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

Abstract

The present invention relates to a negative electrode for a lithium secondary battery and a manufacturing method thereof. More specifically, the negative electrode for a lithium secondary battery includes a protective layer, and the protective layer is formed by solution-coating a solution containing a metal fluoride, a binder, and a lithium salt onto lithium metal. During operation of the battery, a layer containing LiF is further formed between the lithium metal and the protective layer, thereby improving the life characteristics and performance of the battery.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 2022-0033238 filed on March 17, 2022, and all the contents disclosed in the literature of the 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 and a method for manufacturing the same.

Background Art

[0003] With the rapid development of the electronics, communication, and computer industries, the application fields of energy storage technologies have been expanding, including camcorders, mobile phones, notebook computers, PCs, and even electric vehicles. As a result, the development of high-performance secondary batteries that are lightweight, long-lasting, and reliable has been underway. In particular, lithium secondary batteries have been in the spotlight as batteries that meet such requirements.

[0004] A lithium secondary battery has a structure in which an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode is laminated or wound, and this electrode assembly is built into a battery case and a non-aqueous electrolyte is injected therein. The lithium secondary battery produces electrical energy through oxidation and reduction reactions when lithium ions are inserted / desorbed at the positive electrode and the negative electrode.

[0005] Generally, lithium metal, carbon, etc. are used as active materials for the negative electrode of a lithium secondary battery, and lithium oxide, transition metal oxide, metal chalcogen compound, conductive polymer, etc. are used as active materials for the positive electrode.

[0006] In the case of a lithium secondary battery using a high-voltage positive electrode material, a side reaction may occur in which the electrolyte is oxidized on the surface of the positive electrode during charging. In this case, in order to prevent a decrease in battery performance due to oxidation of the electrolyte, the surface of the positive electrode may be made into a protective layer. Since the protective layer is formed by a method (such as ALD, Sol-gel, organic coating, etc.) after coating most of the positive electrode material, the cost may increase accordingly.

[0007] In addition, in a lithium secondary battery using lithium metal as the negative electrode, non-uniformity of electron density may occur on the surface of the lithium metal due to various factors during driving. For this reason, dendritic lithium dendrites are generated on the surface of the electrode, and protrusions are formed or grown on the surface of the electrode, making the surface of the electrode very rough. Such lithium dendrites cause a decrease in battery performance and, in severe cases, damage to the separator and short circuit of the battery. As a result, the temperature inside the battery rises, posing a risk of explosion and fire of the battery.

[0008] In addition, lithium used in the electrode, especially the lithium electrode, has a high reactivity with the electrolyte components. When the electrolyte components come into contact with the lithium metal, a film called a passivation layer is formed by a spontaneous reaction. Since the passivation layer formed on the surface of lithium during charge and discharge repeats destruction and formation, when the battery is repeatedly charged and discharged, the passivation layer components increase in the lithium negative electrode, resulting in a problem of depletion of the electrolyte. In addition, some of the reduced substances in the electrolyte react with the lithium metal in a side reaction, accelerating the consumption of lithium. As a result, the life of the battery is shortened.

[0009] Therefore, extensive research has been carried out to stabilize lithium metal, and as part of such research, a method of forming a protective layer at the position in contact with the negative electrode has been proposed.

[0010] For example, due to its high chemical stability, research on applying LiF to the lithium negative electrode protection layer has been actively carried out. Among them, there are reports of cases where the service life characteristics have been improved by applying LiF to the protection layer of the lithium negative electrode through a substitution reaction with metal fluoride.

[0011] Chinese Patent Publication No. 109671908 presents a method of forming a protective layer on a lithium negative electrode by coating a metal fluoride powder on lithium and then performing heat treatment or hot-pressing. However, there are problems such as non-uniform powder coating and oxidation of lithium by heat treatment, and there are limitations in the formation of a protective layer containing LiF.

[0012] In addition, Korean Patent Publication No. 2018-0041086 presents a method of forming a protective layer on a lithium negative electrode by coating a slurry containing carbon fluoride, metal fluoride, and a binder. Since the metal fluoride has low reactivity and exists in a state of being simply dispersed without being dissolved in a solvent, there is a problem that the formation of LiF by the spontaneous reaction of the metal fluoride does not proceed smoothly.

[0013] Therefore, when the lithium secondary battery is driven, there is a need for technological development to smoothly form LiF, which is a substance showing high chemical stability, on the negative electrode protection layer of the lithium secondary battery and to make the protection layer contain LiF.

Prior Art Documents

Patent Documents

[0014]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0015] As a result of conducting extensive research to solve the above problems, the inventors of the present invention formed a protective layer by solution-coating a solution containing a metal fluoride, a binder, and a lithium salt onto lithium metal. During the operation of the battery, it was confirmed that a layer containing LiF was further formed between the lithium metal and the protective layer, improving the life characteristics of the battery.

[0016] Accordingly, an object of the present invention is to provide a negative electrode for a lithium secondary battery having a protective layer capable of improving the life characteristics of the battery and a method for manufacturing the same.

Means for Solving the Problems

[0017] To achieve the above object, the present invention provides a negative electrode for a lithium secondary battery including a lithium metal layer and a first protective layer, wherein the first protective layer contains a metal fluoride represented by the following Chemical Formula 1, a binder, and a lithium salt: [Chemical Formula 1] MF x In Chemical Formula 1, M is Mg, Zn, Sn, Cu, Al, Ag, Ba, Co, Ca, Ni, Ta, B, Mn, In, or Fe, and x is 1 to 7.

[0018] In one embodiment of the present invention, the negative electrode for a lithium secondary battery further includes a second protective layer formed between the lithium metal layer and the first protective layer, and the second protective layer may contain LiF.

[0019] In one embodiment of the present invention, the second protective layer contains LiN x O y (x is 1 or 2, and y is 2 or 3.), and may further contain one or more selected from the group consisting of Li3N, Li2O, Li2CO3, Li2S, and Li2S2.

[0020] In one embodiment of the present invention, the binder may include one or more selected from the group consisting of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(vinylidene fluoride) (PVDF), poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), poly(methacrylic acid) (PMA), poly(methyl methacrylate) (PMMA), poly(acrylamide) (PAM), poly(methacrylamide) (PMAM), poly(acrylonitrile) (PAN), poly(methacrylonitrile) (PMN), and polyimide (PI).

[0021] In one embodiment of the present invention, the lithium salt may include one or more selected from the group consisting of (CF3SO2)2NLi, (FSO2)2NLi, LiNO3, LiOH, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, LiSCN, and LiC(CF3SO2)3.

[0022] In one embodiment of the present invention, the first protective layer may include 1 to 15% by weight of a metal fluoride, 50 to 70% by weight of a binder, and 20 to 40% by weight of a lithium salt.

[0023] The present invention also provides a method for manufacturing a negative electrode for a lithium secondary battery, including: (S1) dissolving a metal fluoride and a lithium salt in a solvent to produce a solution; (S2) adding a binder to the solution to produce a coating solution for forming a first protective layer; and (S3) coating the coating solution for forming the first protective layer on a lithium metal and drying it.

[0024] In one embodiment of the present invention, the solvent may include one or more selected from the group consisting of dimethoxyethane (DME), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), 1-methyl-2-pyrrolidone (NMP), and dimethylformamide (DMF).

[0025] In one embodiment of the present invention, the coating method may be bar coating, roll coating, spin coating, slit coating, die coating, blade coating, comma coating, slot die coating, lip coating, or solution casting.

[0026] In one embodiment of the present invention, after the formation of the negative electrode for the lithium secondary battery, a second protective layer may be formed between the lithium metal layer and the first protective layer during the operation of the lithium secondary battery.

[0027] The present invention also provides a lithium secondary battery including a positive electrode, the negative electrode, a separator, and an electrolyte.

[0028] In one embodiment of the present invention, the positive electrode may be a lithium-sulfur secondary battery containing sulfur.

[0029] The positive electrode may be a lithium metal battery including one or more positive electrode active materials selected from the group consisting of nickel cobalt manganese oxide (NCM, [Ni, Co, Mn]O2), nickel cobalt manganese aluminum oxide (NCMA, [Ni, Co, Mn, Al]O2), and lithium iron phosphate (LFP, LiFePO4).

Advantages of the Invention

[0030] According to the present invention, the protective layer formed on the negative electrode for the lithium secondary battery can suppress the growth of lithium dendrites and improve the life characteristics of the battery.

[0031] During the operation of the lithium secondary battery, a separate layer containing LiF with high chemical stability is formed between the negative electrode protection layer and the lithium metal, which can improve the performance of the battery.

Brief Description of the Drawings

[0032]

Figure 1a

Figure 1b

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0033] Hereinafter, in order to facilitate the understanding of the present invention, the present invention will be described in more detail.

[0034] The terms and words used in this specification and the claims should not be construed as being limited to their general or dictionary meanings. In accordance with the principle that the inventor can appropriately define the concept of the terms in order to explain his invention in the best way, they should be construed in a meaning and concept consistent with the technical idea of the present invention.

[0035] Negative electrode for lithium secondary battery The present invention relates to a negative electrode for a lithium secondary battery.

[0036] According to an embodiment of the present invention, the negative electrode for a lithium secondary battery includes a lithium metal layer; and a first protection layer formed on the lithium metal layer.

[0037] According to another embodiment of the present invention, the negative electrode for a lithium secondary battery includes a lithium metal layer; a second protective layer formed on the lithium metal layer; and a first protective layer formed on the second protective layer. The second protective layer means a layer containing a metal fluoride which is a fluorine source contained in the first protective layer and LiF formed by lithium contained in the lithium metal layer during battery operation. Due to LiF showing chemically stable properties, the life characteristics of the battery can be further improved.

[0038] Hereinafter, the present invention will be described in more detail with reference to the drawings.

[0039] FIG. 1a is a schematic cross-sectional view showing a longitudinal section of a negative electrode for a lithium secondary battery according to an embodiment of the present invention.

[0040] Referring to FIG. 1a, a negative electrode (1) for a lithium secondary battery according to an embodiment of the present invention includes a lithium metal layer (10); and a first protective layer (20) formed on the lithium metal layer (10).

[0041] In the present invention, the lithium metal layer may be a layer containing ordinary lithium metal used for a lithium negative electrode. For example, the lithium metal layer may be a lithium foil.

[0042] The lithium metal contained in the lithium metal layer can serve as a lithium (Li) source for forming LiF of the second protective layer.

[0043] In the present invention, the first protective layer is formed on the lithium metal layer, can suppress the growth of lithium dendrites during battery operation, prevent side reactions between the lithium metal layer and the electrolyte, and improve the life characteristics of the battery.

[0044] The first protective layer may contain a metal fluoride represented by the following Chemical Formula 1, a binder, and a lithium salt: [Chemical Formula 1] MF x In the above Chemical Formula 1, M is Mg, Zn, Sn, Cu, Al, Ag, Ba, Co, Ca, Ni, Ta, B, Mn, In or Fe, and x is from 1 to 7.

[0045] Also, in the first protective layer, a metal fluoride and a lithium salt may be contained in a mixed state inside the binder matrix.

[0046] Also, the thickness of the first protective layer may be 0.1 μm to 10 μm. Specifically, it may be 0.1 μm or more, 1 μm or more, or 3 μm or more, and may also be 7 or less, 8 or less, or 10 or less. If the thickness of the first protective layer is less than 0.1 μm, the effect of suppressing the growth of lithium dendrites and the effect of preventing side reactions between the lithium metal layer and the electrolyte may decrease. If it exceeds 10 μm, it may act as a resistance during battery operation.

[0047] As described above, the metal fluoride can serve as a fluorine (F) source for LiF, which is a constituent material of the second protective layer formed after battery operation.

[0048] Also, the metal fluoride may be contained in an amount of 1 to 15% by weight based on the total weight of the first protective layer. Specifically, the content of the metal fluoride may be 1% by weight or more, 3% by weight or more, or 5% by weight or more, and may also be 10% by weight or less, 13% by weight or less, or 15% by weight or less. If the content of the metal fluoride is less than 1% by weight, it is difficult to form the second protective layer containing LiF. If it exceeds 15% by weight, the second protective layer may be formed too thick and the ionic conductivity may decrease.

[0049] The binder can play a role in ensuring that the first protective layer is well formed and maintained in a layer form, and in storing the metal fluoride necessary for the formation of the second protective layer well within the first protective layer.

[0050] The binder may include one or more polymer binders selected from the group consisting of poly(vinylidene fluoride - co - hexafluoropropylene) (PVDF - HFP), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polymethacrylic acid (PMA), polymethyl methacrylate (PMMA), polyacrylamide (PAM), polymethacrylamide (PMAM), polyacrylonitrile (PAN), polymethacrylonitrile (PMN), and polyimide (PI).

[0051] Also, the binder may be contained in an amount of 50 to 70% by weight based on the total weight of the first protective layer. Specifically, the content of the binder may be 50% by weight or more, 55% by weight or more, or 60% by weight or more, and may be 65% by weight or less, 68% by weight or less, or 70% by weight or less. If the content of the binder is less than 50% by weight, it may be difficult to form the first protective layer in a layer form, and if it exceeds 70% by weight, it may be difficult to form the first protective layer with a sufficient thickness.

[0052] The lithium salt can increase the solubility of the metal fluoride in the electrolyte by a co - solvent effect and, as described above, can play a role in assisting the formation of LiF, which is a constituent material of the second protective layer formed after the battery is driven.

[0053] The lithium salt may include one or more selected from the group consisting of (CF3SO2)2NLi (LiTFSI, Lithium bis(trifluoromethanesulphonyl)imide), (FSO2)2NLi (LiFSI, Lithium bis(fluorosulfonyl)imide), LiNO3, LiOH, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, LiSCN, and LiC(CF3SO2)3.

[0054] Further, the lithium salt may be contained in an amount of 20 to 40% by weight based on the total weight of the first protective layer. Specifically, the content of the lithium salt may be 20% by weight or more, 25% by weight or more, or 28% by weight or more, and may be 33% by weight or less, 35% by weight or less, or 40% by weight or less. If the content of the lithium salt is less than 20% by weight, it is difficult to form the second protective layer containing LiF. If it exceeds 40% by weight, the second protective layer may be formed excessively thick, and the ionic conductivity may decrease.

[0055] FIG. 1b is a schematic cross-sectional view showing a longitudinal section of a negative electrode for a lithium secondary battery according to another embodiment of the present invention.

[0056] Referring to FIG. 1a, a negative electrode (1) for a lithium secondary battery according to another embodiment of the present invention includes a lithium metal layer (10); a second protective layer (30) formed on the lithium metal layer (10); and a first protective layer (20) formed on the second protective layer (30).

[0057] The second protective layer can be formed when the lithium secondary battery is driven. Specifically, when the lithium secondary battery is driven, the binder contained in the first protective layer swells in the electrolyte, the metal fluoride and the lithium salt are dissociated in the electrolyte, and the dissociated ions begin to be released outside the first protective layer due to the concentration gradient. The released fluoride ions (F−) will spontaneously react with the lithium metal during charge and discharge. At this time, the lithium metal may be the lithium metal contained in the lithium metal layer. As a result, a new layer containing LiF is formed between the lithium metal layer and the first protective layer, and this is referred to as the second protective layer.

[0058] The second protective layer contains LiF and may further contain one or more selected from the group consisting of LiN x O y (where x is 1 or 2, and y is 2 or 3), Li3N, Li2O, Li2CO3, Li2S, and Li2S2.

[0059] In the present invention, the lithium metal layer may be formed on the negative electrode current collector.

[0060] In the present invention, the type of the negative electrode current collector is not particularly limited, and known materials can be used. For example, the negative electrode current collector is not particularly limited as long as it has conductivity without causing a chemical change to the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, those obtained by surface treatment of the surface of copper or stainless steel with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. are used. Further, 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. having fine irregularities formed on the surface.

[0061] 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) a step of dissolving a metal fluoride and a lithium salt in a solvent to produce a solution; (S2) a step of adding a binder to the solution to produce a coating solution for forming a first protective layer; and (S3) a step of applying the coating solution for forming a first protective layer on lithium metal and drying.

[0062] As described above, the method for manufacturing a negative electrode for a lithium secondary battery according to the present invention can form a uniform first protective layer using a simple solution coating process, thereby improving the process efficiency and reducing the cost consumed in the process.

[0063] Hereinafter, the present invention will be described in more detail for each step.

[0064] In the present invention, in the step (S1), a solution can be produced by dissolving a metal fluoride and a lithium salt in a solvent. The types and contents of the metal fluoride and the lithium salt are as described above.

[0065] The solvent is not particularly limited as long as it can dissolve the metal fluoride and the lithium salt and has aprotic properties. For example, the solvent may contain one or more selected from the group consisting of dimethoxyethane (DME), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), 1-methyl-2-pyrrolidone (NMP), and dimethylformamide (DMF).

[0066] The concentration of the solution is not particularly limited as long as it can dissolve the metal fluoride and the lithium salt.

[0067] In the present invention, in the step (S2), a binder can be added to the solution to produce a coating solution for forming a first protective layer. The type and content of the binder are as described above.

[0068] The concentration of the coating solution for forming the first protective layer is not particularly limited as long as it allows the coating process to be carried out smoothly. For example, the concentration of the coating solution for forming the first protective layer may be 15 to 30% by weight.

[0069] In the present invention, in the step (S3), a negative electrode for a lithium secondary battery including a lithium metal layer and a first protective layer can be produced by applying and drying the coating solution for forming the first protective layer on the lithium metal.

[0070] The coating method is not particularly limited as long as it is a coating method commonly used in the art. For example, the coating method may be bar coating, roll coating, spin coating, slit coating, die coating, blade coating, comma coating, slot die coating, lip coating, or solution casting.

[0071] The drying may be performed at room temperature. If the solvent used in the step (S1) is a solvent that is difficult to volatilize at room temperature, vacuum drying may be performed at 80 to 120°C. Among the solvents, DME and THF have high volatility and drying can be performed at room temperature, and in some cases, a separate drying process may not be required. Solvents that are difficult to volatilize at room temperature may be DMSO, NMP, or DMF.

[0072] By the steps (S1) to (S3) as described above, a negative electrode for a lithium secondary battery having a first protective layer formed on a lithium metal layer can be manufactured.

[0073] When a lithium secondary battery including the negative electrode having the first protective layer formed on the lithium metal layer is driven, a second protective layer is formed between the lithium metal layer and the first protective layer. Specifically, when the charge and discharge of the lithium secondary battery are performed, the metal fluoride and lithium salt are gradually released from the inside of the first protective layer having a form in which the metal fluoride and lithium salt are simply mixed inside the binder matrix and react with the lithium of the lithium metal layer, LiF, LiN x O ySubstances such as these are formed, and a layer containing these substances is generated, and the said layer is called the second protective layer. At this time, the lithium salt can play a role in increasing the solubility of the metal fluoride through a co-solvation reaction with the metal fluoride during the production of the coating solution, and assisting in the production of a uniform coating solution. Also, during the formation of the second protective layer, it can also play a role in increasing the solubility of the metal fluoride in the electrolyte through a co-solvation reaction with the metal fluoride to promote the release of fluoride ions and assist in the formation of the second protective layer.

[0074] Lithium secondary battery The present invention also relates to a lithium secondary battery including a positive electrode, a negative electrode, a separator, and an electrolyte.

[0075] Also, the lithium secondary battery may be a lithium-sulfur secondary battery containing sulfur as a positive electrode active material.

[0076] Alternatively, the lithium secondary battery may be a lithium metal battery containing one or more positive electrode active materials selected from the group consisting of nickel-cobalt-manganese oxide (NCM, [Ni, Co, Mn]O2), nickel-cobalt-manganese-aluminum oxide (NCMA, [Ni, Co, Mn, Al]O2), and lithium iron phosphate (LFP, LiFePO4).

[0077] Negative electrode In the lithium secondary battery according to the present invention, the negative electrode is the same as the negative electrode for a lithium secondary battery as described above.

[0078] Positive electrode In the lithium secondary battery according to the present invention, the positive electrode can include a positive electrode current collector; and a positive electrode active material layer formed on the positive electrode current collector.

[0079] The positive electrode active material layer can include a positive electrode active material, a binder, and a conductive material.

[0080] When the positive electrode active material contains sulfur, the positive electrode sulfur material may include elemental sulfur (Elemental sulfur, S8), sulfur-based compounds, or mixtures thereof. Specifically, the sulfur-based compounds may include Li2S n (n ≧ 1), organic sulfur compounds, or sulfur-carbon composites ((C2S x ) n : x = 2.5 to 50, n = 2). Additionally, among the positive electrode active materials, the sulfur-carbon composite is a mixture of a carbon material and sulfur to reduce the outflow of sulfur into the electrolyte and increase the electrical conductivity of the electrode containing sulfur.

[0081] The sulfur-carbon composite may contain sulfur and a carbon material. Specifically, it may contain sulfur and the carbon material in a weight ratio of 55 to 90:45 to 10. When the weight ratio of sulfur to the carbon material contained in the sulfur-carbon composite is satisfied, the capacity of the battery can be improved while maintaining conductivity.

[0082] Also, the sulfur-carbon composite may be contained in an amount of 60 to 95% by weight based on the total weight of the positive electrode active material layer. Specifically, it may be contained in an amount of 60% by weight or more, 70% by weight or more, or 80% by weight or more, or 93% by weight or less, 94% by weight or less, or 95% by weight or less. If it is less than the above range, the battery performance may deteriorate. If it exceeds the above range, the content of the binder and / or metal oxide-based additive other than the positive electrode active material will relatively decrease, resulting in a decrease in durability or a negligible improvement effect on the capacity of the positive electrode or the life characteristics of the battery.

[0083] Also, in the sulfur-carbon composite, the sulfur may be selected from the group consisting of elemental sulfur (elemental sulfur, S8), sulfur-based compounds, and sulfur-carbon composites. Specifically, the sulfur-based compounds may include Li2S n (n ≧ 1), organic sulfur compounds, or carbon-sulfur polymers ((C2S x ) n : x = 2.5 to 50, n ≧ 2), etc.

[0084] In addition, in the sulfur-carbon composite, the carbon material may be a porous carbon material, and generally can be produced by carbonizing various carbon material precursors.

[0085] The porous carbon material contains non-uniform pores inside, and the average diameter of the pores may be 1 to 200 nm. Specifically, it may be 1 nm or more, 5 nm or more, or 10 nm or more, and may also be 100 nm or less, 150 nm or less, or 200 nm or less. Further, the porosity or void fraction of the porous carbon material may be 10% to 90% of the total porous volume. Specifically, it may be 10% or more, 15% or more, or 20% or more, and may also be 70% or less, 80% or less, or 90% or less. If the average diameter and porosity of the pores are less than the above ranges, since the pore size is only at the molecular level, it is impossible to impregnate sulfur. On the contrary, if it exceeds the above ranges, the mechanical strength of the porous carbon decreases, which is not preferable for application in the electrode manufacturing process.

[0086] The form of the porous carbon material is spherical, rod-shaped, needle-shaped, plate-shaped, tube-shaped or bulk-shaped, and can be used without limitation as long as it is commonly used in lithium secondary batteries.

[0087] The porous carbon material may be a porous structure or have a high specific surface area, and any of them is acceptable as long as it is commonly used in the industry. For example, the porous carbon material may include one or more selected from the group consisting of graphite; graphene; carbon black such as Denka black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; carbon nanotubes (CNT) such as single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT); carbon fibers such as graphite nanofibers (GNF), carbon nanofibers (CNF), and activated carbon fibers (ACF); and activated carbon, but is not limited thereto. Preferably, the porous carbon material may be graphite.

[0088] Also, the binder may be SBR (Styrene-Butadiene Rubber) / CMC (Carboxymethyl Cellulose), poly(vinyl acetate), polyvinyl alcohol, polyethylene oxide, polyvinyl pyrrolidone, alkylated polyethylene oxide, cross-linked polyethylene oxide, polyvinyl ether, poly(methyl methacrylate), polyvinylidene fluoride, a copolymer of polyhexafluoropropylene and polyvinylidene fluoride (trade name: Kynar), poly(ethyl acrylate), polytetrafluoroethylene, polyvinyl chloride, polyacrylonitrile, polyvinyl pyridine, polystyrene, polyacrylic acid, their derivatives, blends, copolymers, etc.

[0089] Also, the content of the binder may be 1 to 20% by weight, preferably 3 to 18% by weight, more preferably 5 to 15% by weight based on the total weight of the positive electrode active material layer. If it is less than the above range, the adhesion between the positive electrode active materials or between the positive electrode active material and the current collector may not be greatly improved, and the capacity characteristics may decrease. Also, there may be a possibility of polysulfide leaching due to the interaction between the polysulfide and specific functional groups of the polymer chain used as the binder. If it exceeds the above range, the battery capacity may decrease.

[0090] Also, the conductive material can be included to ensure the lithium ion migration path of the positive electrode.

[0091] The conductive material may be one or more selected from the group consisting of carbon black selected from Super P, Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; carbon derivatives selected from carbon nanotubes and fullerenes; conductive fibers selected from carbon fibers and metal fibers; metal powders selected from carbon fluoride, aluminum, and nickel powders; and conductive polymers selected from polyaniline, polythiophene, polyacetylene, and polyphenol.

[0092] The conductive material may be contained in an amount of 1 to 20% by weight based on the total weight of the positive electrode active material layer. Specifically, the content of the conductive material may be 1% by weight or more, 3% by weight or more, or 5% by weight or more, and may also be 15% by weight or less, 18% by weight or less, or 20% by weight or less. If it is less than the above range, it is difficult to form and maintain a conductive structure and a lithium ion migration path throughout the positive electrode active material layer, and the effect of increasing the discharge capacity and improving the overvoltage of the battery may be negligible. If it exceeds the above range, the battery life may rather decrease.

[0093] As described above, the positive electrode can be manufactured by mixing a positive electrode active material, a binder, and a conductive material in a solvent to produce a positive electrode slurry, and then coating and drying the positive electrode slurry on a positive electrode current collector.

[0094] The solvent used in the production of the positive electrode slurry may be one or more selected from the group consisting of water (distilled water), methanol, ethanol, isopropyl alcohol, acetone, dimethyl sulfoxide, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, acetic acid, 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, and ethyl propionate. In particular, when water (distilled water) or an anhydrous alcohol-based solvent is used, it is preferable because damage to the positive electrode active material can be prevented.

[0095] The concentration of the positive electrode slurry is not particularly limited as long as the coating process can be carried out smoothly.

[0096] Separator In the present invention, the separation membrane may be composed of a porous substrate. Any porous substrate that is usually used in an electrochemical device can be used. For example, a polyolefin-based porous membrane or a non-woven fabric can be used, but it is not particularly limited thereto.

[0097] Examples of the polyolefin-based porous membrane include membranes formed of polyolefin-based polymers such as polyethylene (such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene), polypropylene, polybutylene, and polypentene, either alone or as a mixture thereof.

[0098] Examples of the non-woven fabric include, in addition to polyolefin-based non-woven fabrics, non-woven fabrics formed of polymers such as polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalene, either alone or as a mixture thereof. The structure of the non-woven fabric may be a spunbond non-woven fabric or a meltblown non-woven fabric composed of long fibers.

[0099] The thickness of the porous substrate is not particularly limited, but is 1 μm to 100 μm, or 5 μm to 50 μm.

[0100] The size and porosity of the pores present in the porous substrate are not particularly limited, and may be 0.001 μm to 50 μm and 10% to 95%, respectively.

[0101] Electrolyte In the present invention, the electrolytic solution may be a non-aqueous electrolytic solution, and the electrolyte salt contained in the non-aqueous electrolytic solution is a lithium salt. The lithium salt can be used without limitation those commonly used in electrolytic solutions for lithium secondary batteries. For example, the lithium salt may be one or more selected from the group consisting of LiFSI, LiPF6, LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, lithium chloroborane, and lithium 4-phenylborate.

[0102] As the organic solvent contained in the aforementioned non-aqueous electrolytic solution, those commonly used in electrolytic solutions for lithium secondary batteries and the like can be used without limitation. For example, ether, ester, amide, linear carbonate, cyclic carbonate, etc. can be used alone or in combination of two or more. Among them, typically, it can contain a cyclic carbonate, a linear carbonate, or a carbonate compound that is a slurry thereof.

[0103] Specific examples of the cyclic carbonate compound include any one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinyl ethylene carbonate, and their halides, or a slurry of two or more of these. Examples of these halides include, but are not limited to, fluoroethylene carbonate (FEC).

[0104] Specific examples of the linear carbonate compound include any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, or a slurry of two or more of these, and are not limited thereto. In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are organic solvents with high viscosity, high dielectric constant, and can better dissociate lithium salts in the electrolyte. When such cyclic carbonates are mixed with linear carbonates with low viscosity and low dielectric constant, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio and used, an electrolyte with higher electrical conductivity can be prepared.

[0105] Among the organic solvents, as the ether, any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, and ethyl propyl ether, or a slurry of two or more of these can be used, and are not limited thereto.

[0106] In addition, among the organic solvents, as esters, any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, or a slurry of two or more of these can be used, but it is not limited thereto.

[0107] The injection of the non-aqueous electrolyte can be carried out at an appropriate stage during the manufacturing process of the electrochemical element according to the manufacturing process of the final product and the required physical properties. That is, it can be applied before the assembly of the electrochemical element or at the final stage of the assembly of the electrochemical element.

[0108] The lithium secondary battery according to the present invention enables processes such as lamination and folding of the separator and the electrode, in addition to the general winding process.

[0109] And the shape of the battery case is not particularly limited, and can be made into various shapes such as cylindrical, laminated, rectangular, pouch type, or coin type. Since the structures and manufacturing methods of these batteries are widely known in this field, detailed descriptions are omitted.

[0110] In addition, the lithium secondary battery can be classified into various batteries such as lithium-sulfur secondary batteries, lithium-air batteries, lithium-oxide batteries, and all-solid-state lithium batteries according to the materials of the positive electrode / negative electrode used.

[0111] The present invention also provides a battery module including the lithium secondary battery as a unit cell.

[0112] The battery module can be used as a power source for medium and large-sized devices that require high-temperature stability, long cycle characteristics, and high-capacity characteristics.

[0113] Examples of the medium to large-sized devices include, but are not limited to, power tools powered by battery-operated motors; electric vehicles such as electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV); electric two-wheel vehicles including electric bicycles (E-bike) and electric scooters (E-scooter); electric golf carts; power storage systems, etc.

[0114] Hereinafter, preferred embodiments are presented to assist in the understanding of the present invention. However, it is obvious to those skilled in the art that the following embodiments are merely illustrative of the present invention, and various changes and modifications are possible within the scope of the present invention and its technical concept. It is natural that such changes and modifications belong to the scope of the appended claims.

[0115] Example 1 (1) Manufacture of the negative electrode MgF2, which is a metal fluoride, and LiNO3, which is a lithium salt, were dissolved in THF (Tetrahydrofuran), which is a solvent, to prepare a solution. PVDF-HFP, which is a binder, was added to the solution to prepare a coating solution for forming the first protective layer.

[0116] Using a bar coater, the coating solution for forming the first protective layer was applied to the surface of lithium metal and then dried at room temperature for 1 hour to manufacture a negative electrode with a first protective layer formed on the lithium metal. At this time, the contents of the metal fluoride, binder, and lithium salt in the first protective layer were set to 6.25 wt%, 62.5 wt%, and 31.25 wt%, respectively.

[0117] (2) Manufacture of the positive electrode A mixture of 90 wt% of the positive electrode active material, 5 wt% of the conductive material, and 5 wt% of the binder was mixed with a solvent to produce a positive electrode slurry composition. As the positive electrode active material, a sulfur-carbon composite (S:C = 7:3 (w / w)) was used. As the conductive material, Denka black was used. As the binder, styrene-butadiene rubber (SBR) / carboxymethyl cellulose (CMC) (SBR:CMC = 7:3 (w / w)) was used. As the solvent, H2O was used.

[0118] The positive electrode slurry composition was applied onto an aluminum current collector, dried at 50 °C for 12 hours, and then crimped with a roll press device to produce a positive electrode.

[0119] (3) Electrolyte An electrolyte was produced by dissolving 1 M concentration of LiTFSI and 1 wt% of LiNO3 in an organic solvent containing 1,3-dioxolane (DOL) and dimethyl ether (DME) (DOL:DME = 1:1 (v / v)).

[0120] (4) Separator A polyethylene porous film with a thickness of 20 μm (porosity 68%) was prepared.

[0121] (5) Fabrication of Lithium-Sulfur Secondary Battery After laminating the negative electrode with the first protective layer formed, the separator, and the positive electrode, 0.1 ml of the electrolyte was injected to fabricate a lithium-sulfur secondary battery.

[0122] Example 2 A lithium-sulfur secondary battery was fabricated in the same manner as in Example 1, except that BaF2 was used instead of MgF2 as the metal fluoride.

[0123] Comparative Example 1 A lithium-sulfur secondary battery was fabricated in the same manner as in Example 1, except that lithium metal without a protective layer formed was used as the negative electrode.

[0124] Comparative Example 2 A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except that MgF2, which is a metal fluoride, was coated on lithium metal without using a binder and a lithium salt, then placed in a glove box under an argon atmosphere, and heat-treated at 180 °C for 3 minutes to form a protective layer on the lithium metal.

[0125] Comparative Example 3 A slurry was prepared by dissolving PVDF-HFP, which is a binder, in NMP, which is a solvent, without using a lithium salt, and then adding MgF2, which is a metal fluoride. The slurry was coated on lithium metal and then vacuum-dried at 100 °C for 1 hour to form a protective layer on the lithium metal. A lithium-sulfur secondary battery was manufactured in the same manner as in Example 1, except for this. At this time, the weight ratio of the metal fluoride to the binder was set to 1:10 (MgF2:PVDF-HFP = 1:1 (w / w)).

[0126] Experimental Example 1: Surface Analysis of Anode Protective Layer For the surface analysis of the anode protective layer, the lithium-sulfur secondary batteries manufactured in the examples and comparative examples were repeatedly discharged and charged 3 times at a current density of 0.1C at 25 °C, then discharged and charged once at a current density of 0.2C, and then disassembled for XPS analysis. The equipment used was the Nexsa2 ESCA system (manufactured by Thermo Fisher Scientific).

[0127] Figure 2 is a graph showing the results of X-ray photoelectron spectroscopy (XPS) analysis of the anode protective layer of the lithium-sulfur secondary batteries manufactured in Comparative Example 1 and Example 1.

[0128] Referring to Figure 2, in the anode of the lithium-sulfur battery of Example 1, a layer containing LiF was confirmed, indicating that the second protective layer was formed. It was also found that the second protective layer was formed from F of MgF2, which is the metal fluoride contained in the first protective layer, and Li of LiNO3, which is the lithium salt.

[0129] On the other hand, in Comparative Example 1, LiF was not confirmed.

[0130] Experimental Example 2: Evaluation of Battery Life Characteristics Lithium-sulfur secondary batteries manufactured in lithium examples and comparative examples including a negative electrode with a protective layer were discharged at 0.5C and charged at 0.3C at 25°C, and the discharge capacity retention rate was measured to evaluate the battery life characteristics.

[0131] Table 1 below shows the measurement results of the number of cycle repetitions at the time of reaching 80% discharge capacity retention rate for the lithium-sulfur secondary batteries of Example 1 and Comparative Example 1.

[0132]

Table 1

[0133] Referring to Table 1 above, it was found that Example 1 had significantly more cycle repetitions to maintain 80% discharge capacity compared to Comparative Examples 1 to 3.

[0134] Figure 3 is a graph showing the results of evaluating the life characteristics of the negative electrode protective layers of the lithium-sulfur secondary batteries manufactured in Comparative Example 1 and Example 1.

[0135] Referring to Figure 3, it can be seen that the life characteristics of Example 1 are superior to those of Comparative Example 1.

[0136] As described above, although the present invention has been described with reference to limited examples and drawings, the present invention is not limited thereby, and various modifications and variations can be made within the equivalent scope of the technical idea and claims of the present invention by those having ordinary knowledge in the technical field to which the present invention pertains.

Explanation of Reference Numerals

[0137] 1: Negative electrode for lithium secondary battery 10: Lithium metal layer 20: First protective layer 30: Second protective layer

Claims

1. A negative electrode for a lithium secondary battery comprising a lithium metal layer and a first protective layer, wherein the first protective layer contains a metal fluoride represented by the following Chemical Formula 1, a binder, and a lithium salt: A negative electrode for a lithium secondary battery: [Chemical Formula 1] MF ｘ In Chemical Formula 1, M is Mg, Zn, Sn, Cu, Al, Ag, Ba, Co, Ca, Ni, Ta, B, Mn, In or Fe, and x is 1 to 7.

2. The negative electrode for a lithium secondary battery further includes a second protective layer formed between the lithium metal layer and the first protective layer, and the second protective layer contains LiF. The negative electrode for a lithium secondary battery according to Claim 1.

3. The second protective layer is LiN ｘ O ｙ (where x is 1 or 2, and y is 2 or 3), Li ３ N, Li ２ O, Li ２ CO ３ and Li ２ S and Li ２ S ２ The negative electrode for a lithium secondary battery according to claim 2, further comprising one or more selected from the group consisting of

4. The binder includes one or more selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polymethacrylic acid (PMA), polymethyl methacrylate (PMMA), polyacrylamide (PAM), polymethacrylamide (PMAM), polyacrylonitrile (PAN), polymethacrylonitrile (PMN), and polyimide (PI). The negative electrode for a lithium secondary battery according to Claim 1.

5. The lithium salt is (CF ３ SO ２ ), ２ NLi, (FSO ２ ), ２ NLi, LiNO ３ , LiOH, LiCl, LiBr, LiI, LiClO ４ , LiBF ４ , LiB １０ Cl １０ , LiPF ６ , LiCF ３ SO ３ , LiCF ３ CO ２ , LiAsF ６ , LiSbF ６ , LiAlCl ４ , CH ３ SO ３ Li, CF ３ SO ３ Li, LiSCN and LiC(CF ３ SO ２ ), ３ The negative electrode for a lithium secondary battery according to claim 1, comprising one or more selected from the group consisting of.

6. The first protective layer contains 1 wt% to 15 wt% of a metal fluoride, 50 wt% to 70 wt% of a binder, and 20 wt% to 40 wt% of a lithium salt. The negative electrode for a lithium secondary battery according to Claim 1.

7. A method for manufacturing a negative electrode for a lithium secondary battery comprising a lithium metal layer and a first protective layer, (S1) dissolving a metal fluoride and a lithium salt in a solvent to produce a solution; (S2) adding a binder to the solution to produce a coating solution for forming a first protective layer; and (S3) coating and drying the coating solution for forming a first protective layer on lithium metal. A method for manufacturing a negative electrode for a lithium secondary battery.

8. The solvent includes one or more selected from the group consisting of dimethoxyethane (DME), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), 1-methyl-2-pyrrolidone (NMP), and dimethylformamide (DMF). The method for manufacturing a negative electrode for a lithium secondary battery according to Claim 7.

9. The method for manufacturing a negative electrode for a lithium secondary battery according to claim 7, wherein the coating solution is applied using bar coating, roll coating, spin coating, slit coating, die coating, blade coating, comma coating, slot die coating, lip coating or solution casting.

10. The method for manufacturing a negative electrode for a lithium secondary battery according to claim 7, wherein, during driving of a lithium secondary battery including a negative electrode having a first protective layer formed on the lithium metal layer after formation of the negative electrode for the lithium secondary battery, a second protective layer is formed between the lithium metal layer and the first protective layer.

11. A lithium secondary battery including a positive electrode, a negative electrode according to any one of claims 1 to 6, a separator and an electrolyte.

12. The lithium secondary battery according to claim 11, wherein the positive electrode is a lithium-sulfur secondary battery containing sulfur.

13. The positive electrode contains one or more positive electrode active materials selected from the group consisting of nickel cobalt manganese oxide (NCM, [Ni, Co, Mn]O ２ ), nickel cobalt manganese aluminum oxide (NCMA, [Ni, Co, Mn, Al]O ２ ), and lithium iron phosphate (LFP, LiFePO ４ ), and is a lithium metal battery. The lithium secondary battery according to claim 11.

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