the reaction on the positive electrode side, inside it by repeated charging and discharging. It is possible to suppress the shuttle effect and improve the charge/discharge cycle characteristics of the battery. By using a lithium-titanium-based composite oxide as the lithium-containing oxide, it is possible to further improve charge-discharge cycle characteristics of a battery incorporating the positive electrode.

A lithium-titanium composite oxide is a compound that charges and discharges within a potential range of greater than 1.0 V (vs. Li/Li ⁺ ) and less than 3.0 V (vs. Li/Li ⁺ ) (e.g., Li ₄ Ti ₅ O ₁₂ ), the lithium-containing oxide can function as a secondary positive electrode active material capable of intercalating and deintercalating lithium. Thereby, the discharge capacity of the battery can be increased, and the weight energy density of the battery can be improved.

The content of the lithium-containing oxide is 1% by weight or more and 20% by weight or less with respect to the weight of the sulfur compound layer. By including the lithium-containing oxide in the above content in the sulfur composite material layer, the internal resistance of the sulfur composite material layer can be reduced due to its high ionic conductivity, and the discharge capacity can be improved particularly when a high current is applied.

If the lithium-containing oxide is less than 1% by weight, the internal resistance of the sulfur compound layer increases, the utilization efficiency of the positive electrode active material decreases, and the discharge capacity of the battery may decrease. On the other hand, when the lithium-containing oxide exceeds 20% by weight, the content of the positive electrode active material in the sulfur compound layer is relatively decreased, which may decrease the weight energy density of the battery.
The content of the lithium-containing oxide is preferably 5% by weight or more and 10% by weight or less.

The content of sulfur and/or sulfur compounds is preferably 50% by weight or more, more preferably 55 to 90% by weight, and still more preferably 55 to 65% by weight, based on the weight of the sulfur composite layer. is. If the sulfur and/or sulfur compound content is less than 50% by weight, the content of the positive electrode active material in the sulfur compound layer is low, which is not preferable because the weight energy density of the battery may be lowered.

The sulfur compound layer contains 60 to 90% by weight of sulfur and/or a sulfur compound, 0 to 25% by weight of a conductive agent, 3 to 5% by weight of a binder, and 1 to 20% by weight of a lithium-containing oxide. It is preferable to include in More preferably, the conductive aid is 4 to 25% by weight.

The thickness of the sulfur mixture layer may be a general thickness, for example, the value of the electrode taken out from a 0% SOC battery that has completed initial activation and is in a state that can be distributed as a product is 20 μm to 100 μm. .

The ceramic layer contains ceramic powder and a binder as described above. It may contain a conductive aid.
Ceramic powders are compounds that can trap lithium polysulfides within them. Examples of ceramic powder include TiO ₂ , Li ₄ Ti ₅ O ₁₂ , TiNb ₂ O ₇ , LiTiO ₄ , Li ₂ Ti ₃ O ₇ , Li _1+x+y Al _x (Ti, Ge) _2-x SiyP _3-y O ₁₂ (0≤x≤2, _{0≤y≤3 )} , _{La0.57Li0.29TiO3} , and _{Li1+x+ yAlxTi2 - xSiyP3 - yO12} (0≤x ≤ 1, 0 ≤ y ≤ 1) titanium oxide or titanium-based composite oxide, Al ₂ O ₃ , LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNiCoMnO ₂ , LiNi _0.5 Mn _0.5 O ₂ , LiMPO ₄ (M = transition metal such as Ma, Fe, Co, Ni), LiNi _0.8 Co _0.15 Al _0.05 O ₂ , Li _0.35 La _0.55 TiO ₃ , LiMn _x Fe _1-x PO ₄ (0<x<1), LiNi _1-xy Co _{x Mny} O ₂ (0<x, y, z<1), Li ₇ La ₃ Zr ₂ O ₁₂ and the like.

The ceramic powder preferably consists of a compound that charges and discharges within a potential range of more than 1.0 V (vs. Li/Li ⁺ ) and less than 3.0 V (vs. Li/Li ⁺ ).

By using a compound that charges and discharges within the above potential range for the ceramic powder, the ceramic powder can function as a secondary positive electrode active material capable of intercalating and deintercalating lithium. Sulfur and/or a sulfur compound, which is the main positive electrode active material of the positive electrode, is generally charged and discharged within a potential range of 2.4 to 1.8 V (vs. Li/Li ⁺ ), so the ceramic powder is also used at the same time. This is because it can be charged and discharged. Thereby, the discharge capacity of the battery can be significantly increased, and the weight energy density of the battery can be improved.

The ceramic powder is made of a compound that charges and discharges in the above potential range, and the ceramic layer is laminated on the surface of the sulfur compound layer, so that the sulfur compound dissolved from the sulfur compound layer during discharge is captured on the ceramic powder. Furthermore, sulfur compounds are reduced on the ceramic powder. To enable discharge to be continued until a sulfur compound is sufficiently reduced (discharged) at the same time while ceramic powder is being reduced, improve the utilization efficiency of sulfur, and improve the discharge capacity of a battery incorporating the positive electrode. can be done.

Examples of compounds that charge and discharge in the above potential range include TiO ₂ , Li ₄ Ti ₅ O _{12 ,} TiNb ₂ O ₇ , LiTiO ₄ , Li ₂ Ti ₃ O ₇ , La _0.57 Li _0.29 TiO ₃ , and Li1 _{+x+ yAlxTi2 - xSiyP3 - yO12} (0≤x≤1, 0≤y≤1) and other titanium oxides or titanium-based composite oxides, preferably Li _{4Ti5O12 . _}

The ceramic powder is more preferably made of a compound that undergoes a reduction reaction at a potential lower than 1.8 V (vs. Li/Li ⁺ ). For example, Li ₄ Ti ₅ O ₁₂ is a compound that discharges at a potential of about 1.55 V (vs. Li/Li ⁺ ).

The binder is not particularly limited as long as it is a material that binds the materials contained in the ceramic layers together, and known binders can be used. As the binder, for example, the same binder as the binder for the sulfur compound layer can be used.

The ceramic layer preferably contains a conductive aid to improve electronic conductivity. The conductive aid is not particularly limited as long as it is a material that can improve the electronic conductivity of the ceramic layer, and known materials can be used. As the conductive aid, for example, the same conductive aids as the conductive aids for the sulfur compound layer can be used.

The content of the ceramic powder is preferably 80 wt % or more and 98 wt % or less, more preferably 85 wt % or more and 95 wt % or less, relative to the weight of the ceramic layer.

Since the ceramic layer contains the ceramic powder in the above content, lithium polysulfide, which is a reaction intermediate product on the positive electrode side generated by repeated charging and discharging, can be efficiently trapped in the ceramic layer, resulting in a redx shuttle effect. can be suppressed to improve the charge/discharge characteristics of the battery. If the ceramic powder content is less than 80%, the ionic/electronic conductivity is lowered, and lithium polysulfide may not be trapped inside, which is not preferable.

The ceramic layer preferably contains 80 to 98% by weight of ceramic powder, 0 to 10% by weight of conductive aid, and 1 to 10% by weight of binder.

The thickness of the ceramic layer may be a general thickness, for example, the value of an electrode taken out from a 0% SOC battery after initial activation, which is ready for distribution as a product, is 1 μm to 50 μm.

In addition, the positive electrode can be produced, for example, by the following method. First, the above-mentioned positive electrode active material, conductive aid and binder are dispersed in a solvent to prepare a sulfur compound layer slurry. Next, the sulfur mixture layer slurry is applied to one or both surfaces of the positive electrode current collector, and then dried to obtain a sulfur mixture layer. Next, the above-mentioned ceramics material, conductive aid and binder are dispersed in a solvent to prepare a ceramics layer slurry. Next, after applying the slurry for the ceramics layer onto the sulfur compound layer, it is dried to obtain the ceramics layer. Thereby, a positive electrode in which a ceramic layer is laminated on the surface of the sulfur compound layer can be produced. Solvents used for preparing the slurry for the sulfur compound layer and the slurry for the ceramic layer include, for example, N-methyl-2-pyrrolidone (NMP) or water.

<Lithium sulfur battery>
In some embodiments, a lithium-sulfur battery includes the positive electrode for a lithium-sulfur battery described above, a negative electrode capable of intercalating and deintercalating lithium ions, and a separator impregnated with a non-aqueous electrolyte.

<Negative Electrode>
In some embodiments, the negative electrode comprises a negative electrode active material. A negative electrode current collector may also be included.

The negative electrode current collector is not particularly limited, and known ones can be used. For example, a rolled foil, electrolytic foil, or the like made of copper or a copper alloy can be used. Specifically, it 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 include aluminum-cadmium alloys and the like. In addition, baked carbon, non-conductive polymer surface-treated with a conductive material, conductive polymer, or the like can be used as the negative electrode current collector.

For example, negative electrode active materials include metal materials such as metallic lithium, lithium aluminum alloys, lithium tin alloys, lithium lead alloys, and lithium silicon alloys, as well as natural graphite, artificial graphite, carbon black, acetylene black, graphite, activated carbon, and carbon. It is selected from carbon materials such as fiber, coke, soft carbon and hard carbon, and oxide materials such as lithium titanate. One or more of the negative active materials may be used. When using two or more, their combination and ratio can be arbitrarily selected according to the purpose. The negative electrode active material is preferably metallic lithium or a lithium alloy.

In addition, the negative electrode may further include a conductive aid for allowing electrons to move smoothly within the negative electrode, together with the negative electrode active material.

Examples of conductive aids include carbon-based materials such as carbon black, acetylene black, ketjen black, carbon nanotubes (CNT), graphene, and reduced graphene oxide, or highly conductive materials such as polyaniline, polythiophene, polyacetylene, and polypyrrole. Molecules can be used. The conductive aid is preferably contained in an amount of 0 to 20% by mass with respect to the total weight of the negative electrode active material layer. On the other hand, if the content of the conductive aid exceeds 20% by mass, the content of the negative electrode active material becomes relatively small, which may deteriorate the capacity characteristics of the battery.

In addition, the negative electrode contains a binder capable of serving as a paste of the negative electrode active material, improving adhesion between the active material or between the active material and the negative electrode current collector, and buffering the expansion and contraction of the active material. may further include: Specifically, the same binder as that used for the sulfur mixture layer can be used as the binder.

In some embodiments, the negative electrode comprises only rolled foil made of lithium metal or lithium alloy without a separate negative electrode current collector.

<Non-aqueous electrolyte>
In some embodiments, a non-aqueous electrolyte includes an electrolyte and a non-aqueous solvent.

The electrolyte is, for example, lithium hexafluorophosphate (LiPF ₆ ), lithium bromide (LiBr), lithium perchlorate (LiClO ₄ ), lithium bisoxalate borate (LiB(C ₂ O ₄ )), fluoroborate one or a mixture of two or more selected from the group of lithium (LiBF ₄ ), lithium nitrate (LiNO ₃ ), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), etc. mentioned. When using a mixture, their combination and ratio can be arbitrarily selected according to the purpose.

In some embodiments, the non-aqueous solvent comprises primarily cyclic ethers and linear ethers. Cyclic ethers include, for example, 1,3-dioxolane (DOL). Examples of chain ethers include dimethoxyethane (DME) and the like. The non-aqueous solvent can be, for example, a 1:1 mixture of DOL and DME by volume.

In some embodiments, the non-aqueous solvent is based on sulfolane (SL), dimethylsulfoxide (DMSO), dimethylsulfone, and the like.

In some embodiments, non-aqueous solvents include ethylene carbonate, ethylmethyl carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dimethoxyethane, sulfolane, oxolane, tetraglyme, triglyme, fluoroethylene carbonate, ionic liquids, and the like. be done. Examples of ionic liquids include 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and 1-methyl-3-propylimidazolium. Bis(trifluoromethanesulfonyl)imide, 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, 1-ethyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium chloride, 1-ethyl -3-methylimidazolium methanesulfonate, 1-butyl-3-methylimidazolium methanesulfonate, 1,2,3-trimethylimidazolium methylsulfate, methylimidazolium chloride, methylimidazolium hydrogensulfate, 1 -ethyl-3-methylimidazolium hydrogen sulfate, 1-butyl-3-methylimidazolium hydrogen sulfate, 1-ethyl-3-methylimidazolium tetrachloroaluminate, 1-butyl-3-methylimidazolium Tetrachloroaluminate, 1-ethyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium ethylsulfate, 1-butyl-3-methylimidazolium Lithium methylsulfate, 1-ethyl-3-methylimidazolium thiocyanate, 1-butyl-3-methylimidazolium thiocyanate, 1-ethyl-2,3-dimethylimidazolium ethylsulfate, 1-butylpyridinium Bis(trifluoromethanesulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluoromethanesulfonyl)imide, 1-methyl-1-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide, 1-butyl-1-methylpyrroli dinium bis(trifluoromethanesulfonyl)imide, tetrabutylphosphonium bis(trifluoromethanesulfonyl)imide, tributyldodecylphosphonium bis(trifluoromethanesulfonyl)imide, methyltributylammonium methylsulfate, butyltrimethylammonium bis(trifluoromethanesulfonyl)imide, trimethyl Including hexylammonium bis(trifluoromethanesulfonyl)imide and the like. One or a mixture of two or more selected from the above group can be used as the non-aqueous solvent, and when a mixture is used, the combination and ratio thereof can be arbitrarily selected depending on the purpose.

<Separator>
The separator may be a microporous film or non-woven fabric made of polyolefin resin such as polyethylene resin or polypropylene resin, but is not limited to these. The microporous membrane, non-woven fabric, or the like may have a single-layer structure or a multi-layer structure. Also, it may be a sheet or other shape such as a zigzag fold.

<Charging and discharging conditions>
In some embodiments, the lithium-sulfur battery has a charge cut-off voltage of 2.4-3.0 V (vs. Li/Li ⁺ ) and a discharge cut-off voltage of 1.5-1.0 V (vs. Li/Li ⁺ ) is preferably charged and discharged.

When the charge cut-off voltage is within the above range, the ceramic powder is greater than 1.0 V (vs. Li/Li ⁺ ) and less than 3.0 V (vs. Li/Li ⁺ ) such as Li ₄ Ti ₅ O ₁₂ If it is a compound that charges and discharges within a potential range, it can function as a secondary positive electrode active material that absorbs and releases lithium. Note that sulfur and/or a sulfur compound, which is the main positive electrode active material of the positive electrode, is generally charged and discharged within a potential range of 2.4 to 1.8 V (vs. Li/Li ⁺ ). As a result, the secondary positive electrode active material is charged and discharged simultaneously with sulfur and/or the sulfur compound, which is the main positive electrode active material of the sulfur compound layer, so that the discharge capacity of the battery can be increased.

Since the discharge cutoff voltage is within the range of 1.5 to 1.0 V (vs. Li/Li ⁺ ), sulfur and/or sulfur compounds react for a long time during discharge, and sulfur is reduced. As a result, the utilization efficiency of sulfur as a positive electrode active material can be improved, discharge can be continued until the main positive electrode active material reaches a fully discharged state, and the discharge capacity of a battery incorporating the positive electrode can be improved. .

The shape of the lithium-sulfur battery is not particularly limited, and examples thereof include coin-type, button-type, sheet-type, laminate-type, cylindrical-type, square-type, flat-type, and the like.

Hereinafter, lithium-sulfur batteries according to embodiments will be described with reference to the drawings.

A lithium-sulfur battery 1 comprises a positive electrode 2 , a negative electrode 3 , and a separator 4 interposed between the positive electrode 2 and the negative electrode 3 . These positive electrode 2, negative electrode 3 and separator 4 are housed in an exterior body (not shown).

The positive electrode 2 is composed of a positive electrode current collector 21 , a sulfur composite material layer 22 provided on the surface facing the separator 4 , and a ceramic layer 23 laminated on the surface of the sulfur composite material layer 22 . The negative electrode 3 is composed of a negative electrode current collector 31 and a negative electrode layer 32 provided on the surface facing the separator 4 . The separator 4 is impregnated with, for example, a non-aqueous electrolyte.

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 are possible based on the technical idea of the present invention. .

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

[Examples and Comparative Examples]
[Preparation of positive electrode of Example 1]
Sulfur (S) and Ketjenblack (KB) were mixed at a weight ratio of 70:30. Next, the resulting mixture is heat-treated at 155 ° C. for 12 hours in an inert gas atmosphere to allow sulfur to permeate the pores of Ketjenblack to form a sulfur-carbon composite material (S / KB composite material). made.

Sulfur-carbon composite material (S/KB composite material), acetylene black (AB) and carbon nanotubes (CNT) as conductive aids, carboxymethyl cellulose (CMC) as binder, and Li ₄ Ti as lithium-containing oxide ₅ O ₁₂ was added to ultrapure water as a solvent and mixed and dispersed to prepare a positive electrode slurry.

Next, the prepared positive electrode slurry was applied on a carbon-coated aluminum foil, which was a positive electrode current collector, with a coating machine and dried to form a sulfur compound layer. Drying was vacuum dried at 80° C. for 10 minutes. The composition (weight ratio) of the sulfur compound layer was S/KB composite material: AB: CNT: binder: Li ₄ Ti ₅ O ₁₂ =91:1:2:5:1. The content of the lithium-containing oxide is 1% by weight with respect to the weight of the sulfur composite layer. The content of sulfur is 64% by weight with respect to the weight of the sulfur compound layer.

Next, Li ₄ Ti ₅ O ₁₂ powder as ceramic powder, AB, and CMC as binder were added to ultrapure water as solvent and mixed and dispersed to prepare slurry for ceramic layer. Next, the prepared ceramic layer slurry was applied onto the dried sulfur compound layer with a coating machine and dried to form a ceramic layer. Drying was vacuum dried overnight at 80°C. The composition (weight ratio) of the ceramic layer was Li ₄ Ti ₅ O ₁₂ :AB:binder=90:5:5. Next, it was pressed and stamped.
Through the steps described above, the positive electrode of Example 1 including the sulfur mixture layer and the ceramic layer laminated on the surface of the sulfur mixture layer was produced.

[Preparation of positive electrode of Example 2]
The positive electrode of Example 2 was produced by the same production method as the positive electrode of Example 1 above, except that the content of the lithium-containing oxide in each of the sulfur compound layers was changed to 5% by weight.
The composition (weight ratio) of the sulfur compound layer was S/KB composite material: AB: CNT: binder: Li ₄ Ti ₅ O ₁₂ =87:1:2:5:5. The content of sulfur is 61% by weight with respect to the weight of the sulfur compound layer.

[Preparation of positive electrode of Example 3]
The positive electrode of Example 3 was produced by the same production method as the positive electrode of Example 1 above, except that the content of the lithium-containing oxide in each of the sulfur compound layers was changed to 10% by weight.
The composition (weight ratio) of the sulfur compound layer was S/KB composite material: AB: CNT: binder: Li ₄ Ti ₅ O ₁₂ =82:1:2:5:10. The content of sulfur is 57% by weight with respect to the weight of the sulfur composite layer.

[Preparation of positive electrode of Example 4]
The positive electrode of Example 4 was produced in the same manner as the positive electrode of Example 1 above, except that the content of the lithium-containing oxide in each of the sulfur compound layers was changed to 20% by weight.
The composition (weight ratio) of the sulfur compound layer was S/KB composite material: AB: CNT: binder: Li ₄ Ti ₅ O ₁₂ =72:1:2:5:20. The content of sulfur is 50% by weight with respect to the weight of the sulfur compound layer.

[Preparation of positive electrode of Example 5]
The positive electrode of Example 5 was produced in the same manner as the positive electrode of Example 2 above, except that the ceramic powder constituting the ceramic layer was changed to alumina (Al ₂ O ₃ ).
The content of sulfur is 61% by weight with respect to the weight of the sulfur compound layer.

[Preparation of positive electrode of Example 6]
The positive electrode of Example 5 is the same as the positive _electrode of Example 2 above, except that the lithium-containing oxide in _the sulfur compound layer is _Li7La3Zr2O12 and the content _thereof is changed to 5% by weight. It was produced by the production method of. The content of sulfur is 61% by weight with respect to the weight of the sulfur compound layer.

[Preparation of positive electrode of Example 7]
In the positive electrode of Example 7, the content of the lithium-containing oxide in the sulfur mixture layer was 5% by weight, and the content of sulfur-modified polyacrylonitrile in the sulfur mixture layer was 5% by weight. It was produced by the same production method as the positive electrode of Example 1.
The composition (weight ratio) of the sulfur compound layer was S/KB composite material: AB: CNT: binder: Li ₄ Ti ₅ O ₁₂ : sulfur-modified polyacrylonitrile = 82: 1: 2: 5: 5: 5 . The content of sulfur is 57% by weight with respect to the weight of the sulfur composite layer.

[Preparation of positive electrode of Comparative Example 1]
The positive electrode of Comparative Example 1 was produced by the same production method as that of the positive electrode of Example 1 above, except that the content of the lithium-containing oxide in each of the sulfur compound layers was changed to 0% by weight.
The composition (weight ratio) of the sulfur compound layer was S/KB composite material: AB: CNT: binder: Li ₄ Ti ₅ O ₁₂ =92:1:2:5:0. The content of sulfur is 64% by weight with respect to the weight of the sulfur compound layer.

[Preparation of positive electrode of Comparative Example 2]
The positive electrode of Comparative Example 2 was produced in the same manner as the positive electrode of Example 1 above, except that the content of the lithium-containing oxide in the sulfur compound layer was changed to 0.5% by weight.
The composition (weight ratio) of the sulfur compound layer was S/KB composite material: AB: CNT: binder: Li ₄ Ti ₅ O ₁₂ =91.5:1:2:5:0.5. The content of sulfur is 64% by weight with respect to the weight of the sulfur compound layer.

[Preparation of positive electrode of Comparative Example 3]
The positive electrode of Comparative Example 3 was produced in the same manner as the positive electrode of Example 1 above, except that the content of the lithium-containing oxide in each of the sulfur compound layers was changed to 25% by weight.
The composition (weight ratio) of the sulfur compound layer was S/KB composite material: AB: CNT: binder: Li ₄ Ti ₅ O ₁₂ =67:1:2:5:25. The content of sulfur is 47% by weight with respect to the weight of the sulfur composite layer.

[Preparation of positive electrode of Comparative Example 4]
The positive electrode of Comparative Example 4 was manufactured by the same manufacturing method as that of the positive electrode of Example 2 above, except that the ceramic layer was not provided. The content of sulfur is 61% by weight with respect to the weight of the sulfur compound layer.

[Preparation of non-aqueous electrolyte]
The non-aqueous electrolyte was prepared by mixing a non-aqueous solvent with an electrolyte of lithium bis(trifluoromethane)sulfonimide (LiTFSI) at 1 mol/dm ³ . A mixture of 1,3-dioxolane (DOL) and dimethoxyethane (DME) at a volume ratio of 1:1 was used as the non-aqueous solvent.

[Assembling the test cell]
A coin-shaped test cell was produced using the produced positive electrodes and negative electrodes of Examples 1 to 7 and Comparative Examples 1 to 4. The negative electrode used was a copper foil that was a negative electrode current collector, and lithium metal that was a negative electrode active material was vapor-deposited on the copper foil. In the package, each positive electrode and negative electrode were arranged with a separator impregnated with an electrolytic solution interposed therebetween. The electrode area of each positive electrode is 1.583 cm ² . As the separator, a PP separator manufactured by Celgard was used and impregnated with the prepared non-aqueous electrolyte.

Table 1 below shows the lithium-containing oxide contained in the sulfur compound layer of Examples 1 to 6 and Comparative Examples 1 to 4, the ceramic powder contained in the ceramic layer, and the lithium-containing oxide in the sulfur compound layer. The contents, the results of the following discharge capacity evaluation test 1, and the following charge/discharge cycle test results are shown.

Table 2 below shows the lithium-containing oxide/sulfur-modified polyacrylonitrile (SPAN) contained in the sulfur compound layers of Examples 3 and 7, their contents, and the results of discharge capacity evaluation test 2 below.

<Discharge capacity evaluation test 1>
A discharge capacity evaluation test was performed on each evaluation cell of Examples 1-6 and Comparative Examples 1-4. The test was conducted in an environment with an ambient temperature of 60°C.
First, each test cell was subjected to a constant current discharge at 1.0C until 1.0V was reached. Each test cell was then galvanostatically charged at the same current density until 3.0 V was reached. The discharge capacity was evaluated to be excellent when the capacity per unit weight of the positive electrode was 500 mAhg ⁻¹ or more. The results are shown in Table 1.

From the results in Table 1, in the test cells of Examples 1 to 6, in particular, the ceramic layer was laminated on the surface of the sulfur compound layer, and the content of the lithium-containing oxide in the sulfur compound layer was 1% by weight. A good discharge capacity of 500 mAhg ⁻¹ or more was obtained by setting the content to 20% by weight or less.

On the other hand, in Comparative Examples 1 and 2 in which the content of the lithium-containing oxide in the sulfur composite material layer is less than 1% by weight, the ionic conductivity in the sulfur composite layer is low and the internal resistance is high. Couldn't get capacity.

Further, in Comparative Example 3 in which the content of the lithium-containing oxide in the sulfur composite layer was 25% by weight, the content of the positive electrode active material in the sulfur composite layer was relatively decreased, resulting in good discharge. Couldn't get capacity.

From the results in Table 1, in Examples 1 to 5 in which a ceramic layer is laminated on the surface of the sulfur composite material layer and the content of the lithium-containing oxide in the sulfur composite material layer is 1% by weight or more and 20% by weight or less , it was confirmed that an excellent discharge capacity can be obtained due to the high ionic conductivity in the sulfur compound layer.

<Charge-discharge cycle test>
A charge-discharge cycle test was performed on each evaluation cell of Examples 1-6 and Comparative Examples 1-4. The test was performed in an environment with an ambient temperature of 60°C.

First, each test cell was subjected to a constant current discharge at 1.0C until 1.0V was reached. Each test cell was then galvanostatically charged at the same current density until 3.0 V was reached. This charging/discharging was regarded as one cycle, and 100 cycles of charging/discharging were repeated. The results are shown in Table 1.

From the results in Table 1, the test cells of the examples have a ceramic layer laminated on the surface of the sulfur composite layer, and the content of the lithium-containing oxide in the sulfur composite layer is 1% by weight or more and 20% by weight or less. Some showed a high discharge capacity of 300 mAhg ⁻¹ or more.

In addition, in Example 2 in which the lithium-containing oxide is _Li4Ti5O12 , even better discharge is achieved by having higher lithium ion conductivity than in _{Example 6} in which the _{lithium - containing} oxide is _Li7La3Zr2O12 . capacity was obtained. Further, in Example 2 in which the ceramic powder is Li ₄ Ti ₅ O ₁₂ , a better discharge capacity was obtained than in Example 5 in which the ceramic powder is Al ₂ O ₃ because the ceramic powder undergoes an oxidation-reduction reaction. .

<Discharge capacity evaluation test 2>
For each evaluation cell of Examples 3 and 7, a discharge capacity evaluation test was performed at different current densities. The test was conducted in an environment with an ambient temperature of 60°C.
First, each test cell was galvanostatically discharged at 1.0C, 2.0C, and 3.0C until 1.0V was reached. Each test cell was then galvanostatically charged at the same current density until 3.0 V was reached. The discharge capacity per positive electrode at each current density was compared. The results are shown in Table 2.

From the results in Table 2, the test cell of Example 3 showed almost the same discharge capacity as the test cell of Example 7 to which sulfur-modified polyacrylonitrile was added in the test at 1.0C, but at 3.0C In the test, the result was inferior to that of the test cell of Example 7 with a difference of 100 mAhg ⁻¹ or more. This is probably because in Example 3, the sulfur-modified polyacrylonitrile did not have the effect of adsorbing lithium polysulfide.

Therefore, it was found that a lithium-sulfur battery with better battery performance can be obtained by adding sulfur-modified polyacrylonitrile.

As described above, according to the configuration of the present invention, it is possible to provide a positive electrode for a lithium-sulfur battery and a lithium-sulfur battery that can simultaneously achieve an increase in discharge capacity and an improvement in charge-discharge cycle characteristics, especially when a high current is applied. .

DESCRIPTION OF SYMBOLS 1... Lithium sulfur battery, 2... Positive electrode, 21... Positive electrode collector, 22... Sulfur mixture layer, 23... Ceramic layer, 3... Negative electrode, 31... Negative electrode collector, 32... Negative electrode layer, 4... Separator

Claims (9)

A positive electrode for a lithium-sulfur battery,
a positive electrode current collector;
a sulfur composite layer comprising sulfur and/or a sulfur compound, a binder and a lithium-containing oxide;
a ceramic layer containing ceramic powder and a binder;
The sulfur composite layer is laminated on the surface of the current collector, the ceramic layer is laminated on the surface of the sulfur composite layer,
A positive electrode for a lithium-sulfur battery, wherein the content of the lithium-containing oxide is 1% by weight or more and 20% by weight or less with respect to the weight of the sulfur compound layer. 2. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the content of said lithium-containing oxide is 5% by weight or more and 10% by weight or less with respect to the weight of said sulfur compound layer. 3. The positive electrode for a lithium-sulfur battery according to claim 1, wherein the lithium-containing oxide is a lithium-titanium composite oxide. The positive electrode for a lithium-sulfur battery according to any one of claims 1 to 3, wherein the lithium-containing oxide is Li ₄ Ti ₅ O ₁₂ . 1. The ceramic powder is composed of a compound that charges and discharges within a potential range of more than 1.0 V (vs. Li/Li ⁺ ) and less than 3.0 V (vs. Li/Li ⁺ ). 5. The positive electrode for a lithium-sulfur battery according to any one of 4. The positive electrode for a lithium-sulfur battery according to any one of claims 1 to 5, wherein the ceramic powder is made of titanium oxide or titanium-based composite oxide. The positive electrode for a lithium-sulfur battery according to any one of claims 1 to 6, wherein the ceramic powder comprises Li ₄ Ti ₅ O ₁₂ . The positive electrode for a lithium-sulfur battery according to any one of claims 1 to 7, wherein the sulfur compound layer contains sulfur-modified polyacrylonitrile or a derivative thereof. A lithium-sulfur battery comprising the positive electrode for a lithium-sulfur battery according to any one of claims 1 to 8, a negative electrode capable of intercalating and deintercalating lithium ions, and a separator impregnated with a non-aqueous electrolyte.

Images