lithium secondary batteries

Abstract

The present invention relates to a lithium secondary battery containing tellurium as a positive electrode additive and bis(2,2,2-trifluoroethyl)ether as an electrolyte additive, which has the effect of improving the life characteristics of the lithium secondary battery.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0042148, filed on March 31, 2021, and Korean Patent Application No. 10-2022-0024018, filed on February 24, 2022, which are hereby incorporated by reference in their entirety.

[0002] The present invention relates to a lithium secondary battery, and more particularly, to a lithium secondary battery comprising tellurium as a positive electrode additive and bis(2,2,2-trifluoroethyl)ether as an electrolyte additive. Background Art

[0003] Recently, with the rapid development of miniaturization and lightweighting of electronic products, electronic devices, communication devices, etc., and the related environmental issues, the demand for electric vehicles has increased significantly. There is also a growing demand for improved performance of secondary batteries used as energy sources for these products. Among them, lithium secondary batteries have attracted great attention as high-performance batteries due to their high energy density and high standard electrode potential.

[0004] In particular, lithium-sulfur (Li-S) batteries are secondary batteries that use a sulfur-based material with a sulfur-sulfur bond (S-S bond) as the positive electrode active material and lithium metal as the negative electrode active material. The advantage is that sulfur, the main material for the positive electrode active material, is very abundant, non-toxic, and has a low atomic weight. Furthermore, the theoretical discharge capacity of a lithium-sulfur battery is 1675 mAh / g sulfur, and its theoretical energy density is 2600 Wh / kg. Because the theoretical energy density of a lithium-sulfur battery is much higher than that of other battery systems currently under research (Ni-MH battery: 450 Wh / kg, Li-FeS battery: 480 Wh / kg, Li-MnO2 battery: 1000 Wh / kg, Na-S battery: 800 Wh / kg), the lithium-sulfur battery is the most promising battery developed to date.

[0005] During the discharge of a lithium-sulfur battery, an oxidation reaction of lithium occurs at the negative electrode, and a reduction reaction of sulfur occurs at the positive electrode. Sulfur has a cyclic S8 structure before discharge. During the reduction reaction (discharge), the oxidation number of S decreases as the SS bond breaks, and during the oxidation reaction (charge), as the SS bond is re-formed, an oxidation-reduction reaction in which the oxidation number of S increases is used to store and generate electrical energy. During this reaction, sulfur is converted from the cyclic S8 structure to lithium polysulfide (Li2S) having a linear structure through a reduction reaction. x, x = 8, 6, 4, 2). Ultimately, when the lithium polysulfide is completely reduced, lithium sulfide (Li2S) is ultimately produced. Through the process of reduction to various lithium polysulfides, the discharge behavior of lithium-sulfur batteries is characterized by a step-wise discharge voltage that is different from that of ordinary lithium-ion batteries.

[0006] However, the biggest obstacle to the commercialization of lithium-sulfur batteries is their lifespan. During the charge / discharge process, the charge / discharge efficiency decreases and the battery life is shortened. This shortened lifespan is due to various factors, such as electrolyte side reactions (deposition of byproducts due to decomposition of the electrolyte), lithium metal instability (growth of dendrites on the lithium anode, resulting in short circuits), and deposition of cathode byproducts (lithium polysulfide dissolution from the cathode).

[0007] Specifically, in batteries using sulfur compounds as the positive electrode active material and alkali metals such as lithium as the negative electrode active material, lithium polysulfide dissolution and shuttling occur during charge / discharge, with the lithium polysulfide migrating to the negative electrode, thereby reducing the capacity of the lithium-sulfur battery. Consequently, lithium-sulfur batteries suffer from a shortened lifespan and reduced reactivity. Specifically, because lithium polysulfide leached from the positive electrode has high solubility in organic electrolytes, it can undesirably migrate through the electrolyte toward the negative electrode (PS shuttling). As a result, capacity decline occurs due to irreversible loss of the positive electrode active material, and the battery life is shortened due to the deposition of sulfur particles on the surface of the lithium metal due to side reactions.

[0008] In addition, because lithium metal easily reacts with the electrolyte due to its high chemical / electrochemical reactivity, a passivation layer is formed on the surface of the negative electrode. Because the mechanical strength of this passivation layer is low, its structure collapses as the battery is charged / discharged, resulting in differences in current density in local areas, thereby forming lithium dendrites on the surface of the lithium metal. In addition, the lithium dendrites formed in this way can cause internal short circuits in the battery and inert lithium (dead lithium), resulting in not only increased physical and chemical instability of the lithium-sulfur battery but also reduced battery capacity and shortened cycle life.

[0009] To address this issue and improve the lifespan characteristics of lithium-sulfur batteries, efforts are underway to form a coating on the surface of cathode particles to prevent the dissolution of lithium polysulfides, use cathode additives capable of absorbing lithium polysulfides, form an oxide film on the lithium anode to control shuttle reactions, and use functional electrolytes with novel compositions to inhibit the dissolution of polysulfides into the electrolyte. However, these approaches are somewhat complex. Therefore, there is a need for new technologies that can address these issues and improve the lifespan characteristics of lithium-sulfur batteries.

[0010] Prior art literature

[0011] [Patent Document]

[0012] Korean Patent Publication No. 10-2017-0121047 Summary of the Invention

[0013] [Technical Issues]

[0014] Therefore, the inventors of the present invention conducted various studies to solve the above-mentioned problems, and as a result, confirmed that when the positive electrode additive of the lithium secondary battery contains tellurium (Te) and the electrolyte additive contains bis(2,2,2-trifluoroethyl) ether (BTFE), the life characteristics of the lithium secondary battery are improved, thereby completing the present invention.

[0015] Therefore, an object of the present invention is to provide a lithium secondary battery capable of achieving excellent life characteristics.

[0016] [Technical solution]

[0017] In order to achieve the above object, the present invention provides a lithium secondary battery, which comprises: a positive electrode; a negative electrode; a separator between the positive electrode and the negative electrode; and an electrolyte.

[0018] Wherein, the positive electrode comprises a positive electrode active material and a positive electrode additive,

[0019] Wherein, the positive electrode additive comprises tellurium,

[0020] Wherein, the electrolyte comprises lithium salt, organic solvent and electrolyte additive, and

[0021] Wherein, the electrolyte additive comprises bis(2,2,2-trifluoroethyl) ether.

[0022] The positive electrode may include a current collector and a positive electrode active material layer disposed on at least one surface of the current collector, wherein the positive electrode active material layer may include the positive electrode active material and tellurium.

[0023] The content of tellurium may be 1 to 10 wt % based on 100 wt % of the total positive electrode active material layer.

[0024] The positive active material may include at least one selected from elemental sulfur and a sulfur compound.

[0025] The content of bis(2,2,2-trifluoroethyl)ether may be 1 to 20 volume % based on 100 volume % of the total electrolyte.

[0026] The organic solvent may include cyclic ethers and acyclic ethers.

[0027] The cyclic ether may include at least one selected from the group consisting of furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-propylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5-dimethylfuran, pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzofuran, 2-(2-nitrovinyl)furan, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, tetrahydropyran, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, and 1,4-dimethoxybenzene.

[0028] The acyclic ether may include at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, ethylene glycol diethyl ether, and ethylene glycol ethyl methyl ether.

[0029] The organic solvent comprises 2-methylfuran and dimethoxyethane.

[0030] The lithium salt may include at least one selected from the group consisting of LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiB(Ph)4, LiC4BO8, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(SO2F)2, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate and lithium imide.

[0031] The lithium secondary battery may be a lithium-sulfur battery.

[0032] [Beneficial Effects]

[0033] The lithium secondary battery of the present invention contains tellurium as a positive electrode additive and bis(2,2,2-trifluoroethyl)ether as an electrolyte additive, thereby improving the efficiency of the negative electrode containing lithium metal, inhibiting the dissolution of lithium polysulfide, and forming a protective layer on the surface of the negative electrode, i.e., the lithium metal, thereby inhibiting the formation of lithium dendrites. It can also reduce side reactions with the electrolyte or lithium polysulfide on the negative electrode surface and the corresponding decomposition of the electrolyte. Therefore, the number of cycles required to reach 80% of the initial discharge capacity of the lithium secondary battery can be extended, thereby improving the life characteristics of the lithium secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a graph showing the life characteristics of the lithium-sulfur batteries of Examples 1 to 3 and Comparative Examples 1 to 3.

[0035] Figure 2 This is a graph showing the evaluation of the life characteristics of the lithium-sulfur battery of Example 4.

[0036] Figure 3 This is a graph showing the cycle voltage of the lithium-sulfur battery of Example 1.

[0037] Figure 4 This is a graph showing the cycle voltage of the lithium-sulfur battery of Example 4.

[0038] Figure 5 This is a graph showing the life characteristics of the lithium-sulfur battery of Example 5.

[0039] Figure 6 This is a graph showing the initial coulombic efficiency of the lithium-sulfur battery of Example 3.

[0040] Figure 7 Graph showing the initial coulombic efficiencies of the lithium-sulfur batteries of Examples 1 to 3 and Comparative Example 3. DETAILED DESCRIPTION

[0041] Hereinafter, the present invention will be described in more detail.

[0042] The terms and words used in this specification and claims should not be construed as limited to ordinary or dictionary terms, and should be construed as meanings and concepts consistent with the technical idea of ​​the invention on the basis of the principle that the inventor can appropriately define the concepts of the terms to describe his invention in the best possible manner.

[0043] The terms used herein are only used to describe the purpose of specific embodiments and are not intended to limit the present invention. Unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" include plural referents. It should be understood that terms such as "comprising" or "having" as used in this specification are intended to indicate the presence of the described features, values, steps, operations, components, parts or combinations thereof, but do not exclude the possibility of the presence or addition of one or more other features, values, steps, operations, components, parts or combinations thereof.

[0044] As used herein, the term "composite material" refers to a material in which two or more materials are combined to exhibit more effective functions while forming phases that are physically and chemically different from each other.

[0045] As used herein, the term "polysulfide" includes "polysulfide ions (S x 2- , x=8, 6, 4, 2)" and "lithium polysulfide (Li2S x or LiS x - , x=8, 6, 4, 2)” concept.

[0046] Among several secondary batteries, lithium-sulfur batteries exhibit high theoretical discharge capacity and theoretical energy density and are attracting attention as next-generation secondary batteries due to the advantages that sulfur, mainly used as a positive electrode active material, is abundant in resources, inexpensive, and environmentally friendly.

[0047] Because lithium-sulfur batteries have higher theoretical discharge capacity and theoretical energy density among several secondary batteries, and lithium metal, which is mainly used as the negative electrode active material, has a very small atomic weight (6.94 g / au) and density (0.534 g / cm 3 ), thus lithium-sulfur batteries have become the focus of next-generation batteries due to their easy miniaturization and lightweighting.

[0048] However, as mentioned above, because lithium metal has high reactivity and easily reacts with electrolytes, a passivation film is formed on the surface of lithium metal due to the spontaneous decomposition of the electrolyte, which causes uneven electrochemical reactions on the surface of lithium metal, thereby forming inert lithium and lithium dendrites, thereby reducing the efficiency and stability of the negative electrode. In addition, in lithium-sulfur batteries using sulfur-based materials as positive electrode active materials, lithium polysulfide (Li2S) formed in the positive electrode during battery operation is x , x = 8, 6, 4, 2), lithium polysulfide (Li2S x, typically x>4) due to its high solubility in the electrolyte, it continuously dissolves and dissolves outside the reaction zone of the positive electrode and moves toward the negative electrode. At this time, the lithium polysulfide dissolved from the positive electrode undergoes a side reaction with the lithium metal, causing the lithium sulfide to adhere to the surface of the lithium metal, resulting in electrode passivation. The dissolution of the lithium polysulfide also reduces the utilization rate of sulfur, resulting in a maximum of only about 70% of the theoretical discharge capacity. Moreover, as the cycle progresses, there is a problem of a sharp decline in capacity and charge / discharge efficiency, thereby reducing the life characteristics of the battery.

[0049] To this end, in the prior art, in order to ensure uniform reactivity on the surface of the lithium metal and suppress the growth of lithium dendrites, attempts have been made to form a protective layer on the surface of the lithium metal or to change the composition of the electrolyte. However, when a protective layer is introduced on the surface of the lithium metal, both high mechanical strength for suppressing lithium dendrites and high ion conductivity for transferring lithium ions are required. However, mechanical strength and ion conductivity are in a trade-off relationship, making it difficult to improve both mechanical strength and ion conductivity at the same time. Therefore, the effect of the currently proposed improvement of the stability of lithium in the lithium metal protective layer is not excellent. In addition, due to problems with compatibility with other components constituting the battery, practical application is not easy, as this will cause serious problems in terms of battery performance and operational stability.

[0050] Therefore, in the present invention, an attempt is made to improve the life characteristics of lithium secondary batteries, preferably lithium-sulfur batteries, by incorporating tellurium as a positive electrode additive and bis(2,2,2-trifluoroethyl)ether as an electrolyte additive to suppress the dissolution of lithium polysulfide, and forming a protective film (solid electrolyte interface, SEI layer) on the surface of lithium metal as the negative electrode during the initial discharge stage, thereby solving the above-mentioned problems.

[0051] The present invention relates to a lithium secondary battery, comprising a positive electrode, a negative electrode, a separator between the positive electrode and the negative electrode, and an electrolyte.

[0052] Wherein, the positive electrode comprises a positive electrode active material and a positive electrode additive,

[0053] Wherein, the positive electrode additive comprises tellurium (Te),

[0054] Wherein, the electrolyte comprises lithium salt, organic solvent and electrolyte additive, and

[0055] Wherein, the electrolyte additive includes bis(2,2,2-trifluoroethyl) ether (BTFE).

[0056] The positive electrode may include a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, and the positive electrode active material layer includes a positive electrode active material and a positive electrode additive.

[0057] The positive electrode additive of the present invention contains tellurium (Te).

[0058] Tellurium reacts with lithium polysulfide to form polytellurium sulfide ions (S x Te y 2- ), and polytellurium sulfide ions dissolve in the electrolyte and migrate to the lithium metal serving as the negative electrode, thereby contributing to the formation of a negative electrode protective layer made of lithium telluride sulfide or lithium telluride. Consequently, an improved stripping / plating process can be performed on the surface of the lithium metal serving as the negative electrode. This protective layer allows for more dense plating of the lithium metal and has the effect of suppressing unnecessary decomposition of the electrolyte or loss of lithium. Consequently, the efficiency and stability of the negative electrode are improved, thereby enhancing the lifespan characteristics of a lithium secondary battery, preferably a lithium-sulfur battery, containing the negative electrode.

[0059] In the positive electrode of the lithium secondary battery of the present invention, the content of tellurium as a positive electrode additive may be 1 to 10 weight %, preferably 3 to 7 weight %, based on a total of 100 weight % of the positive electrode active material layer. When the tellurium content is within the range of 1 to 10 weight %, the initial coulombic efficiency may decrease as the tellurium content increases, but even if the initial coulombic efficiency decreases, the life characteristics of the lithium-sulfur battery can be improved. If the tellurium content is less than 1 weight %, the effect of improving the life characteristics of the lithium secondary battery is not obvious. If the tellurium content exceeds 10 weight %, the initial coulombic efficiency decreases excessively, and thus there may be a problem of deterioration of the life characteristics.

[0060] The positive electrode current collector supports the positive electrode active material and is not particularly limited, as long as it has high conductivity and does not cause chemical changes in the battery. Examples of the positive electrode current collector include copper, stainless steel, aluminum, nickel, titanium, palladium, sintered carbon; copper or stainless steel surface-treated with carbon, nickel, silver, or the like; and aluminum-cadmium alloys.

[0061] The positive electrode current collector can improve bonding strength with the positive electrode active material by having fine irregularities on its surface, and can be formed in various forms such as a film, sheet, foil, screen, mesh, porous body, foam, or nonwoven fabric.

[0062] The positive electrode active material may include at least one selected from elemental sulfur (S8) and sulfur compounds. Preferably, the positive electrode active material may include at least one selected from inorganic sulfur, Li2S n (n≥1), disulfide compounds, organic sulfur compounds and carbon-sulfur polymers ((C2S x ) n , x=2.5 to 50, n≥2). Most preferably, the positive electrode active material may contain inorganic sulfur.

[0063] Therefore, the lithium secondary battery of the present invention may be a lithium-sulfur battery.

[0064] Sulfur contained in the positive electrode active material is used in combination with a conductive material such as a carbon material because it does not have conductivity alone. Therefore, sulfur is contained in the form of a sulfur-carbon composite material. Preferably, the positive electrode active material can be a sulfur-carbon composite material.

[0065] The sulfur-carbon composite material includes a porous carbon material, which not only provides a framework capable of uniformly and stably fixing the above-mentioned sulfur, but also compensates for the low electrical conductivity of sulfur, allowing electrochemical reactions to proceed smoothly.

[0066] Porous carbon materials can generally be prepared by carbonizing various carbonaceous precursors. The porous carbon material may contain non-uniform pores with an average pore diameter ranging from 1 to 200 nm, and a porosity ranging from 10 to 90% of the total volume of the porous carbon material. If the average pore diameter is smaller than this range, the pore size is only at the molecular level, and sulfur impregnation is impossible. Conversely, if the average pore diameter exceeds this range, the mechanical strength of the porous carbon material is weakened, which is not preferred for electrode manufacturing processes.

[0067] The shape of the porous carbon material is in the form of spheres, rods, needles, plates, tubes, or blocks, and can be used without limitation as long as it is generally used in lithium-sulfur batteries.

[0068] The porous carbon material may have a porous structure or a high specific surface area, and may be any of the porous carbon materials conventionally used in the art. For example, the porous carbon material may be, but is not limited to, at least one selected from the following: graphite; graphene; carbon black such as Dancar black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); and graphites such as natural graphite, artificial graphite, and expanded graphite; and activated carbon. Preferably, the porous carbon material may be a carbon nanotube.

[0069] In the sulfur-carbon composite material, sulfur is located on at least one of the inner and outer surfaces of the porous carbon material. For example, sulfur may be present in an area of ​​less than 100%, preferably 1 to 95%, and more preferably 40 to 96% of the entire inner and outer surfaces of the porous carbon material. When sulfur is present on the inner and outer surfaces of the porous carbon material within the above range, the maximum effect can be exhibited in terms of the electron migration area and wettability with the electrolyte. Specifically, because sulfur is thinly and uniformly impregnated on the inner and outer surfaces of the porous carbon material within the area within the above range, the contact area for electron migration can be increased during the charge / discharge process. If sulfur is located in an area of ​​100% of the entire inner and outer surfaces of the porous carbon material, the porous carbon material is completely covered with sulfur, resulting in poor wettability to the electrolyte and reduced contact, making it unable to receive electrons and thus unable to participate in the electrochemical reaction.

[0070] Based on 100 wt % of the sulfur-carbon composite material, the sulfur-carbon composite material may contain 65 to 90 wt %, preferably 70 to 85 wt %, and more preferably 72 to 80 wt % of sulfur. If the sulfur content is lower than the above range, as the content of the porous carbon material in the sulfur-carbon composite material increases relatively, the specific surface area increases, thereby increasing the content of the binder during the manufacture of the positive electrode. The increase in the amount of binder may eventually increase the sheet resistance of the positive electrode and act as an insulator that prevents electrons from passing through, thereby deteriorating the performance of the battery. On the contrary, if the sulfur content exceeds the above range, the sulfur that cannot be combined with the porous carbon material accumulates therein or dissolves back into the surface of the porous carbon material, thereby becoming difficult to receive electrons and unable to participate in the electrochemical reaction, resulting in a loss of battery capacity.

[0071] In the present invention, the method for preparing the sulfur-carbon composite material of the present invention is not particularly limited, and a method commonly used in the art can be used. For example, a method of simply mixing sulfur and a porous carbon material and then heat-treating the mixture to form a composite can be used.

[0072] In addition to the above components, the positive electrode active material may further include at least one selected from the group consisting of transition metal elements, Group IIIA elements, Group IVA elements, sulfur compounds of these elements, and alloys of these elements with sulfur.

[0073] The transition metal elements may include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au, Hg, etc., the Group IIIA elements may include Al, Ga, In, Tl, etc., and the Group IVA elements may include Ge, Sn, Pb, etc.

[0074] In the positive electrode for lithium secondary batteries of the present invention, the content of the positive electrode active material may be 50 to 95 weight % based on a total of 100 weight % of the positive electrode active material layer constituting the positive electrode for lithium secondary batteries. In the content of the positive electrode active material, based on a total of 100 weight % of all positive electrode active material layers, the lower limit may be 70 weight % or more or 85 weight % or more, and the upper limit may be 99 weight % or less or 90 weight % or less. The content of the positive electrode active material can be set by a combination of a lower limit value and an upper limit value. If the content of the positive electrode active material is lower than the above range, it is difficult for the positive electrode to fully perform an electrochemical reaction. On the contrary, if the content exceeds the above range, there is a problem of deterioration of the physical properties of the electrode due to a relatively insufficient content of the binder.

[0075] Furthermore, the positive electrode active material layer may further include a binder and a conductive material in addition to the positive electrode active material and tellurium.

[0076] The binder may additionally be used to well adhere the positive active material and tellurium to the positive current collector.

[0077] The binder holds the positive electrode active material in the positive electrode current collector and organically connects the positive electrode active materials to improve binding force therebetween, and any binder known in the art may be used.

[0078] For example, the binder can be, but is not limited to, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyacrylic acid metal salt (metal-PAA), polymethacrylic acid (PMA), polymethyl methacrylate (PMMA), polyacrylamide (PAM), polymethacrylamide, polyacrylonitrile (PAN), polymethacrylonitrile, polyimide (PI), chitosan, starch, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, hydroxypropyl cellulose, regenerated cellulose, various copolymers thereof, and the like.

[0079] Based on a total of 100% by weight of the positive electrode active material layer constituting the positive electrode for a lithium secondary battery, the content of the binder may be 1 to 10% by weight. If the content of the binder is less than the above range, the physical properties of the positive electrode may deteriorate, whereby the positive electrode active material may fall off. If the content of the binder exceeds the above range, the proportion of the positive electrode active material in the positive electrode is relatively reduced, thereby reducing the capacity of the battery. Therefore, it is preferred that the content of the binder is appropriately determined within the above range.

[0080] Furthermore, a conductive material may be additionally used to further improve the conductivity of the positive electrode active material.

[0081] The conductive material is a material that serves as a path for electrons to migrate from the current collector to the positive electrode active material by electrically connecting the electrolyte and the positive electrode active material. The conductive material can be used without limitation as long as it has conductivity.

[0082] For example, as the conductive material, the following substances can be used alone or in combination: graphite such as natural graphite or artificial graphite; carbon black such as Super-P, Danka black, acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal black; carbon derivatives such as carbon nanotubes and fullerenes; conductive fibers such as carbon fibers and metal fibers; carbon fluoride; metal powders such as aluminum powder and nickel powder; or conductive polymers such as polyaniline, polythiophene, polyacetylene and polypyrrole.

[0083] Based on a total of 100% by weight of the positive electrode active material layer constituting the positive electrode, the content of the conductive material may be 1 to 10% by weight, preferably 4 to 7% by weight. If the content of the conductive material is lower than the above range, it is difficult for electrons to migrate between the positive electrode active material and the current collector, thereby reducing the voltage and capacity. On the contrary, if the content exceeds the above range, the proportion of the positive electrode active material is relatively reduced, thereby possibly reducing the total energy (charge) of the battery. Therefore, it is preferred that the content of the conductive material is determined to be an appropriate content within the above range.

[0084] In the present invention, the method of manufacturing the positive electrode is not particularly limited, and various methods known to those skilled in the art or various methods modified therefrom can be used.

[0085] For example, the positive electrode may be prepared by preparing a slurry composition for a positive electrode including the above-mentioned components and then applying the slurry composition to at least one surface of a positive electrode current collector.

[0086] The slurry composition for a positive electrode includes the above-mentioned positive electrode active material and tellurium, and may further include a binder, a conductive material, and a solvent.

[0087] As a solvent, a solvent that can uniformly disperse the positive electrode active material, tellurium, conductive material and binder is used. This solvent is an aqueous solvent, most preferably water, and in this case, water can be distilled water or deionized water. However, it is not necessarily limited to this, and if necessary, lower alcohols that can be easily mixed with water can be used. The example of lower alcohol includes methanol, ethanol, propanol, isopropanol and butanol, and preferably, these lower alcohols can be used in combination with water.

[0088] The content of the solvent may be a level having a concentration that allows easy coating, and the specific content varies depending on the coating method and equipment.

[0089] If necessary, the slurry composition for the positive electrode may further contain materials generally used to improve its functions required in the relevant technical fields, for example, viscosity modifiers, fluidizers, fillers, etc. can be mentioned.

[0090] In the present invention, the method of applying the positive electrode slurry composition is not particularly limited, and methods such as a doctor blade method, a die casting method, a comma coating method, and a screen printing method can be used. In addition, after forming on a separate substrate, the positive electrode slurry can be applied to the positive electrode current collector by pressing or laminating.

[0091] After coating, a drying process for removing the solvent may be performed. The drying process is carried out at a temperature and time at a level at which the solvent can be fully removed, and the conditions may vary depending on the type of solvent, and thus are not particularly limited in the present invention. Examples of drying methods may include: a drying method using warm air, hot air or low-humidity air; a vacuum drying method; and a drying method by irradiating (far) infrared rays or electron beams. The drying rate is generally adjusted in a manner that allows the solvent to be removed as quickly as possible within a speed range that does not cause the positive electrode active material layer to crack due to stress concentration and does not cause the positive electrode active material layer to be delaminated from the positive electrode collector.

[0092] In addition, after drying, the density of the positive electrode active material in the positive electrode can be increased by pressing the current collector. As a pressing method, methods such as mold pressing and roll pressing are mentioned.

[0093] The electrolyte contains lithium salt, organic solvent and electrolyte additives.

[0094] Electrolyte additives include bis(2,2,2-trifluoroethyl) ether (BTFE).

[0095] Bis(2,2,2-trifluoroethyl)ether has low solubility for lithium polysulfide. Therefore, bis(2,2,2-trifluoroethyl)ether inhibits the dissolution of lithium polysulfide and can form a protective layer on the negative electrode surface during the initial discharge phase of lithium secondary batteries, specifically lithium-sulfur batteries. This effectively inhibits side reactions between lithium polysulfide and the lithium metal negative electrode, reducing the shuttling phenomenon caused by lithium polysulfide, thereby improving the lifespan characteristics of lithium-sulfur batteries.

[0096] The content of bis(2,2,2-trifluoroethyl)ether may be 1 to 20 volume %, preferably 5 to 15 volume %, and more preferably 7 to 12 volume %, based on 100 volume % of the total electrolyte.

[0097] If the content of bis(2,2,2-trifluoroethyl)ether is less than 1% by volume, the effect of improving the life characteristics of lithium-sulfur batteries is not significant. If the content of bis(2,2,2-trifluoroethyl)ether exceeds 20% by volume, overvoltage may occur, which may lead to problems such as reduced high-rate discharge capacity and output characteristics.

[0098] The total 100% by volume of the electrolyte refers to the volume of the liquid excluding the lithium salt.

[0099] A lithium salt is contained as an electrolyte salt of the electrolytic solution, and the type of the lithium salt is not particularly limited in the present invention, and a lithium salt can be used without limitation as long as it can be generally used in lithium-sulfur batteries.

[0100] For example, the lithium salt may include at least one selected from the group consisting of LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiB(Ph)4, LiC4BO8, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(SO2F)2, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate and lithium imide, preferably, the lithium salt can be (SO2F)2NLi (lithium bis(fluorosulfonyl)imide, LiFSI).

[0101] The concentration of the lithium salt can be determined based on ion conductivity, solubility, and other factors, and can be, for example, 0.1 to 4.0 M, preferably 0.5 to 2.0 M. If the lithium salt concentration is below the above range, it is difficult to ensure ion conductivity suitable for battery operation. On the other hand, if the concentration exceeds the above range, the viscosity of the electrolyte increases, thereby reducing the mobility of lithium ions, and the decomposition reaction of the lithium salt itself may be accelerated, thereby degrading battery performance. Therefore, the concentration is appropriately adjusted within the above range.

[0102] The organic solvent is a medium through which ions participating in the electrochemical reaction of the lithium secondary battery can move, and contains an organic solvent.

[0103] Organic solvents include cyclic ethers and acyclic ethers.

[0104] The ether compound ensures electrochemical stability within the operating voltage range of the battery while maintaining the solubility of sulfur or sulfur compounds, and has relatively few side reactions with intermediate products during battery operation.

[0105] The cyclic ether may include at least one selected from the group consisting of furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-propylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5-dimethylfuran, pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzofuran, 2-(2-nitrovinyl)furan, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, tetrahydropyran, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, and 1,4-dimethoxybenzene. Preferably, it may include 2-methylfuran.

[0106] Dioxolanes, which are primarily used in cyclic ethers, have very high solubility in lithium polysulfide, which can easily cause shuttling, accelerate the loss of sulfur as a positive electrode active material, and potentially reduce the lifespan of lithium secondary batteries. Therefore, dioxolanes are not preferably used as organic solvents in the lithium secondary battery electrolyte of the present invention.

[0107] The acyclic ether may include at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, ethylene glycol diethyl ether and ethylene glycol ethyl methyl ether, preferably dimethyl ether.

[0108] The cyclic ether and the acyclic ether may be mixed and used in a volume ratio of 1:9 to 9:1, preferably 1:2 to 1:5.

[0109] The electrolyte solution for lithium secondary batteries of the present invention may preferably contain a lithium salt, 2-methylfuran, dimethoxyethane, and bis(2,2,2-trifluoroethyl)ether.

[0110] In addition, the electrolyte for lithium secondary batteries of the present invention may further contain an organic solvent commonly used in lithium secondary battery electrolytes. For example, the electrolyte may further contain at least one selected from ester compounds, amide compounds, linear carbonate compounds, and cyclic carbonate compounds.

[0111] The ester compound may include, but is not limited to, at least one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0112] The linear carbonate compound may include, but is not limited to, at least one selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylmethyl carbonate, methylpropyl carbonate, and ethylpropyl carbonate.

[0113] The cyclic carbonate compound may include, but is not limited to, at least one selected from the group consisting of ethylene carbonate, propylene carbonate, 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinyl ethylene carbonate, and halides thereof.

[0114] In addition to the above components, the electrolyte for lithium-sulfur batteries of the present invention may further contain nitric acid or nitrous acid compounds. Nitric acid or nitrous acid compounds have the function of forming a stable film on the lithium metal electrode as the negative electrode and improving the charge / discharge efficiency.

[0115] In the present invention, the nitric acid or nitrous acid compound is not particularly limited, but can be at least one selected from the following: inorganic nitric acid or nitrous acid compounds, such as lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), barium nitrate (Ba(NO3)2), ammonium nitrate (NH4NO3), lithium nitrite (LiNO2), potassium nitrite (KNO2), cesium nitrite (CsNO2), ammonium nitrite (NH4NO2); organic nitric acid or nitrous acid compounds, such as methyl nitrate, dialkyl imidazole nitrate, Guanidine nitrate, imidazole nitrate Pyridine nitrate Ethyl nitrite, propyl nitrite, butyl nitrite, amyl nitrite, octyl nitrite; organic nitro compounds, such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, dinitrotoluene and combinations thereof, preferably lithium nitrate.

[0116] In addition, the electrolyte of the present invention may further contain other additives for the purpose of improving charge / discharge characteristics, flame retardancy, etc. Examples of the additives may include pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, glycol dimethyl ethers, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted Oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, fluoroethylene carbonate (FEC), propene sultone (PRS) and vinylene carbonate (VC).

[0117] Depending on the manufacturing process and required performance of the final product, the electrolyte injection can be performed at an appropriate stage of the manufacturing process of the electrochemical device. That is, the electrolyte injection can be applied before assembling the electrochemical device or in the final stage of assembling the electrochemical device.

[0118] The negative electrode for a lithium secondary battery of the present invention may include a negative electrode current collector and a negative electrode active material layer coated on one or both surfaces of the negative electrode current collector.

[0119] The negative electrode current collector is used to support the negative electrode active material layer and is as described in the positive electrode current collector.

[0120] In addition to the negative electrode active material, the negative electrode active material layer may further include a conductive material, a binder, etc. In this case, the conductive material and the binder are as described above.

[0121] The negative electrode active material may include: a material capable of reversibly inserting or extracting lithium ions (Li + ) material; a material capable of reversibly forming a lithium-containing compound by reacting with lithium ions; or lithium metal or a lithium alloy.

[0122] Able to reversibly insert or deinsert lithium ions (Li + ) can be, for example, crystalline carbon, amorphous carbon or a mixture thereof. + ) to reversibly form a lithium-containing compound may be, for example, tin oxide, titanium nitrate, or silicon. The lithium alloy may be, for example, an alloy of lithium (Li) and a metal selected from the group consisting of sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra), aluminum (Al), and tin (Sn).

[0123] Preferably, the negative electrode active material may be lithium metal, specifically, may be in the form of a lithium metal thin film or lithium metal powder.

[0124] The method for forming the negative electrode active material is not particularly limited, and methods for forming a layer or film commonly used in the art can be used. For example, methods such as compression, coating, or deposition can be used. In addition, the negative electrode of the present invention also includes the case where a metallic lithium thin film is formed on the metal plate by initial charging after assembling the battery without a lithium thin film in the current collector.

[0125] A separator may be located between the positive electrode and the negative electrode.

[0126] The separator separates or insulates the positive electrode and the negative electrode from each other and enables lithium ions to be transferred between the positive electrode and the negative electrode, and can be made of a porous non-conductive or insulating material. The separator can be used without any particular restrictions as long as it is used as a separator in a lithium secondary battery. Such a separator may be an independent component such as a film, or may be a coating added to the positive electrode and / or the negative electrode.

[0127] As the separator, one having excellent impregnation ability for the electrolyte and low resistance to ion migration in the electrolyte solution is preferred.

[0128] The separator may be made of a porous substrate. Any porous substrate can be used as long as it is a porous substrate commonly used in secondary batteries. A porous polymer film may be used alone or in the form of a laminate. For example, a non-woven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber or a polyolefin-based porous film may be used, but is not limited thereto.

[0129] In the present invention, the material of the porous substrate is not particularly limited, and any material can be used as long as it is a porous substrate commonly used in electrochemical devices. For example, the porous substrate may include at least one material selected from the group consisting of polyolefins (such as polyethylene and polypropylene), polyesters (such as polyethylene terephthalate and polybutylene terephthalate), polyamides, polyacetals, polycarbonates, polyimides, polyetheretherketones, polyethersulfones, polyphenylene oxides, polyphenylene sulfides, polyethylene naphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, poly(p-phenylene benzobis[theta]yl)pyrrolidone ... azoles) and polyarylates.

[0130] The thickness of the porous substrate is not particularly limited, but may be 1 to 100 μm, preferably 5 to 50 μm. Although the thickness range of the porous substrate is not particularly limited to the above range, if the thickness is much thinner than the above lower limit, the mechanical properties deteriorate, and thus the separator may be easily damaged during use of the battery.

[0131] The average diameter and porosity of the pores present in the porous substrate are also not particularly limited, but may be 0.001 μm to 50 μm and 10 to 95%, respectively.

[0132] The lithium secondary battery according to the present invention can be manufactured through lamination, stacking, and folding processes of the separator and electrodes in addition to a general winding process.

[0133] The shape of the lithium secondary battery is not particularly limited, and may be various shapes such as a cylindrical shape, a stacked shape, and a coin shape.

[0134] The lithium secondary battery of the present invention, particularly a lithium-sulfur battery, contains tellurium as a positive electrode additive. Tellurium can help form a protective layer on the negative electrode surface during the initial discharge phase of the lithium-sulfur battery, thereby improving the stripping / plating process on the negative electrode surface. As a result, the efficiency and stability of the negative electrode can be improved, thereby improving the lifespan characteristics of the lithium-sulfur battery.

[0135] Furthermore, the lithium-sulfur battery of the present invention contains bis(2,2,2-trifluoroethyl)ether as an electrolyte additive. Bis(2,2,2-trifluoroethyl)ether can inhibit the dissolution of lithium polysulfide and effectively suppress side reactions between lithium polysulfide and lithium metal as the negative electrode by forming a protective layer on the negative electrode surface during the initial discharge phase of the lithium-sulfur battery, thereby improving the lifespan characteristics of the lithium-sulfur battery.

[0136] Therefore, the lithium secondary battery of the present invention has an effect of improving life characteristics. Specifically, the number of cycles in which the discharge capacity reaches 80% of the initial discharge capacity can be prolonged.

[0137] Preferred Implementation

[0138] Hereinafter, preferred embodiments of the present invention will be described to facilitate understanding of the present invention. However, it will be apparent to those skilled in the art that the following embodiments are provided to illustrate the present invention, and that various modifications and variations can be made within the scope and spirit of the present invention, and naturally, these variations and variations are within the scope of the appended claims.

[0139] <Preparation of lithium-sulfur batteries>

[0140] Example 1

[0141] A sulfur-carbon (CNT) composite (S:C = 75:25 (weight ratio)) serving as the positive electrode active material was mixed with a 3 wt% aqueous lithium-polyacrylic acid (Li-PAA) solution to prepare a mixed solution. Tellurium (Te) powder was then added to the mixed solution. At this point, the weight ratio of the sulfur-carbon composite, the solid content of the lithium-polyacrylic acid, and the tellurium was 90:5:5. Additional water was added and mixed to prepare a positive electrode slurry with a solid content of 32 wt%.

[0142] The positive electrode slurry was coated on an aluminum foil current collector and then coated to a specific thickness using a Mathis coater (Mathis Switzerland, SV-M). Thereafter, the positive electrode was prepared by drying at a temperature of 50° C. for 2 hours.

[0143] The positive electrode loading capacity is 3.3 mAh / cm 2 , and the porosity is 73%.

[0144] 0.75M LiFSI and 4 wt% lithium nitrate (LiNO3) were dissolved in an organic solvent obtained by mixing 2-methylfuran (2-MeF), bis(2,2,2-trifluoroethyl) ether (BTFE), and 1,2-dimethoxyethane (DME) in a volume ratio of 2:1:7 to prepare an electrolyte. At this time, the content of bis(2,2,2-trifluoroethyl) ether was 10% by volume based on the total volume of the electrolyte.

[0145] After the prepared positive electrode and negative electrode were placed to face each other with a polyethylene separator having a thickness of 16 μm and a porosity of 45% interposed therebetween, they were placed in an aluminum bag, which was then injected with an electrolyte and sealed to prepare a lithium-sulfur battery.

[0146] In this case, as the negative electrode, a lithium metal thin film having a thickness of 30 μm was used.

[0147] Example 2

[0148] A lithium-sulfur battery was prepared in the same manner as in Example 1, except that the weight ratio of the sulfur-carbon composite material, the solid content of lithium-polyacrylic acid, and tellurium was 85:5:10.

[0149] Example 3

[0150] A lithium-sulfur battery was prepared in the same manner as in Example 1, except that the weight ratio of the sulfur-carbon composite material, the solid content of lithium-polyacrylic acid, and tellurium was 80:5:15.

[0151] Example 4

[0152] A lithium-sulfur battery was prepared in the same manner as in Example 1, except that 2-methylfuran (2-MeF), bis(2,2,2-trifluoroethyl) ether (BTFE), and 1,2-dimethoxyethane (DME) were mixed in a volume ratio of 20:25:55.

[0153] Example 5

[0154] A lithium-sulfur battery was prepared in the same manner as in Example 1, except that 1,3-dioxolane (DOL), bis(2,2,2-trifluoroethyl) ether (BTFE), and 1,2-dimethoxyethane (DME) were mixed in a volume ratio of 40:10:50.

[0155] Comparative Example 1

[0156] A sulfur-carbon (CNT) composite (S:C = 75:25 (weight ratio)) as a positive electrode active material was mixed with a 3 wt% aqueous lithium-polyacrylic acid (Li-PAA) solution to prepare a mixed solution. The weight ratio of the sulfur-carbon composite to the lithium-polyacrylic acid was 95:5. Additional water was added and mixed to prepare a positive electrode slurry with a solid content of 32 wt%.

[0157] The positive electrode slurry was coated on an aluminum foil current collector and then coated to a specific thickness using a Mathis coater (Mathis Switzerland, SV-M). Thereafter, the positive electrode was prepared by drying at a temperature of 50° C. for 2 hours.

[0158] The positive electrode loading capacity is 3.3 mAh / cm 2 , and the porosity is 73%.

[0159] 0.75 M of LiFSI and 4 wt % of lithium nitrate (LiNO 3 ) were dissolved in an organic solvent obtained by mixing 2-methylfuran (2-MeF) and 1,2-dimethoxyethane (DME) in a volume ratio of 1:4 to prepare an electrolyte.

[0160] After the prepared positive electrode and negative electrode were placed to face each other with a polyethylene separator having a thickness of 16 μm and a porosity of 45% interposed therebetween, an electrolyte was injected to prepare a lithium-sulfur battery.

[0161] In this case, as the negative electrode, a lithium metal thin film having a thickness of 30 μm was used.

[0162] Comparative Example 2

[0163] A lithium-sulfur battery was prepared in the same manner as in Example 1, except that 0.75 M of LiFSI and 4 wt % of lithium nitrate (LiNO 3 ) were dissolved in an organic solvent obtained by mixing 2-methylfuran (2-MeF) and 1,2-dimethoxyethane (DME) in a volume ratio of 1:4 to prepare an electrolyte.

[0164] Comparative Example 3

[0165] A sulfur-carbon (CNT) composite (S:C = 75:25 (weight ratio)) as a positive electrode active material was mixed with a 3 wt% aqueous lithium-polyacrylic acid (Li-PAA) solution to prepare a mixed solution. The weight ratio of the sulfur-carbon composite to the lithium-polyacrylic acid was 95:5. Additional water was added and mixed to prepare a positive electrode slurry with a solid content of 32 wt%.

[0166] A lithium-sulfur battery was prepared in the same manner as in Example 1 except for this.

[0167] Experimental Example 1. Evaluation of Lifespan Characteristics of Lithium-Sulfur Batteries

[0168] For the batteries prepared in Examples 1 to 3 and Comparative Examples 1 to 3, performance was evaluated using a charge / discharge measurement device (PESCO5-0.01, PNE solution, Korea).

[0169] In the first three cycles, the charge / discharge capacity from 2.5 to 1.8 V was measured at a current density of 0.1 C, and from the 4th cycle onwards, the charge / discharge capacity was measured by discharging at 0.3 C and charging at 0.2 C. The capacity retention rate was measured by setting the discharge capacity of the lithium-sulfur battery of Comparative Example 1 at the 4th cycle to 100% and calculating the relative capacity of the subsequent cycles, and the discharge capacity was measured until the discharge capacity reached 80% (the number of cycles at 80%).

[0170] The results are shown in Table 1 and Figure 1 middle.

[0171] Table 1:

[0172] Number of cycles at 80% Improvement rate (%) Comparative Example 1 164 cycles - Comparative Example 2 174 cycles 6.1％ Comparative Example 3 163 cycles - Example 1 202 cycles 23.2％ Example 2 198 cycles 20.7％ Example 3 147 cycles -

[0173] According to the above results, the lithium-sulfur battery of Example 1, in which the positive electrode active material layer contained 5% by weight of tellurium and bis(2,2,2-trifluoroethyl)ether based on a total of 100% by weight, showed significantly improved lifespan characteristics. Furthermore, the lithium-sulfur battery of Example 2, in which the positive electrode active material layer contained 10% by weight of tellurium and bis(2,2,2-trifluoroethyl)ether based on a total of 100% by weight, also showed improved lifespan characteristics.

[0174] In contrast, the lithium-sulfur battery of Example 3, which contained 15% by weight of tellurium based on 100% by weight of the total positive electrode active material layer and also contained bis(2,2,2-trifluoroethyl)ether, contained tellurium in an amount exceeding the desired tellurium content range of 1 to 10% by weight, thereby exhibiting deterioration in lifespan characteristics. Furthermore, the lithium-sulfur battery of Comparative Example 2, which contained only tellurium, showed slightly improved lifespan characteristics, while the lithium-sulfur battery of Comparative Example 3, which contained only bis(2,2,2-trifluoroethyl)ether, showed no effect of improving lifespan characteristics.

[0175] Therefore, it can be seen that when tellurium is included as a positive electrode additive and bis(2,2,2-trifluoroethyl)ether is included as an electrolyte additive, and the content of tellurium is 1 to 10 wt % based on 100 wt % of the total positive electrode active material layer, the life characteristics of the lithium-sulfur battery can be improved.

[0176] Furthermore, the life characteristics of the lithium-sulfur batteries of Examples 4 and 5 were measured in the same manner as described above.

[0177] Example 4 is a lithium-sulfur battery using an electrolyte containing 25% by volume of bis(2,2,2-trifluoroethyl)ether, which shows accelerated life degradation ( Figure 2 ). In addition, the voltages of the lithium-sulfur batteries of Examples 1 and 4 at the 5th, 15th, 25th, and 35th cycles were measured. In the case of the lithium-sulfur battery of Example 1, the cycle capacity remained constant ( Figure 3 ), while in the case of the lithium-sulfur battery of Example 4, the capacity decreases as the cycle progresses, and the overvoltage at the end gradually deepens, thereby accelerating the decline in capacity ( Figure 4 ).

[0178] It can be seen from the above results that when tellurium is included as a positive electrode additive and bis(2,2,2-trifluoroethyl)ether is included as an electrolyte additive, and the content of bis(2,2,2-trifluoroethyl)ether is 1 to 20 volume % based on a total of 100 volume % of the electrolyte, the life characteristics can be further improved.

[0179] Example 5 is a lithium-sulfur battery using 1,3-dioxolane (DOL) instead of 2-methylfuran as the electrolyte, and the electrolyte uses 1,3-dioxolane / 1,2-dimethoxyethane as the electrolyte substrate. From the results of the life characteristics of the lithium-sulfur battery of Example 5, it can be seen that degradation occurs rapidly ( Figure 5 ).

[0180] The above results show that even when tellurium is included as a positive electrode additive and bis(2,2,2-trifluoroethyl)ether is included as an electrolyte additive, the electrolyte solution does not exhibit an effect of improving life characteristics.

[0181] That is, when tellurium is contained as a positive electrode additive in an amount of 1 to 10 wt % based on a total of 100 wt % of the positive electrode active material layer and bis(2,2,2-trifluoroethyl)ether is contained as an electrolyte additive in an amount of 1 to 20 vol % based on a total of 100 vol % of the electrolyte, and no dioxolane compound is used as the electrolyte, an effect of improving the life characteristics of the lithium-sulfur battery can be achieved.

[0182] Experimental Example 2. Evaluation of Initial Coulombic Efficiency of Lithium-Sulfur Batteries

[0183] For the batteries prepared in Examples 1 to 3 and Comparative Example 3, initial coulombic efficiency according to the tellurium content was evaluated using a charge / discharge measurement device (PESCO5-0.01, PNE solution, Korea).

[0184] In the first 3 cycles, the charge and discharge capacity from 2.5 to 1.8 V was measured at a current density of 0.1 C, and from the 4th cycle onwards, the initial coulombic efficiency was measured by discharging at 0.3 C and charging at 0.2 C. The results are shown in Figure 6 and Figure 7 middle.

[0185] The lithium-sulfur battery of Comparative Example 3, which does not contain tellurium, did not show a decrease in initial coulombic efficiency. However, the lithium-sulfur batteries of Examples 1 to 3, which contained tellurium, did show a decrease in initial coulombic efficiency. The lithium-sulfur batteries of Examples 1 and 2, which contained tellurium in amounts of 5% and 10% by weight, respectively, based on a total of 100% by weight of the positive electrode active material layer, showed a decrease in initial coulombic efficiency, but, as in the results of Experimental Example 1, showed improved lifespan characteristics of the lithium-sulfur battery. The lithium-sulfur battery of Example 3 contained tellurium in an amount of 15% by weight, based on a total of 100% by weight of the positive electrode active material layer, which exceeds the tellurium content range of 1 to 10% by weight. Therefore, the lithium-sulfur battery of Example 3 showed an excessive decrease in initial coulombic efficiency and, as in the results of Experimental Example 1, showed no improvement in lifespan characteristics of the lithium-sulfur battery due to the excessive decrease in coulombic efficiency.

[0186] From this, it was found that if the tellurium content exceeds 10 wt % based on 100 wt % of the total positive electrode active material layer, the coulombic efficiency decreases, and it was also found that the tellurium content is preferably 1 to 10 wt %.

Claims

1. A lithium secondary battery, comprising: a positive electrode; a negative electrode; a separator between the positive electrode and the negative electrode; and an electrolyte, in, The positive electrode comprises a positive electrode active material and a positive electrode additive, Wherein, the positive electrode additive comprises tellurium, Wherein, the electrolyte contains lithium salt, organic solvent and electrolyte additives, Wherein, the electrolyte additive comprises bis(2,2,2-trifluoroethyl) ether, Wherein, the organic solvent does not contain dioxolane compounds.

2. The lithium secondary battery according to claim 1, wherein The positive electrode includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector, and The positive electrode active material layer contains the positive electrode active material and tellurium.

3. The lithium secondary battery according to claim 2, wherein The content of tellurium is 1 wt % to 10 wt % based on 100 wt % of the total of the positive electrode active material layer.

4. The lithium secondary battery according to claim 1, wherein The positive electrode active material includes at least one selected from elemental sulfur and a sulfur compound.

5. The lithium secondary battery according to claim 1, wherein The content of bis(2,2,2-trifluoroethyl)ether is 1 to 20 volume % based on 100 volume % of the total electrolyte.

6. The lithium secondary battery according to claim 1, wherein The organic solvent includes cyclic ethers and acyclic ethers.

7. The lithium secondary battery according to claim 6, wherein The cyclic ether includes at least one selected from the group consisting of furan, 2-methylfuran, 3-methylfuran, 2-ethylfuran, 2-propylfuran, 2-butylfuran, 2,3-dimethylfuran, 2,4-dimethylfuran, 2,5-dimethylfuran, pyran, 2-methylpyran, 3-methylpyran, 4-methylpyran, benzofuran, 2-(2-nitrovinyl)furan, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, and tetrahydropyran.

8. The lithium secondary battery according to claim 6, wherein The acyclic ether includes at least one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol ethyl methyl ether, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, and 1,4-dimethoxybenzene.

9. The lithium secondary battery according to claim 1, wherein The organic solvent comprises 2-methylfuran and dimethoxyethane.

10. The lithium secondary battery according to claim 1, wherein The lithium salt includes at least one selected from the following: LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiB(Ph)4, LiC4BO8, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(SO2F)2, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate and lithium imide.

11. The lithium secondary battery according to claim 1, wherein The lithium secondary battery is a lithium-sulfur battery.

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