Negative electrode for lithium secondary battery, method for manufacturing the same, and lithium secondary battery including the same

Abstract

The present invention relates to a negative electrode for a lithium secondary battery, a manufacturing method thereof, and a lithium secondary battery including the same. More specifically, the negative electrode has a salt coating layer containing lithium salt and additives formed between a plurality of lithium thin films. The salt coating layer dissolves during battery operation, replenishing the consumed lithium salt and additives in the electrolyte, thereby maintaining high cooling efficiency and improving the battery's lifespan.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 2021-0079872, filed on June 21, 2021, and all the contents disclosed in the corresponding Korean patent application are incorporated herein by reference in their entirety.

[0002] The present invention relates to a negative electrode for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the same.

Background Art

[0003] Recently, the miniaturization and weight reduction of electronic products, electronic devices, communication devices, etc. have been rapidly progressing. Due to the significant emergence of the need for electric vehicles in relation to environmental issues, the demand for improving the performance of secondary batteries used as power sources for these products is also increasing. Among them, lithium secondary batteries have received considerable attention as high-performance batteries due to their high energy density and high standard electrode potential characteristics.

[0004] In particular, a lithium-sulfur (Li-S) battery is a secondary battery that uses a sulfur-based substance having an S-S bond (Sulfur-Sulfur bond) as a positive electrode active material and lithium metal as a negative electrode active material. Sulfur, which is the main material of the positive electrode active material, is very rich in resources, non-toxic, and has the advantage of having a low weight per unit atom. In addition, the theoretical discharge capacity of a lithium-sulfur secondary battery is 1,675 mAh / g-sulfur, and the theoretical energy density is 2,600 Wh / kg, which is very high compared to the theoretical energy densities of other battery systems currently being studied (Ni-MH battery: 450 Wh / kg, Li-FeS battery: 480 Wh / kg, Li-MnO2 battery: 1,000 Wh / kg, Na-S battery: 800 Wh / kg). Therefore, it is the most promising battery among the batteries developed so far.

[0005] During the discharge reaction of a lithium-sulfur secondary battery, an oxidation reaction of lithium occurs at the negative electrode, and a reduction reaction of sulfur occurs at the positive electrode. Sulfur before discharge has a cyclic S8 structure. During the reduction reaction (discharge), the S-S bond is broken and the oxidation number of S decreases. During the oxidation reaction (charging), the S-S bond is formed again and the oxidation number of S increases. Electrical energy is stored and generated by utilizing such redox reactions. During such reactions, sulfur is converted from cyclic S8 to linear lithium polysulfide (Li2Sx, x = 8, 6, 4, 2) by the reduction reaction. Eventually, if such lithium polysulfide is completely reduced, lithium sulfide (Li2S) is finally generated. The discharge behavior of the lithium-sulfur secondary battery is characterized by showing a discharge voltage step by step, which is different from that of a lithium-ion battery, during the process of being reduced to each lithium polysulfide.

[0006] In addition, lithium sulfide generated at the positive electrode may cause degradation of battery performance through shuttling at the negative electrode. In order to prevent the shuttling phenomenon of the lithium sulfide, a stable SEI (Solid Electrolyte Interphase) must be formed on the lithium negative electrode. The SEI may be formed by the reaction of lithium metal with salts and additives contained in the electrolyte.

[0007] However, the salts and additives in the electrolyte participating in the formation of the SEI are continuously consumed, degrading the performance of the cell. When an excessive amount is introduced, there is a problem that overvoltage occurs and the rate performance decreases. Therefore, in order for a stable SEI that can prevent the shuttling phenomenon of the lithium sulfide to be formed and maintained, it is preferable that the consumption of salts and additives contained in the electrolyte is minimized.

[0008] Korean Patent Publication No. 2004-0026370 discloses a lithium negative electrode that improves the life characteristics of a battery by enhancing the conductivity of lithium ions in the lithium negative electrode. The lithium negative electrode includes organic protective layers on both surfaces of a lithium metal layer formed on a current collector. The organic protective layer has a configuration including a polymer and a lithium salt, and can improve the lithium ion conductivity of the lithium negative electrode, thereby improving the life characteristics of the battery. However, the organic protective layers on both surfaces of the lithium metal layer only serve to simply improve the lithium ion conductivity, and there is a problem in that they cannot perform the role of replenishing the lithium salt and additives in the electrolyte that are consumed as charge and discharge proceed.

[0009] Here, in order to improve the life characteristics of the battery, it is necessary to develop a lithium negative electrode that can replenish an electrolyte substance that can react with lithium to form an SEI.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0011] As a result of conducting extensive research to solve the above problems, the present inventors have confirmed that when a negative electrode in which a salt coating layer containing a lithium salt and an additive is formed between a plurality of lithium thin films is applied to a lithium secondary battery, the salts and additives of the electrolyte consumed during battery operation can be replenished from the salts and additives of the salt coating layer, thereby improving the life characteristics of the battery.

[0012] Accordingly, an object of the present invention is to provide a negative electrode for a lithium secondary battery containing a substance that can replenish the electrolyte consumed during the operation of the lithium secondary battery and a method for manufacturing the same.

[0013] Another object of the present invention is to provide a lithium secondary battery including the negative electrode for a lithium secondary battery.

Means for Solving the Problems

[0014] In order to achieve the above object, the present invention provides a negative electrode for a lithium secondary battery including a plurality of lithium thin films; and a salt coating layer formed between the plurality of lithium thin films.

[0015] The present invention also provides a method for manufacturing a negative electrode for a lithium secondary battery, including: (S1) forming a salt coating layer on one surface of a lithium thin film; (S2) laminating a plurality of the lithium thin films on which the salt coating layer is formed obtained in the step (S1); and (S3) laminating a lithium thin film on the salt coating layer exposed on the outermost shell of the laminate obtained in the step (S2).

[0016] The present invention also provides a lithium secondary battery including the negative electrode, a positive electrode, a separator positioned between the positive electrode and the negative electrode; and an electrolytic solution in which the positive electrode, the negative electrode, and the separator are impregnated.

Effects of the Invention

[0017] In the negative electrode for a lithium secondary battery according to the present invention, an SEI can be stably formed on the negative electrode by a lithium salt and an additive contained in the electrolyte at the initial stage of battery driving. Further, since a salt coating layer containing a lithium salt and an additive is formed between a plurality of lithium thin films in the negative electrode for a lithium secondary battery, when the lithium salt and the additive contained in the electrolyte are consumed as the battery driving proceeds, the lithium salt and the additive in the electrolyte can be replenished from the lithium salt and the additive in the salt coating layer, and the SEI can be stably formed on the negative electrode. Here, the lithium secondary battery can improve its life performance while maintaining a high coulomb efficiency.

[0018] In addition, when the negative electrode is applied to a lithium-sulfur secondary battery, an SEI can be stably formed and maintained on the lithium negative electrode, preventing the shuttle phenomenon of lithium sulfide gushing out from the positive electrode, so that the time when battery performance degrades can be delayed.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0020] Hereinafter, in order to make the present invention easier to understand, the present invention will be described in more detail.

[0021] The terms and words used in this specification and claims should not be construed in a limited sense in accordance with ordinary or dictionary meanings, and the inventor must interpret them in a meaning and concept consistent with the technical idea of the present invention in accordance with the principle that the concept of the terms can be appropriately defined in order to explain his invention in the best way.

[0022] Negative Electrode for Lithium Secondary Battery The present invention relates to a negative electrode for a lithium secondary battery.

[0023] The negative electrode for a lithium secondary battery according to the present invention includes a plurality of lithium thin films and a salt coating layer formed between each of the plurality of lithium thin films. Optionally, one of the plurality of lithium thin films may be formed on the negative electrode current collector. At this time, the plurality means n, and n is an integer of 2 to 4. If the number of lithium thin films is less than 2, a structure in which a salt coating layer is formed between the lithium thin films cannot be obtained. If it exceeds 4, the thickness of the negative electrode becomes too thick, and the energy density may decrease.

[0024] In addition, as charge and discharge proceed, the lithium thin film can become porous with pores formed by electrodeposition and desorption of lithium.

[0025] FIG. 1 shows a schematic diagram of a negative electrode for a lithium secondary battery according to an embodiment of the present invention and a schematic diagram when the salt coating layer is dissolved.

[0026] Referring to FIG. 1, the negative electrode for a lithium secondary battery according to an embodiment of the present invention has a salt coating layer 20 formed between lithium foils (Li foil, 10a, 10b) which are two lithium thin films. As the lithium secondary battery repeats charge and discharge, lithium is repeatedly electrodeposited and desorbed, and the lithium foils 10a, 10b are porous. When the salt coating layer 20 is exposed to the electrolyte between the pores (P) formed by the lithium foils 10a, 10b becoming porous, the salt coating layer 20 is dissolved, and the lithium salt, additive, and electrolyte contained in the salt coating layer 20 dissolve and come out. The lithium salt and additive dissolved in the electrolyte can react with the lithium thin film to stably form SEI.

[0027] At the initial stage of battery operation, the SEI can be formed sufficiently stably only with the lithium salt and additives contained in the electrolyte. However, as the battery operation progresses, if the lithium salt and additives contained in the electrolyte are consumed, as described above, the lithium salt and additives in the salt coating layer will dissolve and come out, and will replenish the lithium salt and additives of the consumed electrolyte, so that the SEI can be formed and maintained stably.

[0028] If the salt coating layer is formed on the exposed surface of the lithium thin film instead of between the lithium thin films, a large amount of the lithium salt and additives contained in the salt coating layer will dissolve into the electrolyte at the initial stage of battery operation, increasing the viscosity of the electrolyte and raising the overvoltage.

[0029] Also, if the salt coating layer is formed between the current collector and the lithium thin film, it may increase the battery resistance or interfere with the electrodeposition of lithium on the current collector during charging.

[0030] In the present invention, the salt coating layer can react with lithium to form a SEI (Solid Electrolyte Interphase), and can play a role in suppressing the shuttling of lithium polysulfide formed in the sulfur-containing positive electrode and improving the battery life. Also, the salt coating layer can contain substances that can replenish the lithium salt and additives consumed by the electrolyte during battery operation.

[0031] The salt coating layer can contain a lithium salt and additives. Since the salt coating layer serves to replenish the lithium salt and additives in the electrolyte consumed when the battery is driven, the lithium salt and additives contained in the salt coating layer can be used without limitation as long as they are the lithium salt and additives that can be contained in the electrolyte for lithium secondary batteries.

[0032] In addition, the salt coating layer can contain 40 to 80% by weight of a lithium salt and 20 to 60% by weight of an additive.

[0033] The lithium salt can be used without limitation as long as it is commonly used in electrolytes for lithium secondary batteries. The lithium salt can include one or more selected from the group consisting of LiN(C2F5SO2)2 (Lithium bis(perfluoroethylsulfonyl)imide, LiBETI), LiN(C2F5SO3)2, LiN(FSO2)2 (Lithium bis(fluorosulfonyl)imide, LiFSI), LiN(CF3SO2)2 (Lithium bis(Trifluoromethanesulfonyl)imide, LiTFSI), LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAl04, LiAlCl4, LiCF3SO3, and LiC4F9SO3. Among them, LiBETI, LiFSI, or LiTFSI may be preferable.

[0034] In addition, the lithium salt can be contained in an amount of 40 to 80% by weight based on the total weight of the salt coating layer. Specifically, the content of the lithium salt may be 40% by weight or more, or 50% by weight or more, and may be 65% by weight or less, 70% by weight or less, or 80% by weight or less. If the content of the lithium salt is less than 40% by weight, the content of the additive becomes relatively high, making it difficult to manufacture a uniform coating solution and difficult to form a salt coating layer. If it exceeds 80% by weight, the content of the additive becomes relatively low, so it may be difficult to form an appropriate SEI protective layer.

[0035] The additive can be used without limitation as long as it is commonly used in the electrolyte for lithium secondary batteries. In particular, an additive that does not react with lithium metal can be used. For example, the additive can include one or more inorganic nitrate compounds selected from lithium nitrate (LiNO3) and lithium nitrite (LiNO2); and one or more organic nitrate compounds including nitromethane (CH3NO2) and methyl nitrate (CH3NO3); and can include one or more selected from the group consisting of these. The additive may contain lithium nitrate (LiNO3) in consideration of its compatibility with the lithium salt.

[0036] Also, the additive can be contained in an amount of 20% by weight to 60% by weight based on the total weight of the salt coating layer. Specifically, the content of the additive may be 20% by weight or more, 30% by weight or more, or 35% by weight or more, and may be 50% by weight or less, 55% by weight or less, or 60% by weight or less. If the content of the additive is less than 20% by weight, it may be difficult to form an appropriate SEI protective layer. If it exceeds 60% by weight, it may be difficult to manufacture a uniform coating solution and it may also be difficult to form the salt coating layer.

[0037] Also, the thickness of the salt coating layer may be 100 nm to 3 μm. Specifically, the thickness of the salt coating layer may be 100 nm or more, 500 nm or more, or 700 nm or more, and may be 1 μm or less, 2 μm or less, or 3 μm or less. If the thickness of the salt coating layer is 100 nm or less, the salt or additive of the electrolyte consumed during battery operation cannot be sufficiently replenished. If it exceeds 3 μm, the electrical resistance may increase and the overvoltage may increase during battery operation.

[0038] In the present invention, the lithium thin film can serve as a negative electrode active material.

[0039] The thickness of the lithium thin film may be 10 μm to 50 μm. Specifically, the thickness of the lithium thin film may be 10 μm or more, 15 μm or more, or 20 μm or more, and may also be 40 μm or less, 45 μm or less, or 50 μm or less. If the thickness of the lithium thin film is less than 10 μm, it may be difficult to proceed with the process of manufacturing the negative electrode using a plurality of lithium thin films. If it exceeds 50 μm, the thickness of the negative electrode including a plurality of lithium thin films may increase, and the energy density may decrease.

[0040] In the present invention, the negative electrode current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, the negative electrode current collector can be copper, stainless steel, aluminum, nickel, titanium, fired carbon, or a surface-treated product of copper or stainless steel with carbon, nickel, titanium, silver, etc. on its surface, or an aluminum-cadmium alloy, etc. can be used. Also, similar to the positive electrode current collector, the negative electrode current collector can be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric body, etc. with fine irregularities formed on its surface.

[0041] Method for manufacturing a negative electrode for a lithium secondary battery The present invention also relates to a method for manufacturing a negative electrode for a lithium secondary battery. The method for manufacturing the negative electrode for a lithium secondary battery includes: (S1) forming a salt coating layer on one surface of a lithium thin film; (S2) laminating a plurality of the lithium thin films on which the salt coating layer is formed obtained in the step (S1); and (S3) laminating a lithium thin film on the salt coating layer exposed on the outermost shell of the laminate obtained in the step (S2).

[0042] Hereinafter, the method for manufacturing a negative electrode for a lithium secondary battery according to the present invention will be described in more detail for each step.

[0043] In the step (S1), a salt coating layer can be formed on one surface of the lithium thin film. In order to form the salt coating layer, after manufacturing a coating solution for forming the salt coating layer, it can be applied to one surface of the lithium thin film and then dried.

[0044] The coating solution for forming the salt coating layer can be manufactured by dissolving a lithium salt and an additive in a solvent. The types of the lithium salt and the additive are as described above. The content of the solvent may be 60 to 80% by weight based on the total weight of the coating solution, and the coating solution can be manufactured using a solvent in an amount that allows the coating process to proceed smoothly.

[0045] Also, the coating is not particularly limited as long as it is a coating method generally used for forming a coating layer. For example, the coating method for forming the coating layer can be selected from the group consisting of bar coating, roll coating, spin coating, slit coating, die coating, blade coating, comma coating, slot die coating, lip coating, and solution casting. When considering the coating efficiency on the lithium thin film, the salt coating layer can be formed by bar coating.

[0046] Moreover, the drying conditions are not particularly limited as long as they can remove the solvent of the coating solution. For example, the drying temperature may be 20°C to 30°C. Specifically, the drying temperature may be 20°C or higher, 22°C or higher, or 24°C or higher, and may also be 26°C or lower, 28°C or lower, or 30°C or lower. If the drying temperature is less than 20°C, the solvent contained in the coating solution may not be completely removed. If it exceeds 30°C, cracks may occur in the salt coating layer, or side reactions between the coating solution and lithium may occur. When drying is performed at room temperature (25°C) under vacuum, side reactions between the coating solution and lithium can be prevented.

[0047] In the step (S2), a plurality of lithium thin films formed with the salt coating layer produced in the step (S1) can be laminated. At this time, they can be laminated in a form in which a salt coating layer is included between the lithium thin films.

[0048] At this time, the plurality means n, and n is an integer from 2 to 4. When the number of lithium thin films is 2 or more, a negative electrode including a structure in which a salt coating layer is formed between the lithium thin films can be manufactured. If it exceeds 4, the thickness of the negative electrode may increase and the energy density may decrease.

[0049] In the step (S3), a lithium thin film can be laminated on the salt coating layer exposed on the outermost shell of the laminate obtained in the step (S2). At this time, the lithium thin film means a lithium thin film on which no salt coating layer is formed. After laminating the lithium thin films, by rolling and laminating them, a negative electrode including a laminate in which a salt coating layer is formed between a plurality of lithium thin films can be manufactured.

[0050] At this time, the method for rolling is not particularly limited as long as it is a method generally used for laminating films, layers, etc. For example, the rolling can be performed by applying a pressure sufficient to laminate each layer, and the pressure is 0.8 Mpa to 15 Mpa.

[0051] Also, when using a lithium thin film attached to the negative electrode current collector, a negative electrode can be manufactured by using a negative electrode current collector (lithium / current collector / lithium) with lithium thin films formed on both sides and a lithium thin film (salt coating layer / lithium) with a salt coating layer formed on one side. For example, a negative electrode can be manufactured by laminating such that the salt coating layer of the lithium thin film with the salt coating layer formed on the lithium thin films formed on both sides of the negative electrode current collector touches (lithium / salt coating layer-lithium / current collector / lithium-salt coating layer / lithium).

[0052] At this time, the current collector to be used is not particularly limited as long as it is used as a negative electrode current collector for a lithium secondary battery. For example, the negative electrode current collector may be copper, nickel, tin, lead, or stainless steel.

[0053] Lithium secondary battery The present invention also relates to a lithium secondary battery including a positive electrode, a negative electrode, a separator interposed therebetween, and an electrolytic solution, and the negative electrode is as described above.

[0054] In the present invention, the positive electrode includes a positive electrode active material layer formed on a positive electrode current collector. The positive electrode active material layer may include a positive electrode active material, a binder, and a conductive material.

[0055] The loading amount of the positive electrode may be 3.0 mAh / cm 2 to 5.0 mAh / cm 2 for consideration of the stable life characteristics of the battery.

[0056] Also, the porosity of the positive electrode may be 60% to 80%.

[0057] As the positive electrode current collector, any material can be used as long as it can be used as a current collector in the technical field, and specifically, it is preferable to use foamed aluminum, foamed nickel, etc. having excellent conductivity.

[0058] The positive electrode active material can include elemental sulfur (Elemental sulfur, S8), sulfur-based compounds, or mixtures thereof. Specifically, the sulfur-based compounds may be, for example, Li2Sn (n≥1), organic sulfur compounds, or carbon-sulfur polymers ((C2Sx)n: x = 2.5 to 50, n≥2). Since these sulfur substances alone have no electrical conductivity, they are applied in combination with a conductive material. The positive electrode active material can be contained in an amount of 50 to 90% by weight based on the total weight of the positive electrode active material layer.

[0059] The conductive material may be porous. Therefore, any conductive material having porosity and conductivity can be used without limitation. For example, a carbon-based material having porosity can be used. As such carbon-based materials, carbon black, graphite, graphene, activated carbon, carbon fiber, etc. can be used. Also, metallic fibers such as metal meshes; metallic powders such as copper, silver, nickel, aluminum, etc.; or organic conductive materials such as polyphenylene derivatives can be used. The conductive materials can be used alone or in combination. The conductive material can be contained in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer.

[0060] The positive electrode can further include a binder for the binding of the positive electrode active material and the conductive material and the binding to the current collector. The binder can include a thermoplastic resin or a thermosetting resin. For example, the binder can be polyethylene, polyethylene oxide, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene rubber, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer, polychlorotrifluoroethylene, vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene-chlorotrifluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymer, ethylene-acrylic acid copolymer, etc., which can be used alone or in combination, but are not necessarily limited thereto, and any material that can be used as a binder in the technical field is possible. The binder can be included in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer.

[0061] The positive electrode as described above can be manufactured by a conventional method. Specifically, a composition for forming a positive electrode active material layer, which is prepared by mixing a positive electrode active material, a conductive material, and a binder in a solvent, is applied and dried on a current collector, and optionally compression-molded onto the current collector to improve the electrode density. At this time, the solvent can be water or an organic solvent. In the case of the organic solvent, it is preferable to use a solvent that can uniformly disperse the positive electrode active material, the binder, and the conductive material and can be easily evaporated. Specifically, acetonitrile, methanol, ethanol, tetrahydrofuran, water, isopropyl alcohol, etc. can be mentioned.

[0062] In the present invention, the separation membrane is a physical separation membrane having a function of physically separating electrodes, and can be used without particular limitation as long as it is a separation membrane normally used. In particular, a separation membrane having low resistance to ion movement of the electrolyte and excellent moisture retention ability of the electrolyte is preferable.

[0063] Further, the separation membrane allows lithium ions to be transported between the positive electrode and the negative electrode while separating or insulating the positive electrode and the negative electrode from each other. Such a separation membrane is made of a porous, non-conductive or insulating material. The separation membrane may be an independent member such as a film, or may be a coating layer applied to the positive electrode and / or the negative electrode.

[0064] Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as a homopolymer of ethylene, a homopolymer of propylene, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer can be used alone or in a laminated form thereof, or a normal porous nonwoven fabric, for example, a nonwoven fabric made of high melting point glass fibers, polyethylene terephthalate fibers, etc. can be used, but it is not limited thereto.

[0065] In the present invention, the electrolyte contains a lithium salt, an additive, and a solvent as a non-aqueous electrolyte containing a lithium salt, and a non-aqueous organic solvent can be used as the solvent.

[0066] The lithium salt of the present invention is a substance that is easily dissolved in a non-aqueous organic solvent. For example, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiB(Ph)4, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, lithium chloroborane, lithium lower aliphatic carboxylic acid, lithium 4-phenylborate, imide, and one or more of them can be included.

[0067] The concentration of the lithium salt may be 0.2 to 2 M, specifically 0.6 to 2 M, more specifically 0.7 to 1.7 M, depending on many 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 lithium battery field. If it is used at less than 0.2 M, the conductivity of the electrolyte may be low and the electrolyte performance may be degraded. If it is used in excess of 2 M, the viscosity of the electrolyte may increase and the mobility of lithium ions (Li+) may be decreased.

[0068] The non-aqueous organic solvent should dissolve the lithium salt well. Examples of the non-aqueous organic solvent of the present invention include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, 4-methyl-1,3-dioxene, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate ester, trimethoxymethane, dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl pyruvate, ethyl propionate, etc. The organic solvent may be a single organic solvent or a mixture of two or more organic solvents.

[0069] In the present invention, the lithium secondary battery may be a lithium-sulfur secondary battery containing sulfur as a positive electrode active material in particular.

[0070] The positive electrode, separator, and electrolyte included in the lithium-sulfur secondary battery can be prepared by ordinary components and manufacturing methods, respectively. Also, although the outer shape of the lithium-sulfur secondary battery is not particularly limited, it may be a cylindrical shape, a rectangular shape, a pouch type, a coin type, etc. using a can.

[0071] Hereinafter, preferred embodiments are presented to assist in understanding the present invention. However, the following embodiments are merely illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications can be made within the scope of the present invention and the scope of the technical idea, and it is natural that such changes and modifications belong to the appended claims.

[0072] Example 1 (1) Manufacture of lithium foil with a salt coating layer formed LiFSI and LiNO3 were dissolved in a 1,2-dimethoxyethane solvent to prepare a coating solution for forming a salt coating layer. At this time, the weight ratio of the LiFSI and LiNO3 was set to 48:52, and the solvent was set to 69% by weight based on the total weight of the coating solution to prepare the coating solution.

[0073] Using a bar coater, the coating solution was coated on a lithium foil with a thickness of 45 μm, and then dried under normal temperature (25 °C) and vacuum conditions to remove the solvent, thereby forming a salt coating layer.

[0074] (2) Manufacture of negative electrode A lithium foil having the same thickness without a salt coating layer was laminated on the lithium foil having the salt coating layer formed thereon, and rolled to manufacture a negative electrode. The negative electrode includes a laminate structure in which a lithium foil, a salt coating layer, and a lithium foil are sequentially laminated (45 μm of Li / salt coating layer / 45 μm of Li).

[0075] (3) Manufacture of lithium-sulfur secondary battery After mixing sulfur-carbon composite (S-CNT), a conductive material, and a binder in a weight ratio of 90:5:5, they were mixed with water as a solvent to produce a slurry. Then, the slurry was coated, dried, and rolled onto an aluminum foil to produce a positive electrode. At this time, the conductive material used was VGCF (Vapor grown carbon fiber), and the binder used was SBR (Styrene Butadiene Rubber). Also, the coating was performed using a doctor blade. The slurry was coated with the doctor blade and rolled to produce a positive electrode. Also, the porosity of the positive electrode was made 70% by the rolling.

[0076] The electrolyte was prepared by dissolving 0.75 M of LiFSI and 5.0 wt% of LiNO3 in a mixed solvent of 2-methylfuran and dimethoxyethane (2ME:DME = 2:8 (v / v)).

[0077] An electrode assembly was manufactured with a porous polyethylene separator having a thickness of 20 μm and a porosity of 45% interposed between the positive electrode and the negative electrode. After positioning the electrode assembly inside the case, the electrolyte was injected into the case to manufacture a lithium-sulfur secondary battery.

[0078] Example 2 A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that a 30-μm-thick lithium foil was used instead of a 45-μm-thick lithium foil, and two salt coating layers were laminated between the three layers of lithium foil (30 μm of Li / salt coating layer / 30 μm of Li / salt coating layer / 30 μm of Li).

[0079] Comparative Example 1 A lithium negative electrode and a lithium-sulfur secondary battery including the same were manufactured in the same manner as in Example 1, except that a single 90-μm-thick lithium foil without a salt coating layer was used.

[0080] Comparative Example 2 The additive was not LiNO3. 2 g of PEO (polyethylene oxide, Mw = 100,000) and 1.303 g of LiTFSI were mixed in an acetonitrile solvent to prepare a coating solution. The concentration of the coating solution was adjusted to 10 wt% based on the solid content concentration. At this time, a lithium-sulfur secondary battery including a lithium anode was manufactured in the same manner as in Example 1, except that the coating solution was coated on a lithium foil to a thickness of 30 μm by a bar coating method to form a polymer coating layer.

[0081] Experimental Example 1: Battery performance experiment After charging and discharging the lithium-sulfur secondary batteries of Example 1 and Comparative Example 1, the excess amount of the injected electrolyte compared to the control, the initial capacity, the nominal voltage at 0.1C discharge, and the life characteristics were measured, and the results are shown in Table 1 below. Charging and discharging were performed at 25 °C under 1.8V - 2.5V cut-off conditions with 0.1C / 0.1C charging and discharging 3 times, 0.2C / 0.2C charging and discharging 3 times, and 0.2C / 0.3C charging and discharging. The charging and discharging results were measured using a charging and discharging measurement device (LAND CT-2001A, manufactured by Wuhan Co., Ltd.).

[0082] In Table 1 below, the excess amount of the injected electrolyte compared to the control means the content of the lithium salt and the additive contained in the coating layer compared to the injected electrolyte. Also, the life (Retention 80%, cycle) means the number of cycles at the time when the capacity retention rate reaches 80% compared to the initial discharge capacity at 0.3C.

[0083]

Table 1

[0084] As shown in Table 1, Figure 2, and Figure 3 above, it can be seen that the lithium-sulfur secondary batteries of Example 1 and Example 2 have the same level of initial capacity and nominal voltage as the lithium-sulfur secondary battery of Comparative Example 1, and exhibit excellent Coulomb efficiency and life performance.

[0085] Experimental Example 2: Experiment on the leaching evaluation of the polymer coating layer and the salt coating layer For the experiment on the leaching evaluation of the polymer coating layer and the salt coating layer, the polymer coating layer and the salt coating layer of Comparative Example 2 and Example 2 were made into Sample 1 and Sample 2 respectively as follows, and the leaching experiment was carried out: - Sample 1: Polymer coating layer formed on a lithium foil - Sample 2: Salt coating layer formed on a lithium foil

[0086] In the process of manufacturing the above-mentioned Sample 1 and Sample 2, Bare Li (g), weight after coating (g), weight of the coating layer (g), and weight after loading on the solvent (g) were measured to confirm the leaching amount of the coating layer. At this time, the loading time was divided into 10 minutes (10 min), 1 hour (1 hr), and 1 day (1 d) respectively for the experiment. At this time, the weight after coating means the sum of the weight of Bare Li and the weight of the coating layer, and it is the weight measured after the coated Li thin film was loaded on the solvent for a specific time, taken out, and dried. The leaching rate of the coating layer was calculated by the following formula 1.

[0087] <Equation 1> (Weight after coating - Weight after loading on the solvent) / (Weight after coating - Weight of Bare Li)

[0088] [Table 2]

[0089] [Table 3]

[0090] As shown in Table 2 and Table 3 above, it was confirmed that for Sample 1, the entire polymer coating layer was desorbed into the solvent within 10 minutes, while for Sample 2, the salt contained in the salt coating layer was gradually dissolved into the solvent.

[0091] Thus, when applying the salt coating layer to the lithium negative electrode, it can be seen that it can serve to supply salt to the electrolyte for a longer period compared to the polymer coating layer, which is advantageous for improving battery performance and life characteristics.

[0092] As described above, although the present invention has been described by way of limited examples and drawings, the present invention is not limited thereby, and it goes without saying that various modifications and variations are possible within the scope equivalent to the technical idea of the present invention and the following claims by those having ordinary knowledge in the technical field to which the present invention pertains.

Explanation of Reference Numerals

[0093] 10a, 10b: Lithium foil 20: Salt coating layer P: Pore

Claims

1. a plurality of lithium thin films; and a salt coating layer formed between the plurality of lithium thin films, the salt coating layer contains a lithium salt and an additive, the lithium salt is LiN(C ２ F ５ SO ２ )( ２ (Lithium bis(perfluoroethylsulfonyl)imide, LiBETI), LiN(C ２ F ５ SO ３ )( ２ , LiN(FSO ２ )( ２ (Lithium bis(fluorosulfonyl)imide, LiFSI), LiN(CF ３ SO ２ )( ２ (Lithium bis(Trifluoromethanesulfonyl)imide, LiTFSI), LiPF ６ , LiClO ４ , LiAsF ６ , LiBF ４ , LiSbF ６ , LiAlO4, LiAlCl ４ , LiCF ３ SO ３ and LiC ４ F ９ SO ３ and contains one or more selected from the group consisting of the additive is an inorganic nitrate compound containing one or more selected from lithium nitrate (LiNO ３ ) and lithium nitrite (LiNO ２ ); and an organic nitrate compound containing one or more selected from nitromethane (CH ３ NO ２ ) and methyl nitrate (CH ３ NO ３ ); and contains one or more selected from the group consisting of The negative electrode for a lithium secondary battery, wherein the salt coating layer contains 40% to 80% by weight of a lithium salt and 20% to 60% by weight of an additive.

2. A plurality of lithium thin films; and a salt coating layer formed between the plurality of lithium thin films, the salt coating layer contains a lithium salt and an additive, the lithium salt contains one or more selected from the group consisting of LiN(C₂F₅SO₂)₂ (Lithium bis(perfluoroethylsulfonyl)imide, LiBETI), LiN(C₂F₅SO₃)₂, LiN(FSO₂)₂ (Lithium bis(fluorosulfonyl)imide, LiFSI), LiN(CF₃SO₂)₂ (Lithium bis(Trifluoromethanesulfonyl)imide, LiTFSI), LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiSbF₆, LiAlO₄, LiAlCl₄, LiCF₃SO₃ and LiC₄F₉SO₃, the additive contains one or more inorganic nitrate compounds selected from lithium nitrate (LiNO₃) and lithium nitrite (LiNO₂); and one or more organic nitrate compounds containing one or more of nitromethane (CH₃NO₂) and methyl nitrate (CH₃NO₃); and contains one or more selected from the group consisting of The negative electrode for a lithium secondary battery, wherein the thickness of the salt coating layer is 100 nm to 3 μm.

3. A plurality of lithium thin films; and a salt coating layer formed between the plurality of lithium thin films, the salt coating layer contains a lithium salt and an additive, The lithium salt contains one or more selected from the group consisting of LiN(C₂F₅SO₂)₂ (Lithium bis(perfluoroethylsulfonyl)imide, LiBETI), LiN(C₂F₅SO₃)₂, LiN(FSO₂)₂ (Lithium bis(fluorosulfonyl)imide, LiFSI), LiN(CF₃SO₂)₂ (Lithium bis(Trifluoromethanesulfonyl)imide, LiTFSI), LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiSbF₆, LiAlO₄, LiAlCl₄, LiCF₃SO₃, and LiC₄F₉SO₃. The additive contains one or more selected from the group consisting of inorganic nitrate compounds containing one or more selected from lithium nitrate (LiNO₃) and lithium nitrite (LiNO₂); and organic nitrate compounds containing one or more selected from nitromethane (CH₃NO₂) and methyl nitrate (CH₃NO₃). The negative electrode for a lithium secondary battery, wherein the thickness of the lithium thin film is 10 μm to 50 μm.

4. The negative electrode for a lithium secondary battery according to any one of claims 1 to 3, wherein the lithium thin film is porous.

5. The negative electrode for a lithium secondary battery according to any one of claims 1 to 3, wherein the plurality is 2 to 4.

6. (S1) Forming a salt coating layer on one surface of a lithium thin film; (S2) Stacking a plurality of the lithium thin films on which the salt coating layer is formed obtained in the step (S1); and (S3) Stacking a lithium thin film on the salt coating layer exposed on the outermost shell of the laminate obtained in the step (S2). The method includes: The salt coating layer contains a lithium salt and an additive. The lithium salt contains one or more selected from the group consisting of LiN(C₂F₅SO₂)₂ (Lithium bis(perfluoroethylsulfonyl)imide, LiBETI), LiN(C₂F₅SO₃)₂, LiN(FSO₂)₂ (Lithium bis(fluorosulfonyl)imide, LiFSI), LiN(CF₃SO₂)₂ (Lithium bis(Trifluoromethanesulfonyl)imide, LiTFSI), LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiSbF₆, LiAlO₄, LiAlCl₄, LiCF₃SO₃, and LiC₄F₉SO₃. The additive contains one or more inorganic nitrate compounds selected from lithium nitrate (LiNO₃) and lithium nitrite (LiNO₂); and one or more organic nitrate compounds selected from nitromethane (CH₃NO₂) and methyl nitrate (CH₃NO₃); and is a method for manufacturing a negative electrode for a lithium secondary battery containing one or more selected from the group consisting of.

7. The negative electrode according to any one of claims 1 to 3; Positive electrode; A separator positioned between the positive electrode and the negative electrode; and An electrolyte impregnating the positive electrode, negative electrode, and separator; A lithium secondary battery comprising.

8. The lithium secondary battery according to claim 7, wherein the lithium secondary battery is a lithium-sulfur secondary battery.

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