Lithium-sulfur secondary batteries containing electrolytes of borate-based lithium salts

Abstract

This invention provides a lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The electrolyte contains a borate lithium salt, specifically lithium difluoro(oxalate)borate, lithium bis(oxalate)borate, or a combination thereof. The content of the borate lithium salt in the electrolyte is greater than 0 ppm and less than 1000 ppm relative to the total weight of the electrolyte. Because the lithium-sulfur secondary battery contains a specific borate lithium salt in the electrolyte, the cycle performance of the lithium-sulfur secondary battery is improved.

Description

Technical Field

[0001] This invention relates to a lithium-sulfur secondary battery comprising an electrolyte containing a borate-based lithium salt. Specifically, this invention relates to a lithium-sulfur secondary battery comprising an electrolyte containing lithium difluoro(oxalate)borate, lithium bis(oxalate)borate, or a combination thereof as a borate-based lithium salt.

[0002] This application claims priority based on Korean Patent Application Nos. 10-2020-0142432 and 10-2020-0142418, filed on October 29, 2020, the entire contents of which are incorporated herein by reference. Background Technology

[0003] As the application of secondary batteries expands to electric vehicles (EVs) or energy storage devices (ESS), lithium-ion secondary batteries, with their relatively low energy storage density relative to weight (~250 Wh / kg), are limiting their use in these products. Alternatively, lithium-sulfur secondary batteries are attracting attention as a next-generation secondary battery technology because they theoretically offer a high energy storage density relative to weight (~2600 Wh / kg).

[0004] Lithium-sulfur secondary batteries refer to battery systems that use sulfur-sulfur materials with sulfur-sulfur bonds (SS bonds) as the positive electrode active material and lithium metal as the negative electrode active material. Sulfur, as the main material of the positive electrode active material, has the advantages of being abundant in resources, non-toxic, and having a low atomic weight.

[0005] In lithium-sulfur secondary batteries, during discharge, lithium, as the negative electrode active material, is oxidized and ionized while releasing electrons, while sulfur, as the positive electrode active material, is reduced while accepting electrons. In this case, the lithium oxidation reaction is the process by which lithium metal releases electrons and converts into lithium cations. Furthermore, the sulfur reduction reaction is the process by which the S-S bond accepts two electrons and converts into sulfur anions. The lithium cations generated from the lithium oxidation reaction are transferred to the positive electrode via the electrolyte and combine with the sulfur anions generated through the sulfur reduction reaction to form a salt. Specifically, sulfur, before discharge, has a cyclic S8 structure, which is converted into lithium polysulfide (LiS) through a reduction reaction. x When the lithium polysulfide is completely reduced, lithium sulfide (Li2S) is formed.

[0006] Sulfur, as a positive electrode active material, suffers from low conductivity, making it difficult to ensure reactivity with electrons and lithium ions in its solid state. In existing lithium-sulfur secondary batteries, Li₂S is generated to improve the reactivity of sulfur. xIntermediate polysulfides are used to induce liquid-phase reactions and improve reactivity. In this case, ether solvents with high solubility for lithium polysulfides, such as dioxolane and dimethoxyethane, are used as the electrolyte solvent. Furthermore, in conventional lithium-sulfur secondary batteries, cathode electrolyte type lithium-sulfur secondary battery systems are constructed to improve reactivity. In this case, the reactivity and lifetime characteristics of sulfur are affected by the electrolyte content due to the characteristics of lithium polysulfides dissolving in the electrolyte. To achieve high energy density, the electrolyte should be injected under low content conditions. However, as the electrolyte content decreases, the concentration of lithium polysulfides in the electrolyte increases, leading to decreased fluidity of the active material and increased side reactions, making it difficult for the battery to operate normally.

[0007] To manufacture lithium-sulfur secondary batteries with high energy density, a battery system capable of operating electrodes with high load and low porosity is needed, and research on such battery systems continues in related technologies.

[0008] Existing technical documents

[0009] [Patent Literature]

[0010] (Patent Document 1) Korean Patent Publication No. 10-2019-0006923 (Pending) Summary of the Invention

[0011] [Technical Issues]

[0012] The purpose of this invention is to provide a lithium-sulfur secondary battery, wherein the cycle performance of the lithium-sulfur secondary battery can be improved by adding a specific borate lithium salt to the electrolyte of the lithium-sulfur secondary battery.

[0013] [Technical Solution]

[0014] The present invention provides a lithium-sulfur secondary battery, which includes a positive electrode, a negative electrode, a separator and an electrolyte, wherein the electrolyte contains a borate lithium salt.

[0015] In one embodiment of the invention, the borate lithium salt is lithium difluoro(oxalate)borate, lithium bis(oxalate)borate, or a combination thereof.

[0016] In one embodiment of the invention, the content of borate lithium salt in the electrolyte is greater than 0 ppm and less than 1000 ppm relative to the total weight of the electrolyte.

[0017] In one embodiment of the invention, the electrolyte further comprises a non-aqueous solvent and a lithium salt, wherein the non-aqueous solvent comprises a fluorinated linear ether.

[0018] In one embodiment of the present invention, the fluorinated linear ether is selected from: 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, bis(fluoromethyl) ether, 2-fluoroethylmethyl ether, bis(2,2,2-trifluoroethyl) ether, propyl 1,1,2,2-tetrafluoroethyl ether, isopropyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl isobutyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, 1H,1H,2'H,3H-decafluorodipropyl ether, 1H,1H,2'H-perfluorodipropyl ether, and combinations thereof.

[0019] In one embodiment of the invention, the non-aqueous solvent contains 50% to 99% by weight of fluorinated linear ether relative to the total weight of the non-aqueous solvent.

[0020] In one embodiment of the present invention, the lithium salt is selected from: LiN(FSO2)2, LiSCN, LiN(CN)2, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiPF6, LiF, LiCl, LiBr, LiI, LiNO3, LiClO4, LiAlO4, LiAlCl4, LiSbF6, LiAsF6, LiBF2C2O4, LiBC4O8, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiC4F9SO3, LiCF3CF2SO3, LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2, and combinations thereof.

[0021] In one embodiment of the present invention, the positive electrode comprises a positive electrode active material layer having a porosity of more than 30% and less than 70%.

[0022] In one embodiment of the present invention, the positive electrode has a capacity of 3.0 mAh / cm³. 2 Up to 10.0mAh / cm 2 The loading of positive electrode active material.

[0023] In one embodiment of the present invention, the positive electrode comprises a sulfur-carbon composite material as the positive electrode active material.

[0024] In one embodiment of the invention, the sulfur-carbon composite material contains 60% to 90% by weight of sulfur relative to the total weight of the sulfur-carbon composite material.

[0025] In one embodiment of the invention, the non-aqueous solvent further comprises a non-fluorinated linear ether, a cyclic ether, a polyether, or a mixture thereof.

[0026] [Beneficial Effects]

[0027] In the lithium-sulfur secondary battery according to the present invention, the cycle performance of the lithium-sulfur secondary battery is improved by adding a specific borate lithium salt to the electrolyte.

[0028] As specific borate lithium salts, lithium difluoro(oxalate)borate or lithium bis(oxalate)borate is added to the electrolyte of lithium-sulfur secondary batteries at amounts less than 1000 ppm. Considering that in related technical fields, it is generally possible to obtain the desired improvement in battery performance when the amount of electrolyte additive is 1% by weight (10000 ppm) or more, lithium difluoro(oxalate)borate or lithium bis(oxalate)borate differs from common electrolyte additives used in the technical field.

[0029] When lithium difluoro(oxalate)borate or lithium bis(oxalate)borate is used in the same amount as common electrolyte additives in related technologies, the effect on improving the cycle performance of lithium-sulfur secondary batteries is not significant or almost non-existent.

[0030] Furthermore, even when used in lithium secondary batteries other than lithium-sulfur secondary batteries, the effect of improving the cycle performance of lithium secondary batteries is not obvious or almost non-existent. Detailed Implementation

[0031] The embodiments provided by the present invention can all be implemented through the following description. It should be understood that the following description is intended to depict preferred embodiments of the present invention, and it should be understood that the present invention is not necessarily limited thereto.

[0032] For the physical properties described herein, unless otherwise specified, the measurement conditions and methods are used by those skilled in the art to measure the physical properties.

[0033] "Lithium secondary battery" is generally a higher-level concept than lithium-sulfur secondary battery and includes lithium-sulfur secondary batteries. However, in this specification, "lithium secondary battery" refers to a conventional lithium secondary battery that uses lithium metal oxide as the positive electrode active material and is used separately from lithium-sulfur secondary batteries.

[0034] This invention provides a lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte contains a borate lithium salt. According to one embodiment of the invention, the borate lithium salt is lithium difluoro(oxalate)borate (LiODFB), lithium bis(oxalate)borate (LiBOB), or a combination thereof. Lithium difluoro(oxalate)borate is a lithium salt having the structure of Formula 1, and lithium bis(oxalate)borate is a lithium salt having the structure of Formula 2. The inventors of this invention completed the invention by confirming that when a specific borate lithium salt is added to the electrolyte of a lithium-sulfur secondary battery using a sulfur-containing material as the positive electrode active material, instead of using a lithium metal oxide as the positive electrode active material, the cycle performance of the battery is improved.

[0035] [Formula 1]

[0036]

[0037] [Equation 2]

[0038]

[0039] Considering that lithium secondary batteries with the same composition except for the positive electrode active material have little or no effect on improving the cycle performance of the battery, it is expected that lithium difluoro(oxalate)borate or lithium bis(oxalate)borate added to the electrolyte will directly interact with the positive electrode active material of the lithium-sulfur secondary battery.

[0040] According to one embodiment of the invention, the electrolyte contains, relative to the total weight of the electrolyte, more than 0 ppm, 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, 300 ppm, 350 ppm, 400 ppm, 450 ppm, or 500 ppm of borate lithium salt, and relative to the total weight of the electrolyte, contains less than 1000 ppm, less than 950 ppm, less than 900 ppm, less than 850 ppm, less than 800 ppm, less than 750 ppm, less than 700 ppm, less than 650 ppm, less than 600 ppm, less than 550 ppm, or less than 500 ppm of borate lithium salt. A feature of this invention is the addition of small amounts of borate lithium salt, such as less than 1000 ppm, to the electrolyte. If large amounts of borate lithium salt, exceeding 1000 ppm, are added, the effect on improving cycle performance may be minimal or nonexistent. In related technologies, the above features are not common in the field, considering that the desired effect of improving battery performance can only be obtained when the amount of electrolyte additive is usually above 1% by weight (10,000 ppm).

[0041] The electrolyte constituting the lithium-sulfur secondary battery according to the present invention includes a non-aqueous solvent and a lithium salt in addition to the aforementioned borate lithium salt. As described above, since the direct interaction between the borate lithium salt and the positive electrode active material of the lithium-sulfur secondary battery allows for improved battery cycle performance through the use of borate lithium salt, the types of non-aqueous solvent and lithium salt are not particularly limited. However, if more suitable non-aqueous solvent and lithium salt are selected for the lithium-sulfur secondary battery, the overall cycle performance of the battery may be excellent.

[0042] According to one embodiment of the present invention, the non-aqueous solvent is an ether solvent. The ether solvent can be a linear ether, a cyclic ether, a polyether, or a mixture thereof.

[0043] The linear ether may be selected from: methyl ethyl ether, methyl propyl ether, methyl butyl ether, ethyl propyl ether, ethyl isopropyl ether, ethyl butyl ether, ethyl isobutyl ether, diethyl ether, dipropyl ether, dibutyl ether, dimethoxymethane (DMM), trimethoxyethane (TMM), dimethoxyethane (DME), diethoxyethane (DEE), dimethoxypropane (DMP) and combinations thereof, but is not limited thereto.

[0044] Cyclic ethers can be selected from: dioxolane (DOL), methyldioxolane, ... Alkane, di Alkane, trialkyl Alkane, tetrahydrofuran (THF), dihydropyran (DHP), tetrahydropyran (THP), methyltetrahydrofuran, furan, methylfuran and combinations thereof, but not limited thereto.

[0045] The polyether may be selected from: diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, ethylene glycol divinyl ether, diethylene glycol divinyl ether, triethylene glycol divinyl ether, dipropylene glycol dimethylene ether, butanediol ether and combinations thereof, but not limited thereto.

[0046] Linear ethers, cyclic ethers, and polyethers can be fluorinated ether compounds. The fluorinated form of the compound can be a fluorinated linear ether, and the fluorinated linear ether can be selected from: 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, bis(fluoromethyl) ether, 2-fluoroethylmethyl ether, bis(2,2,2-trifluoroethyl) ether, propyl 1,1,2,2-tetrafluoroethyl ether, isopropyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl isobutyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, 1H,1H,2'H,3H-decafluorodipropyl ether, 1H,1H,2'H-perfluorodipropyl ether, and combinations thereof, but not limited thereto.

[0047] Fluorinated ether compounds can be used in combination with non-fluorinated linear ethers, cyclic ethers, polyethers, or combinations thereof. According to one embodiment of the invention, the content of the fluorinated ether compound in the electrolyte can be from 50% to 99% by weight, preferably from 60% to 95% by weight, and more preferably from 70% to 90% by weight, relative to the total weight of the solvent constituting the electrolyte. If the content of the fluorinated ether compound in the electrolyte is 50% by weight or more relative to the total weight of the solvent constituting the electrolyte, the battery performance can be improved when used with the positive electrode of a lithium-sulfur secondary battery containing a positive electrode active material with low porosity and high loading.

[0048] Lithium salts are materials that are readily soluble in non-aqueous solvents and can be selected from: LiN(FSO2)2, LiSCN, LiN(CN)2, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiPF6, LiF, LiCl, LiBr, LiI, LiNO3, LiClO4, LiAlO4, LiAlCl4, LiSbF6, LiAsF6, LiBF2C2O4, LiBC4O8, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiC4F9SO3, LiCF3CF2SO3, LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2, and combinations thereof.

[0049] The concentration of the lithium salt can range from 0.1 M to 8.0 M, preferably from 0.5 M to 5.0 M, and more preferably from 1.0 M to 3.0 M, depending on various factors such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, the operating temperature, and other factors known in the field of lithium secondary batteries. If the concentration of the lithium salt is less than the above range, the conductivity of the electrolyte may decrease, thereby potentially degrading the performance of the electrolyte. If the concentration of the lithium salt exceeds the above range, the viscosity of the electrolyte may increase, thereby potentially reducing the concentration of lithium ions (Li). + The migration rate of the lithium salt may decrease. Therefore, it is preferable to select an appropriate lithium salt concentration within the above range.

[0050] The positive electrode constituting the lithium-sulfur secondary battery according to the present invention typically comprises a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. The positive electrode active material layer comprises a positive electrode active material, a conductive material, and a binder.

[0051] There are no particular restrictions on the positive electrode current collector, as long as it has high conductivity without causing chemical changes in the battery. For example, it can be made of stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel with surface treatments using carbon, nickel, titanium, silver, etc. Furthermore, the thickness of the positive electrode current collector can typically range from 3 μm to 500 μm, and forming micro-protrusions on the surface of the current collector can improve its adhesion to the positive electrode active material. For example, the positive electrode current collector can be formed in various forms such as films, sheets, foils, meshes, porous bodies, foams, or nonwoven fabrics.

[0052] Positive electrode active materials are commonly used in lithium-sulfur secondary batteries and contain, for example, elemental sulfur (S₈) and sulfur compounds or mixtures thereof. Specifically, the sulfur compounds can be Li₂S₂. n (n≥1), organic sulfur compounds or sulfur-carbon compounds (C2S) x ) n (x = 2.5~50, n ≥ 2), etc. Because sulfur alone is not conductive, it can be combined with carbon materials and used in the form of sulfur-carbon composites.

[0053] Sulfur-carbon composite materials can have particle sizes ranging from 1 μm to 100 μm. If the particle size of the sulfur-carbon composite material is less than 1 μm, there is an increase in interparticle resistance and an overvoltage problem in the electrodes of lithium-sulfur secondary batteries. If the particle size exceeds 100 μm, the surface area per unit weight decreases, thereby reducing the wetting area with the electrolyte and the reaction sites with lithium ions in the electrode, and reducing the amount of electron transfer relative to the size of the composite material, which may delay the reaction and thus reduce the discharge capacity of the battery.

[0054] In sulfur-carbon composite materials, the sulfur content relative to the total weight of the composite material can be from 60% to 90% by weight, preferably from 70% to 80% by weight. If the sulfur content is less than 60% by weight, the energy density of the battery may be reduced. If the sulfur content exceeds 90% by weight, the conductivity of the electrode and the functionality of the positive electrode active material may be reduced.

[0055] The carbon material (or sulfur carrier material) constituting the sulfur-carbon composite has porosity, especially because the carbon material used as the positive electrode active material of the present invention has a high specific surface area (3000 m²). 2 (above / g) and high porosity (pore volume per unit weight: 0.7 to 3.0 cm³). 3 Due to its properties ( / g), it can be loaded with a large amount of sulfur.

[0056] The carbon material can be, for example, selected from at least one of the following: graphite; graphene; reduced graphene oxide (rGO); carbon black such as Decco black, acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal cracking 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 activated carbon, and its shape can be in the form of spheres, rods, needles, plates, tubes or blocks.

[0057] The content of the positive electrode active material in the positive electrode active material layer can be from 80% to 99% by weight, preferably from 85% to 95% by weight, relative to the total weight of the positive electrode active material layer. If the content of the positive electrode active material in the positive electrode active material layer is less than 80% by weight, there may be a problem of reduced energy density of the battery. If the content of the positive electrode active material in the positive electrode active material layer exceeds 99% by weight, the following problems may exist: due to insufficient binder content, the bonding force between the positive electrode active materials may decrease; and due to insufficient conductive material content, the conductivity in the electrode may decrease.

[0058] Conductive materials are used to impart conductivity to electrodes and can be used without any particular restrictions, provided they are electronically conductive and do not cause chemical changes in the battery in which they are constructed. Specific examples may include: graphite such as natural or artificial graphite; carbonaceous materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, and carbon fibers; metal powders or fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one or more mixtures of these substances may be used.

[0059] The content of conductive material in the positive electrode active material layer can be from 0.1% to 15% by weight, preferably from 0.5% to 10% by weight, relative to the total weight of the positive electrode active material layer. If the content of conductive material in the positive electrode active material layer is less than 0.1% by weight, there may be a problem of reduced conductivity in the electrode due to insufficient conductive material content. If the content of conductive material in the positive electrode active material layer exceeds 15% by weight, there may be a problem of reduced discharge capacity and energy density of the battery due to the relatively small amount of positive electrode active material.

[0060] Adhesives are used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples may include polyvinylidene fluoride (PVDF), PVDF-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and mixtures of one or more of these substances may be used.

[0061] The binder content in the positive electrode active material layer can be from 0.1% to 15% by weight, preferably from 0.5% to 10% by weight, relative to the total weight of the positive electrode active material layer. If the binder content in the positive electrode active material layer is less than 0.1% by weight, there may be a problem of reduced bonding strength between the positive electrode active materials due to insufficient binder content. If the binder content in the positive electrode active material layer exceeds 15% by weight, there may be a problem of reduced battery discharge capacity and energy density due to the relatively small amount of positive electrode active material.

[0062] A positive electrode active material, binder, conductive material, etc., are dispersed in a dispersion medium (solvent) and mixed to form a slurry. The slurry can be coated onto a positive electrode current collector, and then dried and calendered to prepare the positive electrode. The dispersion medium can be, but is not limited to, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), ethanol, isopropanol, water, or mixtures thereof.

[0063] The positive electrode of the lithium-sulfur secondary battery according to the present invention can have a lower porosity than the positive electrode of a conventional lithium-sulfur secondary battery in the related art. Here, porosity is the ratio of the pore volume of the positive electrode to the total volume, usually expressed as a percentage. If the porosity of the positive electrode of a conventional lithium-sulfur secondary battery is low, the battery performance cannot be properly achieved because the material may not move easily with the permeation of the electrolyte. Even so, if the porosity of the positive electrode of a lithium-sulfur secondary battery is increased, it may be undesirable because the volume of the positive electrode increases to load the same amount of positive electrode active material. It may be more preferable to use the electrolyte according to the present invention in a lithium-sulfur secondary battery, because the battery performance can be appropriately achieved in both positive electrodes with high porosity and those with low porosity. According to one embodiment of the present invention, the porosity of the positive electrode active material layer in the positive electrode is 30% or more and less than 70%, preferably 50% to 65%, more preferably 55% to 65%. A porosity of less than 70% is lower than the porosity of the positive electrode active material layer of a typical lithium-sulfur secondary battery in the related art. If battery performance can be appropriately achieved at a certain porosity, the advantage is that the volume of the positive electrode can be reduced for the same amount of positive electrode active material. This porosity can be measured using methods commonly used in the relevant technical field. The thickness of the positive electrode active material layer is measured using a device for measuring material thickness (TESA, u-hite), and then the porosity is calculated using the true density of the positive electrode active material layer measured using a device for measuring the true density of the material (Microtrac, BELPycno).

[0064] The positive electrode of the lithium-sulfur secondary battery according to the present invention can have a higher positive electrode active material loading than the positive electrode of a conventional lithium-sulfur secondary battery in the related art. Generally, if the loading of the positive electrode active material increases, the volume of the positive electrode inevitably increases. However, in the case of the lithium-sulfur secondary battery according to the present invention, because the porosity of the positive electrode can be reduced, as described above, a high positive electrode active material loading can be maintained even in a relatively small volume. According to one embodiment of the present invention, the positive electrode active material loading is 3.0 mAh / cm³. 2 Up to 10.0mAh / cm 2 Preferred capacity: 3.5mAh / cm 2 Up to 7.0mAh / cm 2 More preferably 4.0mAh / cm 2 Up to 7.0mAh / cm 2Theoretically, increasing the loading of the positive electrode active material could improve battery performance. However, limitations exist in increasing the loading of the positive electrode active material due to the increased electrode volume and the difference between theoretical and actual discharge capacity. This is achieved by dividing the theoretical discharge capacity (mAh) of the positive electrode active material loaded on the positive electrode by the surface area (cm²) of the positive electrode active material layer in contact with the positive electrode current collector. 2 The loading of the positive electrode active material can be calculated. For example, in the case of sulfur, it has a theoretical specific discharge capacity of 1675 mAh / g, and the theoretical discharge capacity of sulfur can be calculated by multiplying the theoretical specific discharge capacity by the mass (g) of sulfur loaded on the positive electrode.

[0065] The negative electrode constituting the lithium-sulfur secondary battery according to the present invention comprises a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector.

[0066] The negative electrode active material layer comprises a negative electrode active material, a binder, and a conductive material. The negative electrode active material can be: capable of reversibly inserting or deintercalating lithium ions (Li... + Materials capable of reacting with lithium ions to reversibly form lithium-containing compounds; lithium metal or lithium alloys. The reversibly intercalating or deintercalating lithium ions (Li...) + The material can be, for example, crystalline carbon, amorphous carbon, or a mixture thereof. The material capable of reacting with lithium ions (Li...) + The material used to reversibly form a lithium-containing compound in the reaction can be, for example, tin oxide, titanium nitrate, or silicon. The lithium alloy can be, for example, an alloy of lithium (Li) and metals selected from the following: 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).

[0067] The adhesive, conductive material, and negative electrode current collector can be selected with reference to the above-described positive electrode structure, but are not necessarily limited thereto. Furthermore, the method for forming the negative electrode active material layer on the negative electrode current collector is based on coating methods known in the positive electrode, and is not particularly limited thereto.

[0068] The separator used in the lithium-sulfur secondary battery according to the present invention is a physical separator that functions to physically separate the electrodes. The separator can be used without particular limitation as long as it can be used as a conventional separator. In particular, a separator that exhibits low resistance to ion migration in the electrolyte while possessing excellent electrolyte retention capacity is preferred. The separator is capable of transporting lithium ions between the positive and negative electrodes while isolating or insulating them from each other. Such a separator can be made of a porous, non-conductive, or insulating material with a porosity of 30% to 50%. Specifically, porous polymer membranes can be used, such as porous polymer membranes made from polyolefin polymers like ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, and nonwoven fabrics made from high-melting-point glass fibers can also be used. Among these, porous polymer membranes are preferred.

[0069] If a polymer membrane is used simultaneously as both a buffer layer and a separator, the electrolyte penetration and ion conductivity decrease, and the effects of reducing overvoltage and improving capacity become negligible. Conversely, if nonwoven materials are used simultaneously as both a buffer layer and a separator, mechanical stiffness cannot be guaranteed, leading to battery short circuits. However, if a membrane-type separator and a polymer nonwoven buffer layer are used together, the use of the buffer layer ensures both improved battery performance and guaranteed mechanical strength.

[0070] According to one embodiment of the invention, an ethylene homopolymer (polyethylene) polymer membrane is used as a separator, and a polyimide nonwoven fabric is used as a buffer layer. In this case, it is preferred that the polyethylene polymer membrane has a thickness of 10 μm to 25 μm and a porosity of 40% to 50%.

[0071] The lithium-sulfur secondary battery of the present invention can be manufactured by placing a separator between the positive and negative electrodes to form an electrode assembly, inserting the electrode assembly into a cylindrical or rectangular battery case, and then injecting an electrolyte. Alternatively, the lithium-sulfur secondary battery of the present invention can be manufactured by laminating the electrode assembly, impregnating the electrode assembly with an electrolyte, placing the resulting product into a battery case, and then sealing it.

[0072] In the following description, preferred embodiments will be presented to aid in understanding the invention. However, these embodiments are provided to facilitate understanding, and the invention is not limited thereto.

[0073] Preferred implementation scheme

[0074] Example

[0075] Example 1

[0076] An electrolyte for lithium-sulfur secondary batteries was prepared by adding 1.0 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, LiN(CF3SO2)2) to a solvent obtained by mixing dimethoxyethane (DME) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) in a volume ratio of 3:7, followed by mixing. Then, 500 ppm lithium difluoro(oxalate)borate was added to the mixture.

[0077] 90 parts by weight of a sulfur-carbon composite material (S:C weight ratio = 75:25) was used as the positive electrode active material (in the sulfur-carbon composite material, a pore volume of 1.8 cm³ was used). 3 A slurry composition for the positive electrode was prepared by mixing 5 parts by weight of activated carbon ( / g), 5 parts by weight of Deco Black as a conductive material, and 5 parts by weight of styrene-butadiene rubber / carboxymethyl cellulose (SBR:CMC = 7:3) as a binder. The prepared slurry composition was coated onto an aluminum foil current collector, dried at 50°C for 12 hours, and compressed using a calender to prepare the positive electrode (at this time, the loading was 4.0 mAh / cm³). 2 Furthermore, the porosity of the positive electrode active material layer in the positive electrode is 65%.

[0078] A prepared positive electrode and a 60 μm thick lithium metal negative electrode are placed facing each other, and a polyethylene (PE) separator is inserted between them. Then, the prepared electrolyte is injected to manufacture a coin cell type lithium-sulfur secondary battery. On the other hand, in the manufacture of lithium-sulfur secondary batteries, the positive electrode is stamped into a φ14 circular electrode, the polyethylene separator is stamped into a φ19 separator, and the lithium metal is stamped into a φ16 negative electrode.

[0079] Comparative Example 1

[0080] A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that lithium difluoro(oxalate)borate was not added during the manufacture of the electrolyte.

[0081] Comparative Example 2

[0082] A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that 5000 ppm of lithium difluoro(oxalate)borate was added during the manufacture of the electrolyte.

[0083] Comparative Example 3

[0084] The lithium secondary battery was prepared in the same manner as in Example 1, except that the positive electrode was manufactured using the following method.

[0085] 90 parts by weight of LiNi were used as the positive electrode active material. 0.6 Co 0.2 Mn 0.2O2 (NCM 622), 5 parts by weight of Super-P as a conductive material, and 5 parts by weight of polyvinylidene fluoride (PVDF) as a binder were mixed to prepare a slurry composition for the positive electrode. The prepared slurry composition was coated onto an aluminum foil current collector, dried at 50°C for 12 hours, and compressed using a calender to prepare the positive electrode (at this time, the loading was 3.0 mAh / cm³). 2 Furthermore, the porosity of the positive electrode active material layer in the positive electrode is 30%.

[0086] Comparative Example 4

[0087] A lithium secondary battery was prepared in the same manner as Comparative Example 3, except that lithium difluoro(oxalate)borate was not added during the preparation of the electrolyte.

[0088] Comparative Example 5

[0089] A lithium secondary battery was prepared in the same manner as Comparative Example 3, except that 5000 ppm of lithium difluoro(oxalate)borate was added during electrolyte preparation.

[0090] Comparative Example 6

[0091] A lithium secondary battery was prepared in the same manner as in Comparative Example 3, except that the electrolyte was prepared by the following method.

[0092] An electrolyte for lithium secondary batteries was prepared by adding 1.0 M lithium hexafluorophosphate (LiPF6) to a solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7, and then mixing the mixture with 500 ppm lithium difluoro(oxalate)borate.

[0093] Comparative Example 7

[0094] A lithium secondary battery was prepared in the same manner as Comparative Example 6, except that lithium difluoro(oxalate)borate was not added during the preparation of the electrolyte.

[0095] Comparative Example 8

[0096] A lithium secondary battery was prepared in the same manner as Comparative Example 6, except that 5000 ppm of lithium difluoro(oxalate)borate was added during electrolyte preparation.

[0097] Example 2

[0098] A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, except that lithium bis(oxalate)borate was added instead of lithium difluoro(oxalate)borate during electrolyte preparation.

[0099] Comparative Example 9

[0100] A lithium-sulfur secondary battery was prepared in the same manner as in Example 2, except that lithium bis(oxalate)borate was not added during electrolyte preparation.

[0101] Comparative Example 10

[0102] A lithium-sulfur secondary battery was prepared in the same manner as in Example 2, except that 5000 ppm of lithium bis(oxalate)borate was added during electrolyte preparation.

[0103] Comparative Example 11

[0104] The lithium secondary battery was prepared in the same manner as in Example 2, except that the positive electrode was manufactured using the following method.

[0105] 90 parts by weight of LiNi were used as the positive electrode active material. 0.6 Co 0.2 Mn 0.2 O2 (NCM 622), 5 parts by weight of Super-P as a conductive material, and 5 parts by weight of polyvinylidene fluoride (PVDF) as a binder were mixed to prepare a slurry composition for the positive electrode. The prepared slurry composition was coated onto an aluminum foil current collector, dried at 50°C for 12 hours, and compressed using a calender to prepare the positive electrode (at this time, the loading was 3.0 mAh / cm³). 2 Furthermore, the porosity of the positive electrode active material layer in the positive electrode is 30%.

[0106] Comparative Example 12

[0107] A lithium secondary battery was prepared in the same manner as Comparative Example 11, except that lithium bis(oxalate)borate was not added during the preparation of the electrolyte.

[0108] Comparative Example 13

[0109] A lithium secondary battery was prepared in the same manner as Comparative Example 11, except that 5000 ppm of lithium bis(oxalate)borate was added during electrolyte preparation.

[0110] Comparative Example 14

[0111] A lithium secondary battery was prepared in the same manner as Comparative Example 11, except that the electrolyte was prepared by the following method.

[0112] An electrolyte for lithium secondary batteries was prepared by adding 1.0 M lithium hexafluorophosphate (LiPF6) to a solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7, and then mixing the mixture with 500 ppm lithium bis(oxalate)borate.

[0113] Comparative Example 15

[0114] A lithium secondary battery was prepared in the same manner as Comparative Example 14, except that lithium bis(oxalate)borate was not added during the preparation of the electrolyte.

[0115] Comparative Example 16

[0116] A lithium secondary battery was prepared in the same manner as Comparative Example 14, except that 5000 ppm of lithium bis(oxalate)borate was added during electrolyte preparation.

[0117] Experimental example: Evaluation of the cycle performance of the manufactured battery

[0118] Experimental Example 1: Evaluation of Lithium Difluoro(Oxalic Acid)borate

[0119] The lithium-sulfur secondary battery manufactured in Example 1 and the lithium secondary batteries manufactured in Comparative Examples 1 to 8 were charged and discharged at a rate of 0.3C to evaluate their cycle performance. Considering optimal conditions depending on the battery type, the charging and discharging voltage ranges for the lithium-sulfur secondary battery and the lithium secondary battery were set to 1.0–3.6V and 2.7–4.4V, respectively, and the cycle performance was evaluated at a temperature of 25°C. The cycle performance was evaluated by the number of cycles at which the battery exhibited more than 80% of its initial discharge capacity, and when the relevant number of cycles was exceeded, the discharge capacity decreased to less than 80% relative to the initial discharge capacity. The evaluation results are shown in Table 1 below.

[0120] Table 1:

[0121] Loop count Example 1 72 Comparative Example 1 50 Comparative Example 2 46 Comparative Example 3 20 Comparative Example 4 22 Comparative Example 5 25 Comparative Example 6 52 Comparative Example 7 50 Comparative Example 8 54

[0122] As confirmed in Table 1 above, when 500 ppm of lithium difluoro(oxalate)borate was added to the electrolyte of the lithium-sulfur secondary battery (Example 1), the cycle performance was significantly improved compared to the case without the addition of lithium difluoro(oxalate)borate (Comparative Example 1). However, it was confirmed that when 5000 ppm of lithium difluoro(oxalate)borate was added to the electrolyte of the lithium-sulfur secondary battery (Comparative Example 2), no effect of adding lithium difluoro(oxalate)borate was observed; on the contrary, the cycle performance decreased compared to the case without the addition of lithium difluoro(oxalate)borate (Comparative Example 1).

[0123] Furthermore, in lithium secondary batteries using lithium metal oxide (NCM 622) instead of sulfur-carbon composite material as the positive electrode active material, the ether solvent decomposes in the electrolyte containing the ether solvent as the battery cycles, resulting in generally low measured cycle performance. Unlike lithium-sulfur secondary batteries, in the case of lithium secondary batteries, there was no significant improvement in cycle performance compared to the case with 500 ppm of lithium difluoro(oxalate)borate added to the electrolyte (Comparative Example 3), the case without lithium difluoro(oxalate)borate added (Comparative Example 4), and the case with 5000 ppm of lithium difluoro(oxalate)borate added (Comparative Example 5).

[0124] To confirm whether the results of Comparative Examples 3 to 5 were due to a problem with the electrolyte solvent, other experiments were conducted using an electrolyte containing a carbonate solvent suitable for lithium secondary batteries using lithium metal oxide (NCM 622) as the positive electrode active material. In electrolytes containing carbonate solvents, even during cycling, the carbonate solvent hardly decomposes, thus the cycle performance is generally similar to that of lithium-sulfur secondary batteries. However, even in the case of lithium secondary batteries using electrolytes containing carbonate solvents (Comparative Examples 6 to 8), compared to the case with 500 ppm of lithium difluoro(oxalate)borate added to the electrolyte (Comparative Example 6), the case without adding lithium difluoro(oxalate)borate (Comparative Example 7), and the case with 5000 ppm of lithium difluoro(oxalate)borate added (Comparative Example 8), the cycle performance of the lithium secondary batteries did not show significant improvement, as in the case of lithium secondary batteries using electrolytes containing ether solvents (Comparative Examples 3 to 5).

[0125] Experimental Example 2: Evaluation of Lithium Di(oxalate)borate

[0126] The lithium-sulfur secondary battery manufactured in Example 2 and the lithium secondary batteries manufactured in Comparative Examples 9 to 16 were charged and discharged at a rate of 0.3C to evaluate their cycle performance. Considering optimal conditions depending on the battery type, the charging and discharging voltage ranges for the lithium-sulfur secondary battery and the lithium secondary battery were set to 1.0–3.6V and 2.7–4.4V, respectively, and the cycle performance was evaluated at a temperature of 25°C. The cycle performance was evaluated by the number of cycles at which the battery exhibited more than 80% of its initial discharge capacity, and when the relevant number of cycles was exceeded, the discharge capacity decreased to less than 80% relative to the initial discharge capacity. The evaluation results are shown in Table 2 below.

[0127] Table 2:

[0128] Loop count Example 2 83 Comparative Example 9 50 Comparative Example 10 48 Comparative Example 11 20 Comparative Example 12 21 Comparative Example 13 17 Comparative Example 14 51 Comparative Example 15 50 Comparative Example 16 53

[0129] As confirmed in Table 2 above, when 500 ppm of lithium bis(oxalate)borate (Example 2) was added to the electrolyte of the lithium-sulfur secondary battery, the cycle performance was significantly improved compared to the case without the addition of lithium bis(oxalate)borate (Comparative Example 9). However, it has been confirmed that when 5000 ppm of lithium bis(oxalate)borate was added to the electrolyte of the lithium-sulfur secondary battery (Comparative Example 10), no effect of adding lithium bis(oxalate)borate was observed; on the contrary, the cycle performance decreased compared to the case without the addition of lithium bis(oxalate)borate (Comparative Example 9).

[0130] Furthermore, in lithium secondary batteries using lithium metal oxide (NCM 622) instead of sulfur-carbon composite material as the positive electrode active material, the ether solvent decomposes in the electrolyte containing the ether solvent as the battery cycles, resulting in generally low measured cycle performance. Unlike lithium-sulfur secondary batteries, in the case of lithium secondary batteries, there is no significant improvement in cycle performance compared to the case with 500 ppm of lithium bis(oxalate)borate added to the electrolyte (Comparative Example 11), the case without lithium bis(oxalate)borate added (Comparative Example 12), and the case with 5000 ppm of lithium bis(oxalate)borate added (Comparative Example 13).

[0131] To confirm whether the results of Comparative Examples 11 to 13 were due to a problem with the electrolyte solvent, other experiments were conducted using an electrolyte containing a carbonate solvent suitable for lithium secondary batteries using lithium metal oxide (NCM 622) as the positive electrode active material. In electrolytes containing carbonate solvents, even during cycling, the carbonate solvent hardly decomposes, thus the cycle performance is generally similar to that of lithium-sulfur secondary batteries. However, even in the case of lithium secondary batteries using electrolytes containing carbonate solvents (Comparative Examples 14 to 16), compared to the case with 500 ppm of lithium bis(oxalate)borate added to the electrolyte (Comparative Example 14), the case without adding lithium bis(oxalate)borate (Comparative Example 15), and the case with 5000 ppm of lithium bis(oxalate)borate added (Comparative Example 16), the cycle performance of the lithium secondary batteries did not show a significant improvement, as in the case of lithium secondary batteries using electrolytes containing ether solvents (Comparative Examples 11 to 13).

[0132] All simple variations and modifications of this invention fall within the scope of this invention, and the specific scope of protection of this invention is defined by the claims.

Claims

1. A lithium-sulfur secondary battery, the lithium-sulfur secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The electrolyte contains a non-aqueous solvent and a borate lithium salt, wherein the borate lithium salt is lithium difluoro(oxalate)borate, lithium bis(oxalate)borate, or a combination thereof. The content of the borate lithium salt in the electrolyte is greater than 0 ppm and less than 1000 ppm relative to the total weight of the electrolyte. The non-aqueous solvent comprises 50% to 99% by weight of fluorinated linear ethers relative to the total weight of the non-aqueous solvent.

2. The lithium-sulfur secondary battery according to claim 1, wherein the electrolyte further comprises a lithium salt.

3. The lithium-sulfur secondary battery according to claim 1, wherein the fluorinated linear ether is selected from: 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether, 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether, bis(fluoromethyl) ether, 2-fluoroethylmethyl ether, bis(2,2,2-trifluoroethyl) ether, propyl 1,1,2,2-tetrafluoroethyl ether, isopropyl 1,1,2,2-tetrafluoroethyl ether, 1,1,2,2-tetrafluoroethyl isobutyl ether, 1,1,2,3,3,3-hexafluoropropyl ethyl ether, 1H,1H,2'H,3H-decafluorodipropyl ether, 1H,1H,2'H-perfluorodipropyl ether, and combinations thereof.

4. The lithium-sulfur secondary battery according to claim 2, wherein the lithium salt is selected from: LiN(FSO2)2, LiSCN, LiN(CN)2, LiN(CF3SO2)2, LiN(CF3CF2SO2)2, LiPF6, LiF, LiCl, LiBr, LiI, LiNO3, LiClO4, LiAlO4, LiAlCl4, LiSbF6, LiAsF6, LiBF2C2O4, LiBC4O8, Li(CF3)2PF4, Li(CF3)3PF3, Li(CF3)4PF2, Li(CF3)5PF, Li(CF3)6P, LiCF3SO3, LiC4F9SO3, LiCF3CF2SO3, LiCF3CF2(CF3)2CO, Li(CF3SO2)2CH, LiCF3(CF2)7SO3, LiCF3CO2, LiCH3CO2, and combinations thereof.

5. The lithium-sulfur secondary battery according to claim 1, wherein the positive electrode comprises a positive electrode active material layer, the positive electrode active material layer having a porosity of more than 30% and less than 70%.

6. The lithium-sulfur secondary battery according to claim 1, wherein the positive electrode has a 3.0 mAh / cm³. 2 Up to 10.0 mAh / cm 2 The loading of positive electrode active material.

7. The lithium-sulfur secondary battery according to claim 1, wherein the positive electrode comprises a sulfur-carbon composite material as the positive electrode active material.

8. The lithium-sulfur secondary battery according to claim 7, wherein the sulfur-carbon composite material contains 60% to 90% by weight of sulfur relative to the total weight of the sulfur-carbon composite material.

9. The lithium-sulfur secondary battery according to claim 1, wherein the non-aqueous solvent further comprises non-fluorinated linear ethers, cyclic ethers, polyethers, or mixtures thereof.

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