Anode for lithium secondary batteries, manufacturing method thereof, and lithium secondary batteries comprising thereof

Abstract

This invention relates to a negative electrode for lithium secondary batteries, a method for manufacturing the same, and a lithium secondary battery comprising the same. More specifically, in the negative electrode, a salt coating containing lithium salt and additives is formed between multiple lithium films, such that the salt coating dissolves during battery operation and replenishes the lithium salt and additives consumed in the electrolyte, thereby improving battery life characteristics while maintaining high coulombic efficiency.

Description

Technical Field

[0001] This application claims priority to Korean Patent Application No. 2021-0079872, filed on June 21, 2021, the entire contents of which are incorporated herein by reference.

[0002] This invention relates to a negative electrode for lithium secondary batteries, a method for manufacturing the same, and a lithium secondary battery comprising the same. Background Technology

[0003] Recently, with the rapid miniaturization and lightweighting of electronic products, devices, and communication equipment, and the significant increase in demand for electric vehicles related to environmental issues, the need for improved performance of secondary batteries used as energy sources for these products has been growing. Among them, lithium secondary batteries, as high-performance batteries, have always attracted much attention due to their high energy density and high standard electrode potential characteristics.

[0004] Specifically, lithium-sulfur (Li-S) batteries are secondary batteries that use sulfur-sulfur based 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 for the positive electrode active material, has the advantages of being abundant, non-toxic, and having a low atomic weight. Furthermore, the theoretical discharge capacity of lithium-sulfur secondary batteries is 1,675 mAh / g sulfur, and its theoretical energy density is 2,600 Wh / kg. Since the theoretical energy density of lithium-sulfur secondary batteries is much higher than that of other battery systems currently under research (Ni-MH battery: 450 Wh / kg, Li-FeS battery: 480 Wh / kg, Li-MnO2 battery: 1000 Wh / kg, Na-S battery: 800 Wh / kg), lithium-sulfur secondary batteries are the most promising batteries developed to date.

[0005] During the discharge of a lithium-sulfur secondary battery, lithium oxidation occurs at the negative electrode, and sulfur reduction occurs at the positive electrode. Sulfur has a cyclic S8 structure before discharge. During the reduction reaction (discharge), the oxidation number of sulfur (S) decreases as the S-S bonds are broken. During the oxidation reaction (charging), the S-S bonds reform, and electrical energy is stored and generated through a redox reaction in which the oxidation number of sulfur (S) increases. During this reaction, sulfur is converted from a cyclic S8 structure to lithium polysulfide (Li2S) through reduction. x (x = 8, 6, 4, 2), and finally, when the lithium polysulfides are completely reduced, lithium sulfide (Li2S) is produced. Unlike lithium-ion batteries, the discharge behavior of lithium-sulfur secondary batteries is characterized by staged discharge voltages through the process of reduction to various lithium polysulfides.

[0006] Furthermore, lithium sulfide generated at the positive electrode can shuttle to the negative electrode, leading to battery performance degradation. To prevent lithium sulfide shuttle, a stable solid electrolyte interface (SEI) should be formed in the lithium negative electrode. The SEI can be formed by the reaction of lithium metal with salts and additives contained in the electrolyte.

[0007] However, there are problems such as the continuous consumption of salts and additives involved in SEI formation in the electrolyte, which degrades battery performance and causes overvoltage under excessive input, thus reducing rate performance. Therefore, in order to stably form and maintain an SEI that can prevent lithium sulfide shuttle phenomenon, it is preferable to minimize the consumption of salts and additives contained in the electrolyte.

[0008] Korean Patent Publication No. 2004-0026370 discloses a lithium anode that improves battery life characteristics by enhancing lithium-ion conductivity in the lithium anode. The lithium anode includes organic protective layers on both surfaces of a lithium metal layer formed on a current collector. These organic protective layers are configured to contain a polymer and a lithium salt, thereby improving the lithium-ion conductivity of the lithium anode and thus enhancing battery life characteristics. However, the presence of organic protective layers on both surfaces of the lithium metal layer only improves lithium-ion conductivity but does not replenish the lithium salt or additives in the electrolyte consumed during charging / discharging.

[0009] Therefore, in order to improve the lifespan characteristics of batteries, it is necessary to develop a lithium anode that can supplement the electrolyte material that can react with lithium to form an SEI.

[0010] [Existing technical documents]

[0011] [Patent Literature]

[0012] (Patent Document 1) Korean Patent Publication No. 2004-0026370 Summary of the Invention

[0013] Technical issues

[0014] The inventors of this invention have conducted research in various ways to solve the above problems, and have confirmed that if a negative electrode in which a salt coating containing lithium salt and additives is formed between multiple lithium films is applied to a lithium secondary battery, the salt and additives in the electrolyte consumed during battery operation can be replenished from the salt and additives in the salt coating, thereby improving the battery's lifespan characteristics.

[0015] Therefore, the object of the present invention is to provide a negative electrode for a lithium secondary battery and a method for manufacturing the same, wherein the negative electrode comprises a material capable of replenishing the electrolyte consumed during operation of the lithium secondary battery.

[0016] Another object of the present invention is to provide a lithium secondary battery comprising the negative electrode for the lithium secondary battery.

[0017] Technical solution

[0018] To achieve the above objective, the present invention provides a negative electrode for a lithium secondary battery, comprising a plurality of lithium films and a salt coating formed between the plurality of lithium films.

[0019] Furthermore, the present invention provides a method for manufacturing a negative electrode for a lithium secondary battery, comprising the following steps: (S1) forming a salt coating on one surface of a lithium film; (S2) stacking multiple lithium films with the salt coating formed thereon obtained in step (S1); and (S3) exposing the lithium film stacked in step (S2) to the outermost salt coating.

[0020] The present invention also provides a lithium secondary battery comprising the negative electrode, the positive electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte impregnating the positive electrode, the negative electrode and the separator.

[0021] Beneficial effects

[0022] In the negative electrode for lithium secondary batteries according to the present invention, during the initial stage of battery operation, an electrolyte electrolyte interphase (SEI) can be stably formed on the negative electrode by the lithium salt and additives contained in the electrolyte. Furthermore, in the negative electrode for lithium secondary batteries, since a salt coating containing lithium salt and additives is formed between multiple lithium films, if the lithium salt and additives contained in the electrolyte are consumed during battery operation, the lithium salt and additives in the electrolyte can be replenished from the lithium salt and additives in the salt coating, thereby stably forming an SEI on the negative electrode. This improves the lifespan performance of the lithium secondary battery while maintaining high coulombic efficiency.

[0023] Furthermore, if the negative electrode is applied to a lithium-sulfur secondary battery, an SEI can be stably formed and maintained on the lithium negative electrode, thereby preventing the shuttle phenomenon of lithium sulfide dissolved from the positive electrode, and thus delaying the time of battery performance degradation. Attached Figure Description

[0024] Figure 1 A schematic diagram of a negative electrode for a lithium secondary battery according to an embodiment of the present invention, and a schematic diagram of the salt coating dissolving are shown.

[0025] Figure 2 The graph shows the initial charge / discharge capacity of the lithium-sulfur secondary batteries of Examples 1 and 2 and Comparative Example 1.

[0026] Figure 3The graph shows the lifespan characteristics of the lithium-sulfur secondary batteries of Examples 1 and 2 and Comparative Example 1. Detailed Implementation

[0027] The invention will now be described in more detail to aid in understanding it.

[0028] The terms and words used in this specification and claims should not be considered as limited to ordinary or dictionary terms, but should be interpreted in a meaning and concept consistent with the technical idea of ​​the invention, based on the principle that the inventor can appropriately define the concepts of the terms in order to describe his invention in the best possible way.

[0029] Negative electrode for lithium secondary batteries

[0030] This invention relates to a negative electrode for lithium secondary batteries.

[0031] The negative electrode for a lithium secondary battery according to the present invention comprises a plurality of lithium films and a salt coating formed between each of the plurality of lithium films. Furthermore, if necessary, one of the plurality of lithium films may be formed on the negative electrode current collector. In this case, "plural" means n, where n can be an integer from 2 to 4. If the number of lithium films is less than 2, a structure with a salt coating formed between the lithium films cannot be obtained; and if the number of lithium films exceeds 4, the thickness of the negative electrode may become excessive, thus potentially reducing the energy density.

[0032] Furthermore, as charging / discharging proceeds, the lithium film may become porous, with pores formed through lithium electrodeposition and desorption.

[0033] Figure 1 A schematic diagram of a negative electrode for a lithium secondary battery according to an embodiment of the present invention, and a schematic diagram of the salt coating dissolving are shown.

[0034] refer to Figure 1 According to one embodiment of the present invention, the negative electrode for a lithium secondary battery may have a salt coating 20 formed between two lithium films, namely lithium foils (Li foils) 10a and 10b. As the lithium secondary battery is repeatedly charged / discharged, lithium electrodeposition and desorption occur repeatedly, causing the lithium foils 10a and 10b to become porous. When the salt coating 20 is exposed to the electrolyte between the pores (P) formed as the lithium foils 10a and 10b become porous, the salt coating 20 dissolves, and the lithium salts and additives contained in the salt coating 20 dissolve into the electrolyte. The lithium salts and additives dissolved in the electrolyte can react with the lithium films to stably form an SEI (Sediment Intercalation).

[0035] When the battery starts operating, the SEI can be formed stably and sufficiently by the lithium salts and additives contained in the electrolyte alone. However, as the battery continues to operate, if the lithium salts and additives contained in the electrolyte are consumed, as mentioned above, the lithium salts and additives in the salt coating can be dissolved and replenished by the consumed electrolyte lithium salts and additives, thereby enabling the stable formation and maintenance of the SEI.

[0036] If the salt coating forms on the exposed surface of the lithium film rather than between the lithium films, then when the battery starts running, the large amount of lithium salt and additives contained in the salt coating dissolves in the electrolyte, thereby increasing the viscosity of the electrolyte and thus increasing the overvoltage.

[0037] Furthermore, if a salt coating is formed between the current collector and the lithium film, it can prevent the battery resistance from increasing and can prevent lithium from depositing on the current collector during charging.

[0038] In this invention, the salt coating can react with lithium to form an SEI (solid electrolyte interface) and can suppress the shuttling of lithium polysulfides formed in the sulfur-containing cathode, thereby improving battery life. Furthermore, the salt coating may contain materials capable of replenishing the lithium salts and additives consumed in the electrolyte during battery operation.

[0039] The salt coating may contain lithium salts and additives. Since the salt coating is used to replenish the lithium salts and additives in the electrolyte consumed during battery operation, the lithium salts and additives contained in the salt coating can be used without restriction, as long as they are lithium salts and additives that can be contained in the electrolyte for lithium secondary batteries.

[0040] In addition, the salt coating may contain 40% to 80% by weight of the lithium salt and 20% to 60% by weight of the additives.

[0041] The lithium salt can be used without restriction, as long as it is commonly used in electrolytes for lithium secondary batteries. The lithium salt may contain at least one 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, wherein LiBETI, LiFSI, or LiTFSI are preferred.

[0042] Furthermore, based on the total weight of the salt coating, the lithium salt content can be from 40% to 80% by weight. Specifically, the lithium salt content can be more than 40% by weight or more than 50% by weight, and can be less than 65% by weight, less than 70% by weight, or less than 80% by weight. If the lithium salt content is less than 40% by weight, it may be difficult to prepare a uniform coating solution due to the relatively high content of the additive, and therefore it may be difficult to form a salt coating. If the lithium salt content exceeds 80% by weight, the content of the additive is relatively low, and therefore it may be difficult to form a suitable SEI protective layer.

[0043] The additive can be used without restriction, as long as it is commonly used as an electrolyte for lithium secondary batteries. In particular, additives that do not react with lithium metal can be used. For example, the additive may contain at least one selected from the group consisting of: inorganic nitrate compounds, including at least one selected from lithium nitrate (LiNO3) and lithium nitrite (LiNO2); and organic nitrate compounds, including at least one selected from nitromethane (CH3NO2) and methyl nitrate (CH3NO3). Considering compatibility with lithium salts, the additive may contain lithium nitrate (LiNO3).

[0044] Furthermore, based on the total weight of the salt coating, the content of the additive can be from 20% to 60% by weight. Specifically, the content of the additive can be more than 20% by weight, more than 30% by weight, or more than 35% by weight, and less than 50% by weight, less than 55% by weight, or less than 60% by weight. If the content of the additive is less than 20% by weight, it may be difficult to form a suitable SEI protective layer. If the content of the additive exceeds 60% by weight, it is difficult to prepare a uniform coating solution, and therefore it may also be difficult to form a salt coating.

[0045] Furthermore, the thickness of the salt coating can be from 100 nm to 3 μm. Specifically, the thickness of the salt coating can be greater than 100 nm, greater than 500 nm, or greater than 700 nm, and less than 1 μm, less than 2 μm, or less than 3 μm. If the thickness of the salt coating is less than 100 nm, the salt or additives of the electrolyte consumed during battery operation cannot be adequately replenished. If the thickness of the salt coating exceeds 3 μm, the resistance may increase, and therefore the overvoltage may increase during battery operation.

[0046] In this invention, the lithium film can serve as a negative electrode active material.

[0047] The thickness of the lithium film can be from 10 μm to 50 μm. Specifically, the thickness of the lithium film can be greater than 10 μm, greater than 15 μm, or greater than 20 μm, and less than 40 μm, less than 45 μm, or less than 50 μm. If the thickness of the lithium film is less than 10 μm, the process of manufacturing the negative electrode using multiple lithium films may be difficult to perform. If the thickness of the lithium film exceeds 50 μm, the thickness of the negative electrode containing the multiple lithium films will become thicker, which may reduce the energy density.

[0048] In this invention, the negative electrode current collector is not particularly limited, as long as it is conductive and will not cause chemical changes in the battery. For example, the negative electrode current collector can be copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., or an aluminum-cadmium alloy. Furthermore, similar to the positive electrode current collector, the negative electrode current collector can be formed in various forms, such as films, sheets, foils, meshes, porous bodies, foams, or nonwoven fabrics, with fine irregularities formed on its surface.

[0049] Method for manufacturing negative electrodes for lithium secondary batteries

[0050] The present invention also relates to a method for manufacturing a negative electrode for a lithium secondary battery, wherein the method for manufacturing a negative electrode for a lithium secondary battery includes the following steps: (S1) forming a salt coating on one surface of a lithium film; (S2) stacking a plurality of lithium films having the salt coating formed on them obtained in step (S1); and (S3) exposing the lithium film stacked in step (S2) to the outermost salt coating.

[0051] The steps of the method for manufacturing a negative electrode for a lithium secondary battery according to the present invention will be described in detail below.

[0052] In step (S1), a salt coating can be formed on one surface of the lithium film. To form the salt coating, a coating solution for salt coating formation can be prepared, and then applied to one surface of the lithium film, followed by drying.

[0053] The coating solution for forming the salt coating can be prepared by dissolving lithium salt and additives in a solvent. The types of lithium salt and additives are the same as described above. Based on the total weight of the coating solution, the solvent content can be 60 to 80% by weight, and the coating solution can be prepared using a solvent sufficient to smoothly carry out the coating process.

[0054] Furthermore, the coating is not particularly limited, as long as it is a coating method commonly used to form a coating. For example, the coating method for forming the coating can be selected from the group consisting of bar coating, roller coating, spin coating, slit coating, die coating, doctor blade coating, comma coating, groove die coating, lip coating, and solution coating. Considering the coating efficiency on lithium films, the salt coating can be formed by bar coating.

[0055] Furthermore, drying conditions are not particularly limited, as long as the solvent in the coating solution is removed. For example, the drying temperature can be between 20°C and 30°C; specifically, it can be above 20°C, above 22°C, or above 24°C, and below 26°C, below 28°C, or below 30°C. If the drying temperature is below 20°C, the solvent contained in the coating solution cannot be completely removed. If the drying temperature exceeds 30°C, the salt coating may crack, or side reactions may occur between the coating solution and lithium. If drying is carried out at room temperature (25°C) and under vacuum, side reactions between the coating solution and lithium can be prevented.

[0056] In step (S2), multiple lithium films with a salt coating formed on them, prepared in step (S1), can be stacked. In this case, the stacking can be performed in such a way that a salt coating is included between the lithium films.

[0057] In this context, "multiple" refers to n, where n can be an integer from 2 to 4. If the number of lithium films is two or more, a negative electrode with a salt coating formed between the lithium films can be manufactured. If the number of lithium films exceeds four, the thickness of the negative electrode may increase, potentially reducing the energy density.

[0058] In step (S3), the lithium films can be stacked to expose the laminate obtained in step (S2) to the outermost salt coating. In this case, the lithium film refers to a lithium film on which no salt coating is formed. By stacking the lithium films, then calendering and combining them, a negative electrode comprising a laminate in which a salt coating is formed between multiple lithium films can be manufactured.

[0059] At this point, the calendering method is not particularly limited, as long as it is a method commonly used for combining films or layers. For example, calendering can be performed by applying a pressure sufficient to fully combine the individual layers, said pressure being from 0.8 MPa to 15 MPa.

[0060] Furthermore, when using a lithium film attached to the negative electrode current collector, a negative electrode can be manufactured using a negative electrode current collector with lithium films formed on both sides (lithium / current collector / lithium) and a lithium film with a salt coating formed on one side (salt coating / lithium). For example, a negative electrode (lithium / salt coating-lithium / current collector / lithium-salt coating / lithium) can be manufactured by laminating the salt coating of the lithium film with the salt coating formed on it in contact with the lithium films formed on both sides of the negative electrode current collector.

[0061] At this time, the current collector used as the negative electrode current collector for lithium secondary batteries is not particularly limited. For example, the negative electrode current collector can be made of copper, nickel, tin, lead or stainless steel.

[0062] Lithium secondary batteries

[0063] The present invention also relates to a lithium secondary battery comprising a positive electrode, a negative electrode, a separator inserted therebetween, and an electrolyte, wherein the negative electrode is as described above.

[0064] In this invention, the positive electrode comprises a positive electrode active material layer formed on a positive electrode current collector. The positive electrode active material layer may comprise a positive electrode active material, a binder, and a conductive material.

[0065] Considering the stable lifespan characteristics of the battery, the positive electrode loading can be 3.0 mAh / cm². 2 Up to 5.0mAh / cm 2 .

[0066] In addition, the porosity of the positive electrode can be 60% to 80%.

[0067] Materials used as current collectors within this technical field can all be used as the positive current collector. Specifically, foamed aluminum, foamed nickel, etc., which have excellent conductivity, are preferred.

[0068] The positive electrode active material may comprise elemental sulfur (S₈), sulfide compounds, or mixtures thereof. Specifically, the sulfide compounds may comprise Li₂Sn (n≥1), organic sulfur compounds, carbon-sulfur polymers ((C₂Sx)n: x=2.5~50, n≥2), etc. In the case of these sulfur materials, since they are not conductive on their own, they are used in combination with conductive materials. Based on the total weight of the positive electrode active material layer, the content of the positive electrode active material may be 50% to 90% by weight.

[0069] The conductive material can be porous. Therefore, the conductive material can be used without limitation, as long as it is porous and conductive; for example, porous carbon-based materials can be used. As the carbon-based material, carbon black, graphite, graphene, activated carbon, carbon fibers, etc., can be used. In addition, metal fibers such as metal mesh; metal powders such as copper, silver, nickel, and aluminum; or organic conductive materials such as polyphenylene derivatives can also be used. The above conductive materials can be used alone or in combination. Based on the total weight of the positive electrode active material layer, the content of the conductive material can be from 1% to 30% by weight.

[0070] The positive electrode may further include an adhesive for bonding the positive electrode active material and the conductive material, as well as an adhesive for bonding with the current collector. The adhesive may comprise a thermoplastic resin or a thermosetting resin. For example, polyethylene, polyethylene oxide, polypropylene, polytetrafluoroethylene (PTEE), 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, and ethylene-acrylic acid copolymer, etc., may be used alone or in combination, but are not limited to these, and any adhesive available in the art as an adhesive may be used. The adhesive content may be 1 to 30% by weight, based on the total weight of the positive electrode active material layer.

[0071] The aforementioned positive electrode can be prepared using conventional methods. Specifically, a positive electrode active material layer forming composition, prepared by mixing positive electrode active material, conductive material, and binder in a solvent, can be coated onto a current collector and dried. Optionally, it can be compressed and shaped onto the current collector to improve electrode density, thereby manufacturing the positive electrode. In this case, the solvent can be water or an organic solvent. As an organic solvent, it is preferred to use an organic solvent that can uniformly disperse the positive electrode active material, binder, and conductive material and is easily evaporated. Examples include acetonitrile, methanol, ethanol, tetrahydrofuran, water, and isopropanol.

[0072] In this invention, the diaphragm is a physical diaphragm that functions as a physical separator of electrodes and can be used without particular restriction, as long as it is used as a conventional diaphragm. In particular, a diaphragm that has low resistance to ion migration in the electrolyte and excellent impregnation ability in the electrolyte is preferred.

[0073] Furthermore, the separator is capable of transporting lithium ions between the positive and negative electrodes while separating or insulating them from each other. The separator can be made of a porous, non-conductive, or insulating material. The separator can be a separate component, such as a membrane, or a coating added to the positive and / or negative electrodes.

[0074] Specifically, porous polymer membranes, such as those made from polyolefin polymers like ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, can be used alone or by laminating them. Furthermore, conventional porous nonwoven fabrics, such as those made from high-melting-point glass fibers and polyethylene terephthalate fibers, can be used, but are not limited to these.

[0075] In this invention, the electrolyte is a non-aqueous electrolyte containing lithium salt, and includes lithium salt, additives and solvent, and a non-aqueous organic solvent can be used as the solvent.

[0076] The lithium salt of the present invention is a material that is well soluble in non-aqueous organic solvents, and can be, for example, selected from LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 At least one of the following groups: LiB(Ph)4, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, lithium chloroborane, lithium lower aliphatic carboxylate, lithium tetraphenylborate, and lithium imino.

[0077] The concentration of the lithium salt can be from 0.2 to 2 M, preferably from 0.6 to 2 M, and more preferably from 0.7 to 1.7 M, depending on various factors such as the exact composition of the electrolyte, 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 batteries. If the concentration of the lithium salt is below 0.2 M, the conductivity of the electrolyte may decrease, and therefore the performance of the electrolyte may deteriorate. If the concentration of the lithium salt exceeds 2 M, the viscosity of the electrolyte may increase, and therefore the mobility of lithium ions (Li+) may decrease.

[0078] The non-aqueous organic solvent should dissolve lithium salts well. The non-aqueous organic solvent of this invention may include, for example, aprotic organic solvents such as N-methyl-2-pyridinone, propylene carbonate, ethylene carbonate, butyl carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, 4-methyl-1,3-dioxolane, etc. Alkenes, diethyl ethers, formamides, dimethylformamides, dioxolane, acetonitrs, nitromethanes, methyl formate, methyl acetate, triphosphates, trimethoxymethanes, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketones, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate, and these organic solvents may be used in the form of a mixture of one or more of these solvents.

[0079] In this invention, the lithium secondary battery can be a lithium-sulfur secondary battery containing sulfur as the positive electrode active material.

[0080] The positive electrode, separator, and electrolyte contained in the lithium-sulfur secondary battery can be prepared according to conventional components and manufacturing methods, and there are no particular restrictions on the external shape of the lithium-sulfur secondary battery, but it can be cylindrical, prismatic, pouch-shaped, or coin-shaped.

[0081] Methods for implementing the invention

[0082] Preferred embodiments of the invention will be described below to aid in understanding the invention. However, it will be apparent to those skilled in the art that the following embodiments exemplify the invention and that various changes and modifications can be made within the scope and spirit of the invention, and such changes and modifications are naturally within the scope of the appended claims.

[0083] Example 1

[0084] (1) Preparation of lithium foil with salt coating

[0085] LiFSI and LiNO3 were dissolved in a 1,2-dimethoxyethane solvent to prepare a coating solution for salt coating formation. The coating solution was prepared such that the weight ratio of LiFSI to LiNO3 was 48:52, and the solvent accounted for 69% by weight based on the total weight of the coating solution.

[0086] The coating solution was applied to a 45 μm thick lithium foil using a bar coater, and then dried at room temperature (25 °C) and under vacuum to remove the solvent, thereby forming a salt coating.

[0087] (2) Manufacturing of the negative electrode

[0088] A lithium foil of the same thickness without a salt coating is stacked on a lithium foil with a salt coating, and then rolled to prepare a negative electrode. The negative electrode comprises a stacked structure (45 μm Li / salt coating / 45 μm Li) in which lithium foil, salt coating and lithium foil are sequentially stacked.

[0089] (3) Manufacturing of lithium-sulfur secondary batteries

[0090] A sulfur-carbon composite (S-CNT), conductive material, and binder were mixed in a weight ratio of 90:5:5, and then mixed with water as a solvent to prepare a slurry. The slurry was then coated onto aluminum foil, dried, and calendered to fabricate the positive electrode. Vapor-grown carbon fiber (VGCF) was used as the conductive material, and styrene-butadiene rubber (SBR) was used as the binder. Furthermore, a doctor blade was used for coating; the slurry was coated with a doctor blade and calendered to prepare the positive electrode. The calendering process resulted in a porosity of 70% for the positive electrode.

[0091] The electrolyte was prepared by dissolving 0.75 M LiFSI and 5.0 wt% LiNO3 in a mixed solvent of 2-methylfuran and dimethoxyethane (2ME:DME = 2:8 (volume / volume)).

[0092] An electrode assembly is prepared by inserting a porous polyethylene membrane with a thickness of 20 μm and a porosity of 45% between the positive and negative electrodes, placing the electrode assembly inside a shell, and then injecting the electrolyte into the shell to prepare a lithium-sulfur secondary battery.

[0093] Example 2

[0094] A lithium-sulfur secondary battery was prepared in the same manner as in Example 1, but a 30 μm thick lithium foil was used instead of a 45 μm thick lithium foil, and two salt coating layers were stacked between three lithium foil layers (30 μm Li / salt coating / 30 μm Li / salt coating / 30 μm Li).

[0095] Comparative Example 1

[0096] The lithium anode and the lithium-sulfur secondary battery containing it were prepared in the same manner as in Example 1, except that a single lithium foil with a thickness of 90 μm and without a salt coating was used.

[0097] Comparative Example 2

[0098] Without using LiNO3 as an additive, 2 g of polyethylene oxide (PEO, Mw = 100,000) and 1.303 g of LiTFSI were mixed in acetonitrile solvent to prepare a coating solution. The concentration of the coating solution was set to 10% by weight based on the solid components. A lithium anode and a lithium-sulfur secondary battery containing the anode were then prepared in the same manner as in Example 1, except that the polymer coating was formed by applying the coating solution to a lithium foil to a thickness of 30 μm using a rod coating method.

[0099] Experiment Example 1: Battery Performance Experiment

[0100] The lithium-sulfur secondary batteries of Example 1 and Comparative Example 1 were charged / discharged, and the excess amount, initial capacity, nominal 0.1C discharge voltage, and lifetime characteristics compared to the injected electrolyte were measured. The results are shown in Table 1 below. Charge / discharge was performed at 25°C under 1.8V-2.5V cutoff conditions, using three 0.1C / 0.1C charge / discharge cycles, three 0.2C / 0.2C charge / discharge cycles, and 0.2C / 0.3C charge / discharge cycles. The charge / discharge results were measured using a charge / discharge measurement device (LANDCT-2001A, manufactured by Wuhan Company).

[0101] In Table 1 below, "excess amount compared to injected electrolyte" refers to the content of lithium salts and additives contained in the coating compared to the injected electrolyte. Furthermore, lifetime (80% capacity retention, cycles) refers to the number of cycles required to achieve 80% capacity retention compared to the initial 0.3C discharge capacity.

[0102] Table 1:

[0103]

[0104] As shown in Table 1 and Figure 2 and Figure 3 As shown, the lithium-sulfur secondary batteries of Examples 1 and 2 have the same initial capacity and nominal voltage as the lithium-sulfur secondary battery of Comparative Example 1, and exhibit excellent coulombic efficiency and lifetime performance.

[0105] Experiment Example 2: Leaching Evaluation Experiment of Polymer Coating and Salt Coating

[0106] For the leaching evaluation experiments of the polymer coating and the salt coating, the polymer coating and salt coating of Comparative Example 2 and Example 2 were used as Sample 1 and Sample 2, respectively, and the leaching experiments were conducted as follows:

[0107] Sample 1: Polymer coating formed on lithium foil

[0108] Sample 2: Salt coating formed on lithium foil

[0109] During the preparation of Samples 1 and 2, the weight of bare Li (g), the weight after coating (g), the weight of the coating (g), and the weight after immersion in the solvent (g) were measured to confirm the leaching amount of the coating. The immersion time was divided into 10 minutes (10 min), 1 hour (1 h), and 1 day (1 d) for the experiment. Here, the weight after coating refers to the sum of the weights of the bare Li and the coating. The coated Li film was immersed in the solvent for a specified time, then removed, dried, and its weight was measured. The leaching rate of the coating was calculated using Equation 1 below.

[0110] <Formula 1>

[0111] (Weight after coating - Weight after immersion in solvent) / (Weight after coating - Weight of bare Li)

[0112] Table 2:

[0113]

[0114] Table 3

[0115]

[0116] As shown in Tables 2 and 3, it was confirmed that in the case of Sample 1, the entire polymer coating was desorbed into the solvent within 10 minutes, while in the case of Sample 2, the salt contained in the salt coating was gradually dissolved into the solvent.

[0117] Therefore, compared with polymer coatings, if salt coatings are applied to lithium anodes, they can be used to supply salt to the electrolyte for a long time, which will help improve the performance and lifespan of the battery.

[0118] While the present invention has been described above with reference to limited embodiments and accompanying drawings, the invention is not limited thereto, and it is obvious that those skilled in the art can make various modifications and changes within the scope of the technical spirit of the invention and the equivalents of the appended claims.

[0119] [Symbol Explanation]

[0120] 10a, 10b: Lithium foil

[0121] 20: Salt coating

[0122] P: Kong

Claims

1. A negative electrode for a lithium secondary battery, comprising: Multiple lithium thin films; and A salt coating is formed between the plurality of lithium films. The salt coating comprises 40 to 80% by weight of lithium salt and 20 to 60% by weight of additives. The lithium salt comprises at least one selected from the group consisting of: LiN(C2F5SO2)2, LiN(C2F5SO3)2, LiN(FSO2)2, LiN(CF3SO2)2, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, and LiC4F9SO3. The additive comprises at least one selected from the group consisting of: inorganic nitrate compounds, including at least one selected from lithium nitrate and lithium nitrite; and organic nitrate compounds, including at least one selected from nitromethane and methyl nitrate.

2. The negative electrode for a lithium secondary battery according to claim 1, wherein the thickness of the salt coating is from 100 nm to 3 μm.

3. The negative electrode for a lithium secondary battery according to claim 1, wherein the thickness of the lithium film is 10 μm to 50 μm.

4. The negative electrode for a lithium secondary battery according to claim 1, wherein the lithium film is porous.

5. The negative electrode for a lithium secondary battery according to claim 1, wherein the plurality refers to 2 to 4.

6. A method for manufacturing a negative electrode for a lithium secondary battery according to claim 1, comprising the following steps: (S1) A salt coating is formed on one surface of a lithium film; (S2) Stack multiple lithium films with the salt coating formed thereon obtained in step (S1); and (S3) The lithium film is stacked on the outermost salt coating layer to expose the laminate obtained in step (S2).

7. A lithium secondary battery, comprising: The negative electrode according to claim 1; positive electrode; The membrane located between the positive electrode and the negative electrode; and Electrolyte impregnating the positive electrode, the negative electrode, and the membrane.

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

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