Method for manufacturing a lithium metal electrode, lithium metal electrode manufactured thereby, and lithium secondary battery including the same

Abstract

The present invention relates to a method for manufacturing a lithium metal electrode, a lithium metal electrode manufactured using the method, and a lithium secondary battery including the lithium metal electrode. More specifically, the present invention relates to a method for manufacturing a lithium metal electrode, the method comprising the steps of: (a) coating fluoroethylene carbonate on at least one surface of a lithium metal layer; and (b) rolling the lithium metal layer coated with the fluoroethylene carbonate to form a lithium fluoride protective layer on at least one surface of the lithium metal layer.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2020-0118835, filed on September 16, 2020, and Korean Patent Application No. 10-2021-0120225, filed on September 9, 2021, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a method for manufacturing a lithium metal electrode, a lithium metal electrode manufactured by the method, and a lithium secondary battery including the lithium metal electrode. Background Art

[0003] As the use range of lithium secondary batteries has expanded to electric vehicles (EVs), power storage systems (ESSs), and portable electronic devices, the demand for lithium secondary batteries having high capacity, high energy density, and long life has increased.

[0004] Among various lithium secondary batteries, a lithium-sulfur battery is a battery system that uses a sulfur-based material having a sulfur-sulfur bond as a positive electrode active material and uses lithium metal, a carbon-based material in which lithium ions are intercalated / deintercalated, silicon or tin that forms an alloy with lithium, etc. as a negative electrode active material.

[0005] Advantages are that sulfur used for the positive electrode in a lithium-sulfur battery has a low atomic weight, is very abundant in resources, is thus easy to supply and receive, and is inexpensive, non-toxic, and environmentally friendly.

[0006] In addition, a lithium-sulfur battery has a theoretical specific capacity of 1675 mAh / g derived from the conversion reaction of lithium ions and sulfur in the positive electrode (S8 + 16Li + + 16e - → 8Li2S). If lithium metal is used as the negative electrode, the theoretical energy density is 2600 Wh / kg. Since the energy density of a lithium-sulfur battery is much higher than the theoretical energy densities 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) and lithium-ion batteries (250 Wh / kg), among the secondary batteries developed so far, lithium-sulfur batteries have received much attention as lithium secondary batteries having high capacity, being environmentally friendly, and being inexpensive.

[0007] In such a lithium-sulfur battery, if lithium metal is used as the negative electrode, since the theoretical specific capacity is a very high 3860 mAh / g and the standard hydrogen electrode (SHE) is also a very low -3.045 V, a battery having high capacity and high energy density can thus be achieved, and thus various studies are being conducted as a next-generation battery system.

[0008] However, because of the high chemical / electrochemical reactivity of lithium metal, as charging / discharging proceeds, the growth and porosity of lithium dendrites occur, resulting in poor battery life characteristics. For this reason, extensive research is being conducted on the protective layer formed on the lithium metal surface.

[0009] Among the protective layers formed on the lithium metal surface, a lithium fluoride (LiF) protective layer inhibits the growth of lithium dendrites with high ionic conductivity, thereby ensuring the effect of improving capacity and life characteristics when applied to a battery. Therefore, in the related art, various methods for forming a lithium fluoride protective layer on the lithium metal surface have been studied.

[0010] In the related art, the lithium fluoride protective layer is formed by immersing the lithium metal in fluoroethylene carbonate (FEC) for a predetermined time or longer or by coating a solution containing fluoroethylene carbonate on the lithium metal surface.

[0011] However, an oxide layer (natural layer) such as LiOH, Li2O, Li2CO3, etc. exists on the surface of the lithium metal. Therefore, because the reaction between fluoroethylene carbonate or the solution containing fluoroethylene carbonate and the lithium metal is uneven, the formed lithium fluoride protective layer has differences in physical properties such as composition, thickness, density, etc.

[0012] The differences in the physical properties of the lithium fluoride protective layer result in differences in local current density during charging / discharging, thereby accelerating the formation of lithium dendrites on the lithium metal surface. In addition, the lithium dendrites formed in this way cause internal short circuits and dead lithium in the battery, which not only increases the physical and chemical instabilities of the lithium secondary battery but also causes the problems of accelerated decline in the capacity and cycle life of the battery.

[0013] Therefore, there is a need to develop a technology that can solve the problem of lithium metal reactivity by forming a uniform lithium fluoride protective layer on the lithium metal surface through a simple method.

[0014] Prior art documents

[0015] [Patent documents]

[0016] Korean Patent Laid-Open Publication No. 2019-0071618 (June 24, 2019) "Continuous Manufacturing Method of Lithium Secondary Battery Forming Passive layer on the Surface of Lithium Metal Electrode and Lithium Secondary Battery prepared by the Same"

[0017] [Non-Patent Document]

[0018] Ngoc Duc Trinh et al., "An Artificial Lithium Protective Layer that Enables the Use of Acetonitrile-Based Electrolytes in Lithium Metal Batteries", Angewandte Chemie, 2018, 57(18), 5072 - 5075. Summary of the Invention

[0019] [Technical Problem]

[0020] Therefore, as a result of various studies conducted to solve the above problems, the inventors of the present invention have confirmed that when a lithium fluoride protective layer is formed by directly coating fluoroethylene carbonate on the surface of lithium metal and then rolling, the effect of suppressing the growth of lithium dendrites is excellent, thereby completing the present invention.

[0021] Therefore, an object of the present invention is to provide a method for manufacturing a lithium metal electrode, which can form a uniform lithium fluoride protective layer with a high fluorine content on the surface of lithium metal.

[0022] In addition, another object of the present invention is to provide a lithium metal electrode manufactured by the manufacturing method and a lithium secondary battery including the lithium metal electrode.

[0023] [Technical Solution]

[0024] To achieve the above object, the present invention provides a method for manufacturing a lithium metal electrode, the method comprising the following steps: (a) coating fluoroethylene carbonate on at least one surface of a lithium metal layer; and (b) rolling the lithium metal layer coated with the fluoroethylene carbonate to form a lithium fluoride protective layer on at least one surface of the lithium metal layer.

[0025] The lithium metal layer may comprise lithium metal or a lithium alloy.

[0026] The rolling may be carried out by placing the lithium metal layer coated with fluoroethylene carbonate between rolling units and applying pressure.

[0027] The rolling may be carried out under temperature conditions of 10 to 80 °C.

[0028] The thickness reduction rate of the lithium metal layer before and after rolling may be 10% or more.

[0029] In addition, the present invention provides a lithium metal electrode manufactured by the above manufacturing method, the lithium metal electrode comprising: a lithium metal layer; and a lithium fluoride protective layer formed on at least one surface of the lithium metal layer.

[0030] The lithium fluoride protective layer may have a thickness of 10 to 500 nm.

[0031] The lithium fluoride protective layer may have a fluorine content of 0.1 to 10 atomic %.

[0032] In addition, the present invention provides a lithium secondary battery comprising the lithium metal electrode.

[0033] [Advantageous Effects]

[0034] According to the method for manufacturing a lithium metal electrode of the present invention, by directly coating fluoroethylene carbonate on the surface of lithium metal and rolling, a uniform lithium fluoride protective layer with a high fluorine content can be formed, thereby effectively suppressing the generation of lithium dendrites, and enabling the lithium secondary battery comprising the lithium metal electrode to have a higher capacity and a longer lifespan. Description of the Drawings

[0035] Figure 1 is a diagram showing the XPS spectrum of the lithium fluoride protective layer according to Experimental Example 1.

[0036] Figure 2 is a diagram showing the XPS depth analysis result of the lithium fluoride protective layer according to Example 1.

[0037] Figure 3 is a diagram showing the XPS depth analysis result of the lithium fluoride protective layer according to Comparative Example 2.

[0038] Figure 4 FIG. Figure 4 is a graph showing the performance evaluation results of another lithium secondary battery according to Experimental Example 2. DETAILED DESCRIPTION

[0039] Hereinafter, the present invention will be described in more detail.

[0040] The terms and words used in this specification and the claims should not be construed as limited to ordinary or dictionary meanings, but should be construed as meanings and concepts consistent with the technical idea of the present invention on the basis of the principle that the inventor can appropriately define the concept of the term to describe his / her own invention in the best way.

[0041] The terms used in the present invention are only for describing specific embodiments and are not intended to limit the present invention. As used herein, unless the context clearly indicates otherwise, the singular forms include the plural forms. In the present invention, it should be understood that the terms "comprising" or "having" indicate the presence of the features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0042] The term "composite material" used herein refers to a material formed by combining two or more materials to exhibit a more effective function while forming physically and chemically different phases.

[0043] The term "polysulfide" used herein includes the concepts of "polysulfide ion (S x 2- , x = 8, 6, 4, 2)" and "lithium polysulfide (Li2S x or LiS x - , x = 8, 6, 4, 2)".

[0044] Among various secondary batteries, lithium-sulfur batteries exhibit high theoretical discharge capacity and theoretical energy density, and lithium metal, which is mainly used as a negative electrode active material, has a very small atomic weight (6.94 g / a.u.) and density (0.534 g / cm 3 ), making lithium-sulfur batteries attract attention as next-generation batteries due to their easy miniaturization and light weight.

[0045] However, when lithium metal is used as the negative electrode in a battery, due to several factors that change with battery operation, non-uniformity of current density may occur on the surface of the lithium metal, and thus, lithium dendrites are generated to cause battery short circuit, which causes problems of reducing the overall performance of the battery.

[0046] In addition, in the case of a lithium-sulfur battery, lithium polysulfide (Li2S) formed in the positive electrode during battery operationx , among x = 8, 6, 4, 2), polysulfide lithium (Li2S with a higher oxidation number of sulfur x , usually x > 4) continuously dissolves due to its high solubility in the electrolyte and dissolves out of the reaction zone of the positive electrode to move towards the negative electrode. At this time, the polysulfide lithium dissolved out from the positive electrode undergoes a side reaction with the lithium metal electrode as the negative electrode, and as lithium sulfide adheres to the surface of the lithium metal electrode, this not only causes the passivation of the lithium metal electrode, but also reduces the utilization rate of sulfur due to the dissolution of polysulfide lithium, so that the theoretical discharge capacity can only reach about 70% at most, which causes the problems of accelerating capacity and cycle life decline with the progress of cycling.

[0047] In the related art, in order to prevent the growth of lithium dendrites, a lithium fluoride (LiF) protective layer with excellent ionic conduction characteristics is introduced on the surface of the lithium metal. Specifically, in the related art, in order to form a lithium fluoride protective layer on the surface of the lithium metal, methods such as immersing the lithium metal in fluoroethylene carbonate (FEC) or coating a solution containing fluoroethylene carbonate are used.

[0048] However, in the case of the method used in the related art, because it is difficult to uniformly form a lithium fluoride protective layer due to the inevitable presence of an oxide layer in the lithium metal, there is a problem that it is difficult to sufficiently ensure the effect of suppressing the generation of lithium dendrites.

[0049] Therefore, the present invention provides a method for manufacturing a lithium metal electrode, which uniformly forms a lithium fluoride protective layer on the surface of the lithium metal by directly coating fluoroethylene carbonate on the surface of the lithium metal and performing rolling, thereby having an excellent effect of suppressing the growth of lithium dendrites.

[0050] The method for manufacturing a lithium metal electrode according to the present invention includes the following steps:

[0051] (a) Coating fluoroethylene carbonate on at least one surface of the lithium metal layer; and

[0052] (b) Rolling the lithium metal layer coated with the fluoroethylene carbonate to form a lithium fluoride protective layer on at least one surface of the lithium metal layer.

[0053] In the case of the present invention, fluoroethylene carbonate is directly coated on the surface of lithium metal and rolled to form a lithium fluoride protective layer with a high fluorine content and a uniform thickness on the surface of the lithium metal. In the related art, methods such as immersing lithium metal in fluoroethylene carbonate or coating a solvent containing fluoroethylene carbonate on the surface of lithium metal are used. However, the method of forming a lithium fluoride protective layer in the related art cannot effectively prevent the growth of lithium dendrites because the thickness is uneven due to the oxide layer formed on the surface of the lithium metal, or a lithium fluoride protective layer with a low fluorine content is formed. In addition, a method of forming a lithium fluoride protective layer by a pressing (rolling combination) method using a fluorine-containing polymer film (such as polytetrafluoroethylene and polyvinylidene fluoride) is provided. Thus, in the case of the method of forming a lithium fluoride protective layer by pressing a fluorine-containing polymer film and lithium, there is a problem that since a residual polymer film remains after reacting with lithium, the growth of lithium dendrites cannot be prevented by blocking the migration of lithium ions.

[0054] In contrast, in the present invention, since fluoroethylene carbonate is directly coated on the surface of lithium metal and the surface of the lithium metal is rolled, a uniform reaction between lithium metal and fluoroethylene carbonate can be carried out on the surface of the lithium metal. Therefore, a lithium fluoride protective layer is uniformly formed on the surface of the lithium metal, and the content of fluorine contained in the lithium fluoride protective layer is also high. The lithium fluoride protective layer formed in this way effectively inhibits the formation of lithium dendrites, thereby improving the stability of the lithium metal electrode and the capacity and life characteristics of the lithium secondary battery including the lithium metal electrode. In addition, the manufacturing method of the lithium metal electrode according to the present invention has the advantages that the process is very simple and easy to apply.

[0055] Hereinafter, the present invention will be described in more detail for each step.

[0056] First, step (a) is used to coat fluoroethylene carbonate on the surface of the lithium metal layer, wherein the fluoroethylene carbonate is coated on at least one surface of the lithium metal layer.

[0057] The lithium metal layer may contain lithium metal or a lithium alloy (Li-M).

[0058] The metal M contained in the lithium alloy may be at least one selected from the following: aluminum (Al), magnesium (Mg), zinc (Zn), boron (B), silicon (Si), tin (Sn), germanium (Ge), strontium (Sr), lanthanum (La), silver (Ag), indium (In), tantalum (Ta), and niobium (Nb).

[0059] In the present invention, the coating is carried out by directly coating fluoroethylene carbonate on at least one surface of the lithium metal layer.

[0060] The coating can be carried out by conventional methods that are generally known, such as various methods like pouring, spraying, atomizing, etc.

[0061] There is no limit to the number of coating times of fluoroethylene carbonate, and it can be carried out once or multiple times.

[0062] Subsequently, step (b) is used to roll the lithium metal layer coated with fluoroethylene carbonate in the above step (a) to form a lithium fluoride protective layer on at least one surface of the lithium metal layer.

[0063] By reacting lithium metal with fluoroethylene carbonate coated on the surface of the lithium metal layer through the rolling in step (b), a uniform lithium fluoride protective layer can be formed on the surface of the lithium metal layer.

[0064] There is no limit to the number of rolling times, and it can be carried out once or multiple times.

[0065] The rolling can be carried out by placing the lithium metal layer coated with fluoroethylene carbonate between rolling units and applying pressure.

[0066] The rolling unit is not particularly limited to the conventional units used in the art. For example, a rolling mill, a laminator, etc. can be used as the rolling unit.

[0067] The rolling can be carried out under temperature conditions of 10 to 80 °C. During rolling, the temperature and time can vary according to process conditions. When the rolling temperature and time are lower than the above range, since the reaction between fluoroethylene carbonate and lithium metal is not fully carried out, the formed lithium fluoride protective layer may be uneven. On the contrary, when the rolling temperature and time exceed the above range, there is a problem of lithium metal deformation.

[0068] The thickness of the lithium metal layer is reduced through the rolling in step (b). Specifically, the thickness reduction rate according to the following formula 1 can be 10% or more, preferably 10% to 30%. When the thickness reduction rate of the lithium metal layer through rolling is within the above range, the advantage is that a uniform lithium fluoride protective layer can be formed on the surface of the lithium metal layer, thereby more effectively suppressing the generation of lithium dendrites.

[0069] [Formula 1]

[0070]

[0071] In the present invention, after the rolling in the above step (b), a step of removing the remaining fluoroethylene carbonate and drying the lithium fluoride protective layer to firmly fix it on the surface of the lithium metal layer can be further carried out.

[0072] The drying is carried out at a temperature and for a time that can sufficiently remove fluoroethylene carbonate, and since the conditions may vary depending on the process conditions, there are no particular limitations on them in the present invention.

[0073] For example, the drying can be carried out by the following methods: drying using warm air, hot air, and low-humidity air; vacuum drying; and irradiation with (far) infrared rays and electron beams.

[0074] As described above, in the method for manufacturing a lithium metal electrode according to the present invention, by directly coating fluoroethylene carbonate on the surface of the lithium metal layer and performing rolling, a lithium metal electrode including a uniform lithium fluoride protective layer having a high fluorine content can be easily manufactured. The lithium fluoride protective layer manufactured by the manufacturing method of the present invention not only has an excellent effect of suppressing the growth of lithium dendrites on the lithium metal surface due to excellent uniformity, but also exhibits a predetermined level of ionic conductivity, thereby improving the capacity and life characteristics of the lithium secondary battery including it.

[0075] In addition, the present invention provides a lithium metal electrode manufactured by the above manufacturing method.

[0076] The lithium metal electrode includes: a lithium metal layer; and a lithium fluoride protective layer formed on at least one surface of the lithium metal layer.

[0077] The lithium metal layer may include lithium metal or a lithium alloy. In addition, the lithium metal layer may be a lithium metal thin film, or a lithium metal plate including lithium metal powder or a lithium metal thin film formed on at least one surface of a negative electrode current collector.

[0078] The negative electrode current collector is used to support the lithium metal layer as the negative electrode active material, and there are no particular limitations as long as the negative electrode current collector has high conductivity and does not cause chemical changes in the corresponding battery. For example, the following can be used: copper, stainless steel, aluminum, nickel, titanium, palladium, plastic carbon; copper or stainless steel surface-treated with carbon, nickel, silver, etc.; and aluminum-cadmium alloys, etc.

[0079] The negative electrode current collector may form fine irregularities on its surface to enhance the bonding force with the lithium metal thin film as the negative electrode active material, and various forms such as films, sheets, foils, sieves, meshes, porous bodies, foams, non-woven bodies, etc. may be used.

[0080] The method for forming the lithium metal thin film is not particularly limited, and methods for forming layers or films commonly used in the art can be used. For example, methods such as pressing, coating, deposition, etc. can be used.

[0081] The lithium fluoride protective layer can be formed according to the manufacturing method described above, and its thickness can be 10 to 500 nm, preferably 50 to 200 nm. When the thickness of the lithium fluoride protective layer is less than the above range, the effect of the above lithium fluoride protective layer is not obvious. On the contrary, when the thickness of the lithium fluoride protective layer exceeds the above range, problems of overvoltage may occur during charging and discharging.

[0082] As a result of X-ray photoelectron spectroscopy (XPS) analysis, the fluorine content of the lithium fluoride protective layer can be 0.1 to 10 atomic % of all the elements contained in the lithium fluoride protective layer, preferably 0.5 to 5 atomic %. In the lithium metal electrode according to the present invention, the lithium fluoride protective layer is uniformly formed according to the above manufacturing method, so that the fluorine content in the lithium fluoride protective layer is higher than that of the lithium metal electrode including the lithium fluoride protective layer in the related art.

[0083] The ionic conductivity of the lithium fluoride protective layer can be in the range of 10 -20 to 10 -4 S / cm, preferably 10 -12 to 10 -7 S / cm.

[0084] In addition, the surface resistance in the electrolyte of the lithium metal electrode formed with the lithium fluoride protective layer is 100 to 300 Ω, preferably 150 to 200 Ω, and does not change within 1 to 72 hours, preferably 24 to 65 hours, more preferably 36 to 60 hours. This means that, different from the related art, a continuous electrochemical reaction of the battery is maintained on the surface of the lithium metal electrode, and the lithium fluoride protective layer does not react with the electrolyte but remains in a stable state.

[0085] In addition, the present invention provides a lithium secondary battery including the above lithium metal electrode.

[0086] The lithium secondary battery includes a positive electrode, a negative electrode, and an electrolyte interposed between the positive electrode and the negative electrode, and includes the lithium metal electrode according to the present invention as the negative electrode.

[0087] Preferably, the lithium secondary battery can be a lithium-sulfur battery including sulfur as the positive electrode active material.

[0088] The positive electrode may include a positive electrode current collector and a positive electrode active material layer coated on at least one surface of the positive electrode current collector.

[0089] The positive electrode current collector is used to support the positive electrode active material and is as described in the negative electrode current collector.

[0090] The positive electrode active material layer includes a positive electrode active material, and optionally, may further include a conductive material, a binder, an additive, etc.

[0091] The positive electrode active material may include a sulfur compound, specifically, one or more selected from elemental sulfur and a sulfur compound. The elemental sulfur may include inorganic sulfur (S8). In addition, the sulfur compound may be at least one selected from the following: Li2S n (n≥1), disulfide compounds, organic sulfur compounds and carbon-sulfur polymers ((C2S x ) n , x=2.5 to 50, n≥2). Preferably, the positive electrode active material may include inorganic sulfur (S8).

[0092] When sulfur is included in the positive electrode active material, sulfur is used in combination with a conductive material such as a carbon material because sulfur alone does not have conductivity. Therefore, sulfur may be included in the form of a sulfur-carbon composite material. Preferably, the positive electrode active material may be a sulfur-carbon composite material.

[0093] The carbon contained in the sulfur-carbon composite material is a porous carbon material, and provides a skeleton that can uniformly and stably fix sulfur and compensates for the low conductivity of sulfur, so that an electrochemical reaction can proceed smoothly.

[0094] The porous carbon material can be prepared generally by carbonizing the precursors of various carbon materials. The porous carbon material may contain non-uniform pores therein, the average diameter of the pores being in the range of 1 to 200 nm, and the porosity may be in the range of 10 to 90% of the total volume of the porous carbon material. When the average diameter of the pores is less than the range, the pore size is only at the molecular level, whereby the impregnation of sulfur is impossible, and conversely, when the average diameter exceeds the range, the mechanical strength of the porous carbon material is weakened, whereby this is not preferred for the manufacturing process applied to the electrode.

[0095] The porous carbon material may be in the form of a sphere, a rod, a needle, a plate, a tube, or a block, and may be used without limitation as long as the porous carbon material is generally used in a lithium-sulfur battery.

[0096] The porous carbon material has a porous structure or a structure with a high specific surface area, and can be any porous carbon material, as long as the porous carbon material is commonly used in the art. For example, the porous carbon material can be one or more selected from the following: graphite; graphene; carbon black, such as Deco black, acetylene black, Ketjen black, channel black, furnace black, lamp black and 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); graphite, such as natural graphite, artificial graphite and expanded graphite; and activated carbon, but not limited thereto. Preferably, the porous carbon material can be a carbon nanotube.

[0097] Based on 100% by weight of the sulfur-carbon composite material, the content of sulfur in the sulfur-carbon composite material can be 60 to 90% by weight, preferably 65 to 85% by weight, more preferably 70 to 80% by weight. When the content of sulfur is below the above range, as the content of the porous carbon material in the sulfur-carbon composite material relatively increases, the specific surface area increases, resulting in an increase in the content of the binder during the manufacture of the positive electrode. The increased usage of the binder may ultimately increase the surface resistance of the positive electrode and act as an insulator to prevent electrons from passing through, thereby deteriorating the performance of the battery. On the contrary, when the content of sulfur exceeds the above range, the sulfur not combined with the porous carbon material aggregates or redissolves onto the surface of the porous carbon material, making it difficult to receive electrons, and thus the sulfur cannot participate in the electrochemical reaction, resulting in a possible loss of battery capacity.

[0098] In addition, in the sulfur-carbon composite material, sulfur is located in at least one of the inner surface and the outer surface of the porous carbon material. At this time, sulfur can be present in an area less than 100%, preferably 1 to 95%, more preferably 60 to 90% of the entire inner surface and outer surface of the porous carbon material. When sulfur is present on the inner surface and the outer surface of the porous carbon material within the above range, the maximum effect can be exhibited in terms of the electron migration area and the wettability with the electrolyte. Specifically, since sulfur is thinly and uniformly impregnated on the inner surface and the outer surface of the porous carbon material within the above range, the electron migration contact area can increase during the charge / discharge process. If sulfur is located in 100% of the area of the entire inner surface and outer surface of the porous carbon material, the carbon material is completely covered by sulfur, thereby reducing the wettability with the electrolyte and decreasing the contact with the conductive material contained in the electrode, making it impossible for sulfur to receive electrons and thus unable to participate in the electrochemical reaction.

[0099] The manufacturing method of the sulfur-carbon composite material of the present invention is not particularly limited, and a method commonly used in the art can be adopted. For example, a method of simply mixing sulfur and a porous carbon material and then heating the mixture to make it composite can be used.

[0100] In addition to the above composition, the positive electrode active material may further contain at least one additive selected from the following: transition metal elements, Group IIIA elements, Group IVA elements, sulfur compounds of these elements, and alloys of these elements and sulfur.

[0101] As the transition metal elements, it may include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Os, Ir, Pt, Au, Hg, etc. As the Group IIIA elements, it may include Al, Ga, In, Tl, etc., and as the Group IVA elements, it may include Ge, Sn, Pb, etc.

[0102] Based on 100% by weight of the entire cathode active material layer constituting the cathode, the content of the cathode active material can be 40 to 95% by weight, preferably 50 to 90% by weight. When the content of the cathode active material is below the above range, it is difficult to fully exhibit the electrochemical reaction of the cathode. On the contrary, when the content of the cathode active material exceeds the above range, the problem is that the content of the conductive material and the binder described below is relatively insufficient, resulting in an increase in the resistance of the cathode and deterioration of the physical properties of the cathode.

[0103] The cathode active material layer may optionally further contain: a conductive material that enables electrons to move smoothly within the cathode (specifically, the cathode active material); and a binder that is used to well adhere the cathode active material to the current collector.

[0104] The conductive material is a material that electrically connects the electrolyte and the cathode active material to serve as a path for electrons to move from the current collector to the cathode active material, and any conductive material can be used without limitation as long as the material has conductivity.

[0105] For example, as the conductive material, the following substances can be used alone or in combination: graphite, such as natural graphite and artificial graphite; carbon black, such as super-P, Degussa black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; carbon derivatives, such as carbon nanotubes and fullerenes; conductive fibers, such as carbon fibers and metal fibers; carbon fluoride; metal powders, such as aluminum powder and nickel powder; or conductive polymers, such as polyaniline, polythiophene, polyacetylene, and polypyrrole.

[0106] Based on 100% by weight of the entire cathode active material layer constituting the cathode, the content of the conductive material can be 0.01 to 30% by weight. If the content of the conductive material is below the above range, the voltage and capacity decrease because electrons are difficult to migrate between the cathode active material and the current collector. On the contrary, if the content of the conductive material exceeds the above range, the total energy (charge amount) of the battery decreases because the proportion of the cathode active material relatively decreases. Therefore, it is preferable to determine an appropriate content within the above range.

[0107] The binder holds the cathode active material in the cathode current collector and organically connects the cathode active materials to further increase the binding strength between the cathode active materials. Any binder known in the art can be used.

[0108] For example, the binder may be one or a mixture or copolymer of two or more selected from the following: fluororesin binders, including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); rubber binders, including styrene-butadiene rubber (SBR), nitrile rubber, and styrene-isoprene rubber; cellulose binders, including carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, and regenerated cellulose; polyol binders; polyolefin binders, including polyethylene and polypropylene; polyimide binders; polyester binders; and silane binders.

[0109] Based on 100% by weight of all the positive electrode active material layers constituting the positive electrode, the content of the binder may be 0.5 to 30% by weight. When the content of the binder is less than the above range, the physical properties of the positive electrode deteriorate, and thus the positive electrode active material and the conductive material may separate. When the content exceeds the above range, the ratio of the positive electrode active material and the conductive material in the positive electrode relatively decreases, and thus the capacity of the battery may decrease. Therefore, it is preferable to determine an appropriate content within the above range.

[0110] In the present invention, the method for manufacturing the positive electrode is not particularly limited, and various methods known to those skilled in the art or modified therefrom can be used.

[0111] For example, the positive electrode can be manufactured by the following method: preparing a positive electrode paste composition having the above composition, and then coating the prepared positive electrode paste composition on at least one surface of a positive electrode current collector.

[0112] The positive electrode paste composition may contain a positive electrode active material, a conductive material, and a binder, and may also contain a solvent.

[0113] The solvent may be a solvent capable of uniformly dispersing the positive electrode active material, the conductive material, and the binder. As the solvent, water as an aqueous solvent is most preferred, and the water may be distilled water and deionized water. However, the solvent is not limited thereto, and if necessary, lower alcohols that can be easily miscible with water can be used. The lower alcohols may include methanol, ethanol, propanol, isopropanol, and butanol. Preferably, these lower alcohols can be used as a mixture with water.

[0114] The content of the solvent may be at a level having a concentration sufficient to facilitate coating, and the specific content varies depending on the coating method and apparatus.

[0115] If necessary, the positive electrode paste composition may additionally contain materials commonly used in the art to improve its function. For example, a viscosity control agent, a fluidizing agent, a filler, etc. may be included.

[0116] In the present invention, the method of coating the positive electrode paste composition is not particularly limited, and examples thereof may include methods such as a doctor blade method, a casting method, a comma coating method, and a screen printing method. Further, after forming on a separate substrate, the positive electrode paste can be coated on the positive electrode current collector by a pressing or lamination method.

[0117] After coating, a drying process for removing the solvent can be carried out. The drying process is carried out at a temperature and for a time at a level capable of sufficiently removing the solvent, and the conditions can vary depending on the type of the solvent, and thus there is no particular limitation in the present invention. As examples, drying methods using warm air, hot air, or low-humidity air; vacuum drying methods; and drying methods by irradiating (far) infrared rays and electron beams can be used. The drying rate is adjusted in a range where the positive electrode active material layer is not broken due to normal stress concentration or in a range where the positive electrode active material layer is not peeled off from the positive electrode current collector, in such a manner that the solvent can be removed as quickly as possible.

[0118] In addition, after drying, the density of the positive electrode active material in the positive electrode can be increased by pressing the current collector. As pressing methods, methods such as molding and rolling are mentioned.

[0119] The porosity of the positive electrode, specifically the positive electrode active material layer, manufactured by the above composition and manufacturing method can be 50 to 80%, preferably 60 to 75%. When the porosity of the positive electrode is less than 50%, since the filling degree of the positive electrode paste composition containing the positive electrode active material, the conductive material, and the binder becomes too high, and thus the electrolyte sufficient to exhibit ionic conductivity and / or electrical conductivity between the positive electrode active materials cannot be maintained, there is a problem that the output characteristics or cycle characteristics of the battery may deteriorate, and the overvoltage and discharge capacity of the battery decrease sharply. On the contrary, when the porosity of the positive electrode exceeds 80% and thus has an excessively high porosity, there is a problem that the physical connection and electrical connection with the current collector decrease, and thus the bonding force decreases and the reaction is difficult to carry out, and more pores are filled with the electrolyte, and thus the energy density of the battery may be reduced. Therefore, the porosity of the positive electrode is appropriately adjusted within the above range.

[0120] In addition, the sulfur loading amount in the positive electrode according to the present invention, that is, the mass of sulfur per unit area of the positive electrode active material layer in the positive electrode can be 0.5 to 15 mg / cm 2 , preferably 1 to 10 mg / cm 2 .

[0121] The negative electrode is as described above.

[0122] The electrolyte contains lithium ions and is intended to cause an electrochemical oxidation or reduction reaction between the positive electrode and the negative electrode by the lithium ions.

[0123] The electrolyte can be a non-aqueous electrolyte or a solid electrolyte that does not react with lithium metal, but is preferably a non-aqueous electrolyte and contains an electrolyte salt and an organic solvent.

[0124] The electrolyte salt contained in the non-aqueous electrolyte is a lithium salt. Lithium salts can be used without limitation as long as they are commonly used in electrolytes for lithium secondary batteries. For example, the lithium salt can be LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 、LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, LiN(SO2F)2, lithium chloroborane, lithium lower aliphatic carboxylate, tetraphenylborate, lithium imide, etc.

[0125] The concentration of the lithium salt can be 0.2 to 2 M, specifically 0.4 to 2 M, and more specifically 0.4 to 1.7 M, depending on various factors such as the exact composition of the electrolyte solvent mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, the operating temperature, and other factors known in the field of lithium batteries. When the concentration of the lithium salt is less than 0.2 M, the conductivity of the electrolyte decreases, and thus the electrolyte performance may deteriorate. When the concentration of the lithium salt exceeds 2 M, the viscosity of the electrolyte increases, and the mobility of lithium ions may decrease.

[0126] As the organic solvent contained in the non-aqueous electrolyte, organic solvents commonly used in electrolytes for lithium secondary batteries can be used without limitation, and for example, ethers, esters, amides, linear carbonates, cyclic carbonates, etc. can be used alone or in combination of two or more. Among them, as a representative example, ether compounds can be included.

[0127] Ether compounds can include acyclic ethers and cyclic ethers.

[0128] For example, acyclic ethers can be, but are not limited to, at least one selected from the following: dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, dimethoxyethane, diethoxyethane, ethylene glycol ethyl methyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methyl ethyl ether, triethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methyl ethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methyl ethyl ether, polyethylene glycol dimethyl ether, polyethylene glycol diethyl ether, and polyethylene glycol methyl ethyl ether.

[0129] For example, the cyclic ether may be, but is not limited to, at least one selected from the following: 1,3-dioxolane, 4,5-dimethyl-dioxolane, 4,5-diethyl-dioxolane, 4-methyl-1,3-dioxolane, 4-ethyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, 2,5-dimethoxytetrahydrofuran, 2-ethoxytetrahydrofuran, 2-methyl-1,3-dioxolane, 2-vinyl-1,3-dioxolane, 2,2-dimethyl-1,3-dioxolane, 2-methoxy-1,3-dioxolane, 2-ethyl-2-methyl-1,3-dioxolane, tetrahydropyran, 1,4-di alkane, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, and isosorbide dimethyl ether.

[0130] Examples of the ester of the organic solvent may be, but are not limited to, any one selected from the following or a mixture of two or more thereof: methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0131] Specific examples of the linear carbonate compound may typically be, but are not limited to, any one selected from the following or a mixture of two or more thereof: dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate.

[0132] In addition, specific examples of the cyclic carbonate compound may be any one selected from the following or a mixture of two or more thereof: ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, ethylene vinylene carbonate, and halides of the above materials. Examples of these halides include, but are not limited to, fluoroethylene carbonate (FEC), etc.

[0133] In addition to the above electrolyte salts and organic solvents, the electrolyte may further contain a nitrate or nitrite compound as an additive. The nitrate or nitrite compound has the effect of forming a stable film on the lithium metal electrode serving as the negative electrode and improving the charge / discharge efficiency.

[0134] In the present invention, the nitric acid or nitrous acid compounds are not particularly limited, but may be at least one selected from the following: inorganic nitric acid or nitrous acid compounds such as lithium nitrate (LiNO3), potassium nitrate (KNO3), cesium nitrate (CsNO3), barium nitrate (Ba(NO3)2), ammonium nitrate (NH4NO3), lithium nitrite (LiNO2), potassium nitrite (KNO2), cesium nitrite (CsNO2) and ammonium nitrite (NH4NO2); organic nitric acid or nitrous acid compounds such as methyl nitrate, dialkylimidazole nitrate guanidine nitrate, imidazole nitrate pyridine nitrate ethyl nitrite, propyl nitrite, butyl nitrite, amyl nitrite and octyl nitrite; organic nitro compounds such as nitromethane, nitropropane, nitrobutane, nitrobenzene, dinitrobenzene, nitropyridine, dinitropyridine, nitrotoluene, dinitrotoluene and combinations of the above materials. Lithium nitrate is preferably used.

[0135] According to the manufacturing process of the final product and the required performance, the electrolyte can be injected at an appropriate stage in the manufacturing process of the electrochemical device. That is, it can be injected before assembling the electrochemical device or at the final stage of assembling the electrochemical device.

[0136] A separator may be additionally included between the positive electrode and the negative electrode.

[0137] The separator can be made of a porous non-conductive or insulating material that separates or insulates the positive electrode and the negative electrode from each other and allows lithium ions to be transmitted between the positive electrode and the negative electrode. The separator can be used without particular limitation as long as it is used as a separator in a conventional lithium secondary battery. The separator can be an independent member such as a film, or can be a coating added to the positive electrode and / or the negative electrode.

[0138] Preferably, the separator has a low resistance to ion migration of the electrolyte and excellent wettability to the electrolyte.

[0139] The separator can be made of a porous substrate, and as long as the porous substrate is a porous substrate commonly used in secondary batteries, it can be used. A porous polymer film can be used alone or by laminating them. For example, a non-woven fabric or a polyolefin porous film made of glass fibers with a high melting point, polyethylene terephthalate fibers, etc. can be used, but it is not limited thereto.

[0140] In the present invention, the material of the porous substrate is not particularly limited, and any material can be used as long as it is a porous substrate commonly used in electrochemical devices. For example, the porous substrate may include at least one material selected from the following: polyolefins (such as polyethylene and polypropylene), polyesters (such as polyethylene terephthalate and polybutylene terephthalate), polyamides, polyacetals, polycarbonates, polyimides, polyether ether ketones, polyether sulfones, polyphenylene ethers, polyphenylene sulfides, polyethylene naphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, poly(p-phenylene benzobis oxazoles) and polyarylate.

[0141] The thickness of the porous substrate is not particularly limited, but may be 1 to 100 μm, preferably 5 to 50 μm. Although the thickness range of the porous substrate is not particularly limited to the above range, if the thickness is much thinner than the above lower limit, the mechanical properties deteriorate, and thus the separator may be easily damaged during battery use.

[0142] The average diameter and porosity of the pores present in the porous substrate are also not particularly limited, but may be 0.001 μm to 50 μm and 10% to 95%, respectively.

[0143] In addition to the usual winding process, the lithium secondary battery according to the present invention can also be manufactured by processes such as lamination, stacking, and folding of the separator and the electrode.

[0144] The shape of the lithium secondary battery is not particularly limited, and may be various shapes such as cylindrical, laminated, and coin-shaped.

[0145] Furthermore, the present invention provides a battery module including the above lithium secondary battery as a unit cell.

[0146] The battery module can be used as a power source for medium and large-sized devices that require high-temperature stability, long cycle characteristics, high-capacity characteristics, etc.

[0147] Examples of these medium and large-sized devices may include, but are not limited to: power tools powered by an electric motor and moving; electric vehicles, including electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), etc.; electric two-wheel vehicles, including electric bicycles (E-bicycles) and electric scooters (E-scooters); electric golf carts; power storage systems, etc.

[0148] Preferred Embodiments

[0149] In the following, in order to facilitate the understanding of the present invention, preferred embodiments of the present invention will be described. However, it will be apparent to those skilled in the art that the following embodiments are only for illustrating the present invention, and various modifications and variations can be made within the scope and spirit of the present invention, and such variations and modifications are within the scope of the appended claims.

[0150] Examples and Comparative Examples

[0151] [Example 1]

[0152] 10 ml of fluoroethylene carbonate was sprayed on the surface of a lithium metal film with a thickness of 60 μm, and then calendered at room temperature (25 °C) using a calender, and dried in a vacuum oven at 80 °C for 12 hours to fabricate a 45-μm-thick lithium metal electrode having a lithium fluoride protective layer formed on the surface of the lithium metal film.

[0153] [Example 2]

[0154] 90 wt% of a sulfur-carbon composite (S:C = 75:25 (weight ratio)) as a positive electrode active material, 5 wt% of Super P as a conductive material, and 5 wt% of styrene-butadiene rubber / carboxymethyl cellulose (SBR:CMC = 70:30 (weight ratio)) as a binder were mixed to prepare a positive electrode paste composition.

[0155] The prepared positive electrode paste composition was coated on a 20-μm-thick aluminum current collector, dried at 50 °C for 12 hours, and pressed using a calender to fabricate a positive electrode. At this time, the loading amount of the positive electrode active material was 5.4 mAh / cm 2 hereinafter, and the porosity of the positive electrode was 68%.

[0156] The fabricated positive electrode and the lithium metal electrode fabricated in Example 1 were arranged to face each other, and a polyethylene separator with a thickness of 20 μm and a porosity of 68% was inserted therebetween, and then 0.1 ml of an electrolyte was injected to fabricate a lithium secondary battery.

[0157] At this time, as the electrolyte, a mixed solution formed by dissolving 1.0 M of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 1.0 wt% of lithium nitrate (LiNO3) in an organic solvent composed of 1,3-dioxolane and dimethyl ether (DOL:DME = 1:1 (volume ratio)) was used.

[0158] [Comparative Example 1]

[0159] A lithium metal electrode was fabricated using a 45-μm-thick lithium metal film on which no lithium fluoride protective layer was formed.

[0160] [Comparative Example 2]

[0161] A lithium metal thin film with a thickness of 45 μm was immersed in fluoroethylene carbonate for 1 hour, and then dried in a vacuum oven at 80 °C for 12 hours to fabricate a lithium metal electrode having a lithium fluoride protective layer formed on the surface of the lithium metal thin film.

[0162] [Comparative Example 3]

[0163] A lithium secondary battery was fabricated in the same manner as in Example 2, except that the lithium metal electrode of Comparative Example 1 was used instead of the lithium metal electrode of Example 1.

[0164] [Comparative Example 4]

[0165] A lithium secondary battery was fabricated in the same manner as in Example 2, except that the lithium metal electrode of Comparative Example 2 was used instead of the lithium metal electrode of Example 1.

[0166] Experimental Example 1. X-ray photoelectron spectroscopy analysis

[0167] X-ray photoelectron spectroscopy (XPS) analysis was performed on the surfaces of the lithium metal electrodes fabricated in Example 1 and Comparative Examples 1 and 2. The XPS equipment used for the analysis was K-ALPHA from Thermo Fisher Scientific. At this time, the corrosion rate was 0.09 nm / s, and the obtained data was analyzed using Avantage software. At this time, the obtained results are shown in Tables 1 to 3 and Figures 1 to 3 in.

[0168] [Table 1]

[0169]

[0170] [Table 2]

[0171]

[0172] [Table 3]

[0173]

[0174] Figure 1 Table 1 shows the XPS spectra of the fluoride protective layer, and in the case of Example 1, it was confirmed that the fluorine content was 1.6 times that of Comparative Example 2.

[0175] In addition, Figure 2 and 3 Tables 2 and 3 show the XPS depth profiles of the lithium fluoride protective layer. It can be seen that the lithium fluoride protective layer was 90 nm (corrosion time: 1000 seconds) in Example 1, while it was 18 nm (corrosion time: 200 seconds) in Comparative Example 2.

[0176] It can be confirmed from the results that a thick lithium fluoride protective layer with a high fluorine content is formed on the lithium metal electrode manufactured by the manufacturing method of the present invention.

[0177] Experimental Example 2. Evaluation of battery performance

[0178] For the batteries manufactured in Example 2 and Comparative Examples 3 and 4, the performance was evaluated using a charge / discharge measurement device (LAND CT-2001A, a product of Wuhan company).

[0179] Specifically, the discharge current rate at 25 °C was set to 0.5C, and then 110 charge / discharge cycles were repeated to measure the capacity and life characteristics. The results obtained at this time are shown in Table 4 and Figure 4 in.

[0180] [Table 4]

[0181] Retention rate of capacity (%) relative to the 7th cycle Example 2 87.0 Comparative Example 3 50.0 Comparative Example 4 67.0

[0182] As Figure 4 shown in and Table 4, in the case of the battery of Example 2, it can be seen that the overall performance is more excellent compared with the batteries of Comparative Examples 3 and 4.

[0183] Specifically, through Figure 4 and Table 4, it can be confirmed that in the case of the battery of Example 2 using the lithium metal electrode including the lithium fluoride protective layer manufactured according to the present invention, compared with Comparative Example 3 using the lithium metal electrode without the lithium fluoride protective layer and Comparative Example 4 using the lithium metal electrode with the lithium fluoride protective layer formed by the conventional impregnation method, its capacity and retention rate are higher.

[0184] It can be seen from the results that in the case of the lithium secondary battery including the lithium metal electrode manufactured by the manufacturing method of the present invention, excellent capacity and life characteristics are exhibited.

Claims

1. A method for manufacturing a lithium metal electrode, the method comprising the following steps: (a) Directly coating fluoroethylene carbonate onto at least one surface of a lithium metal layer; and (b) Rolling the lithium metal layer coated with the fluoroethylene carbonate to form a lithium fluoride protective layer on at least one surface of the lithium metal layer.

2. The manufacturing method of the lithium metal electrode according to claim 1, wherein, The lithium metal layer contains lithium metal or a lithium alloy.

3. The manufacturing method of the lithium metal electrode according to claim 1, wherein, The rolling is performed by placing the lithium metal layer coated with the fluoroethylene carbonate between rolling units and applying pressure.

4. The manufacturing method of the lithium metal electrode according to claim 1, wherein, The rolling is performed under a temperature condition of 10 °C to 80 °C.

5. The manufacturing method of the lithium metal electrode according to claim 1, wherein, The thickness reduction rate of the lithium metal layer before and after rolling is 10% or more.

6. A lithium metal electrode, the lithium metal electrode being manufactured according to the manufacturing method described in claim 1, and the lithium metal electrode includes: a lithium metal layer; and a lithium fluoride protective layer formed on at least one surface of the lithium metal layer.

7. The lithium metal electrode according to claim 6, wherein, The lithium fluoride protective layer has a thickness of 10 nm to 500 nm.

8. The lithium metal electrode according to claim 6, wherein, The lithium fluoride protective layer has a fluorine content of 0.1 atomic % to 10 atomic %.

9. The lithium metal electrode according to claim 6, wherein, The lithium metal electrode is a negative electrode for a lithium secondary battery.

10. A lithium secondary battery, the lithium secondary battery includes: a positive electrode, the positive electrode including a positive electrode active material; a negative electrode, the negative electrode including the lithium metal electrode described in claim 6; and an electrolyte.

11. The lithium secondary battery according to claim 10, wherein, The positive electrode active material includes at least one selected from elemental sulfur and sulfur compounds.

12. The lithium secondary battery according to claim 11, wherein, The positive electrode active material includes at least one selected from the following: inorganic sulfur S8; Li2S n , where n≥1; and organic sulfur compounds.

13. The lithium secondary battery according to claim 11, wherein, The positive electrode active material includes a disulfide compound.

14. The lithium secondary battery according to claim 11, wherein, The positive electrode active material contains a carbon-sulfur polymer (C2S x ) n , where x = 2.5 to 50 and n ≥ 2.

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