Negative electrode for lithium secondary battery and lithium secondary battery comprising the same

Abstract

The present invention relates to a negative electrode for a lithium secondary battery, which comprises a three-dimensional carbon structure coated with a fluorine-based polymer; and lithium metal introduced into the outer surface and interior of the three-dimensional carbon structure coated with the fluorine-based polymer, thereby suppressing the generation of lithium dendrites and improving the life characteristics of the lithium secondary battery comprising the negative electrode.

Description

Technical Field

[0001] This application claims priority based on Korean Patent Application No. 10-2021-0071885, filed on June 3, 2021, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery comprising the negative electrode, and more particularly to a negative electrode for a lithium secondary battery, the negative electrode comprising lithium metal introduced into the outer surface and interior of a three-dimensional carbon structure coated with a fluorine-based polymer; and a lithium secondary battery comprising the negative electrode. Background Art

[0003] Recently, as miniaturization and lightweighting of electronic products, electronic devices, communication devices, etc. have been rapidly progressing and demand for electric vehicles has greatly increased in relation to environmental issues, there is a growing demand for performance improvement of secondary batteries used as energy sources for these products.

[0004] In particular, lithium-sulfur (Li-S) batteries are secondary batteries that use a sulfur-based material with sulfur-sulfur bonds (S-S bonds) as the positive electrode active material and lithium metal as the negative electrode active material. As the primary positive electrode active material, sulfur has the advantages of being abundant, non-toxic, and having a low atomic weight. Furthermore, the theoretical discharge capacity of a lithium-sulfur battery is 1675 mAh / g sulfur, and its theoretical energy density is 2,600 Wh / kg. Because the theoretical energy density of lithium-sulfur batteries is much higher than that of other battery systems currently under research (Ni-MH batteries: 450 Wh / kg, Li-FeS batteries: 480 Wh / kg, Li-MnO2 batteries: 1000 Wh / kg, Na-S batteries: 800 Wh / kg), lithium-sulfur batteries are the most promising batteries developed to date.

[0005] During the discharge reaction of a lithium-sulfur battery, a lithium oxidation reaction occurs at the negative electrode and a sulfur reduction reaction occurs at the positive electrode. Sulfur has a cyclic S8 structure before discharge. During the reduction reaction (discharge), the oxidation number of S decreases as the SS bond is cut, and during the oxidation reaction (charge), the SS bond is reformed, and an oxidation-reduction reaction in which the oxidation number of S increases is used to store and generate electrical energy. During this reaction, sulfur is converted from the cyclic S8 structure to a linear lithium polysulfide (Li2S) through a reduction reaction. x , x = 8, 6, 4, 2). Finally, when the lithium polysulfide is completely reduced, lithium sulfide (Li2S) is ultimately produced. Unlike lithium-ion batteries, the discharge behavior of lithium-sulfur batteries is characterized by a gradual display of discharge voltage through the process of reduction to various lithium polysulfides.

[0006] However, when lithium metal is used as the negative electrode, due to the high reactivity during battery operation, there may be electron density unevenness on the surface of the lithium metal. As a result, dendritic lithium dendrites are generated on the electrode surface, and protrusions are formed or grown on the electrode surface, making the electrode surface very rough. Such lithium dendrites lead to deterioration of battery performance and, in severe cases, damage to the separator and battery short circuit. As a result, the temperature inside the battery rises, so there is a risk of battery explosion and fire, and there is a problem of limited battery life.

[0007] To address these issues, studies have been conducted to introduce polymer protective layers or inorganic solid protective layers into the lithium metal layer, increase the salt concentration in the electrolyte, or apply suitable additives. However, the suppression effect on lithium dendrites in these studies was insufficient.

[0008] [Prior art literature]

[0009] [Patent Document]

[0010] (Patent Document 1) Korean Patent Publication No. 10-2017-0117649, “Passivation layer for lithium electrode, electrode and lithium secondary battery comprising the same”

[0011] (Patent Document 2) Korean Patent Publication No. 10-2016-0052351, “LITHIUM METAL ELECTRODE FOR LITHIUM SECONDARY BATTERY WITH SAFE PROTECTIVE LAYER AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME” Summary of the Invention

[0012] Technical issues

[0013] The inventors of the present invention have conducted various studies to solve the above-mentioned problems. As a result, the inventors of the present invention have confirmed that if lithium metal constituting the negative electrode of a lithium secondary battery is introduced into the outer surface and interior of a three-dimensional carbon structure coated with a fluorine-based polymer, the fluorine-based polymer and the lithium metal react at the interface between the three-dimensional carbon structure coated with the fluorine-based polymer and the lithium metal to form lithium fluoride, and the lithium fluoride can protect the lithium metal to inhibit the formation of lithium dendrites and improve the life of the lithium secondary battery, thereby improving the life of the lithium secondary battery, thereby completing the present invention.

[0014] Therefore, an object of the present invention is to provide a negative electrode for a lithium secondary battery that can suppress the generation of lithium dendrites and improve the life of the lithium secondary battery, and a lithium secondary battery including the negative electrode.

[0015] Technical Solution

[0016] In order to achieve the above-mentioned object, the present invention provides a negative electrode for a lithium secondary battery, which comprises a three-dimensional carbon structure coated with a fluorine-based polymer; and lithium metal introduced into the outer surface and interior of the three-dimensional carbon structure coated with the fluorine-based polymer.

[0017] Furthermore, the present invention provides a lithium secondary battery comprising a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

[0018] The negative electrode is the negative electrode of the present invention.

[0019] Beneficial effects

[0020] In the negative electrode for a lithium secondary battery of the present invention, lithium fluoride having excellent ion conductivity can be formed at the interface between the three-dimensional carbon structure coated with the fluorine-based polymer and lithium metal to protect the lithium metal, thereby suppressing lithium dendrite growth.

[0021] Therefore, a lithium secondary battery including the negative electrode for a lithium secondary battery of the present invention can have improved lifespan characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic diagram illustrating a method for producing a negative electrode for a lithium secondary battery according to the present invention.

[0023] Figure 2 This is a schematic diagram of a negative electrode for a lithium secondary battery in Comparative Example 1.

[0024] Figure 3 Schematic diagram showing a method for producing a negative electrode for a lithium secondary battery in Comparative Example 2.

[0025] Figure 4is a SEM photograph of the surface of the three-dimensional carbon structure coated with PTFE prepared in Example 1.

[0026] Figure 5 is a SEM photograph of the surface of the three-dimensional carbon structure coated with PTFE prepared in Example 2.

[0027] Figure 6 This is a SEM photograph of the surface of the three-dimensional carbon structure of Comparative Example 2.

[0028] Figure 7 These are photographs of negative electrodes for lithium secondary batteries of Examples 1 and 2 and Comparative Example 2.

[0029] Figure 8 This is the XPS chart of Experimental Example 3. DETAILED DESCRIPTION

[0030] Hereinafter, the present invention will be described in more detail.

[0031] In lithium secondary batteries, preferably lithium-sulfur batteries, that use lithium metal as a negative electrode, the high reactivity of lithium during operation causes lithium dendrites to grow on the surface of the negative electrode. As a result, there is a problem of the negative electrode becoming porous and the lifespan characteristics of the lithium secondary battery containing the negative electrode being degraded. Furthermore, unlike other negative electrode active materials such as graphite, lithium negative electrodes do not have a host material capable of storing lithium. Therefore, there is a problem of large changes in the volume of the negative electrode during the charging and discharging processes of the lithium secondary battery, resulting in uneven lithium utilization.

[0032] To address these issues, researchers are hoping to improve the reversibility of lithium and the lifespan of lithium secondary batteries by introducing three-dimensional carbon structures capable of supporting lithium metal into the lithium anode. However, due to the lack of affinity between carbon and lithium, it is difficult to uniformly introduce lithium into the pores of the three-dimensional carbon structure, making it difficult to address these issues.

[0033] Therefore, in the present invention, an attempt is made to provide a negative electrode for a lithium secondary battery that can inhibit lithium dendrite growth and improve the life characteristics of the lithium secondary battery by uniformly introducing lithium metal into the outer surface and interior of the three-dimensional carbon structure using a three-dimensional carbon structure coated with a fluorine-based polymer.

[0034] That is, the present invention relates to a negative electrode for a lithium secondary battery, the negative electrode comprising a three-dimensional carbon structure coated with a fluorine-based polymer;

[0035] and lithium metal introduced into the outer surface and the interior of the three-dimensional carbon structure coated with the fluorine-based polymer.

[0036] The three-dimensional carbon structure is a porous carbon structure, specifically meaning that cylindrical carbon materials are interconnected in a three-dimensional manner.

[0037] In this case, the three-dimensional structure may mean that intersection points where two or more lines intersect are distributed in three dimensions.

[0038] In addition, the three-dimensional structure may mean that each basic unit wound in two dimensions is wound again in three dimensions, and finally has a three-dimensional structure. "Twisting" may mean that two or more lines cross each other through physical contact.

[0039] The cylindrical carbon material is not particularly limited in type, but may include at least one selected from the group consisting of carbon nanofibers, carbon nanotubes, graphite nanofibers, and activated carbon fibers.

[0040] In addition, the porous carbon structure may include at least one selected from the group consisting of carbon paper, carbon felt, and carbon mat. Even when lithium metal is introduced into the outer surface and the interior of the porous carbon structure, the structure must be maintained, so the porous carbon structure may preferably be carbon paper.

[0041] Since the three-dimensional carbon structure is in a state in which cylindrical carbon materials are entangled in three dimensions, empty spaces, i.e., pores, can exist in the carbon structure. Therefore, lithium metal can be introduced into the outer surface and interior of the three-dimensional carbon structure.

[0042] In the present invention, the three-dimensional carbon structure can serve not only as a carrier for loading lithium on the outer surface and inside, but also as a current collector.

[0043] In the three-dimensional carbon structure, carbon does not have a high affinity for lithium metal, so there is a problem that lithium cannot be electrochemically plated uniformly on the surface of the three-dimensional carbon structure during the operation of the lithium secondary battery. Therefore, in the present invention, in order to induce lithium to be uniformly plated inside the three-dimensional carbon structure, the surface of the three-dimensional carbon structure is coated with a fluorine-based polymer. If lithium metal is introduced into the outer surface and interior of the three-dimensional carbon structure coated with a fluorine-based polymer, when lithium metal is introduced, the fluorine-based polymer reacts with the lithium metal to spontaneously form lithium fluoride (LiF). Lithium fluoride has excellent ion conductivity, so it can suppress the growth of lithium dendrites, thereby stabilizing lithium metal. Therefore, the life characteristics of the lithium secondary battery comprising the lithium secondary battery negative electrode of the present invention can be improved.

[0044] There is no particular limitation on the type of the fluorine-based polymer, but the fluorine-based polymer may include at least one selected from the group consisting of polytetrafluoroethylene, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, perfluoroalkoxyalkane, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer, tetrafluoroethylene-chlorotrifluoroethylene copolymer, and ethylene-chlorotrifluoroethylene copolymer, and may preferably include polytetrafluoroethylene.

[0045] There is no particular limitation on the coating method of the three-dimensional carbon structure coated with the fluorine-based polymer, but it can be prepared by melting and coating the fluorine-based polymer or coating an aqueous solution of the fluorine-containing polymer. In the present invention, the three-dimensional carbon structure coated with the fluorine-based polymer can be preferably prepared by coating the three-dimensional carbon structure with an aqueous solution of the fluorine-containing polymer.

[0046] The content of the fluorine-based polymer may be 2 wt % or more and less than 20 wt %, preferably 5 wt % to 15 wt %, based on the total weight of the aqueous solution of the fluorine-based polymer.

[0047] If the content of the fluorine-based polymer is less than 2% by weight, the coating on the surface of the three-dimensional carbon structure is insufficient, and thus there may be a problem of uneven formation of lithium fluoride at the interface between the lithium metal and the three-dimensional carbon structure. If the content of the fluorine-based polymer is greater than 20% by weight, there may be a problem of excessive reaction between the lithium metal and the fluorine-based polymer, resulting in a large loss of lithium metal, and thus the life characteristics of the lithium secondary battery cannot be improved.

[0048] The three-dimensional carbon structure coated with the fluorine-based polymer may contain 5 to 30 parts by weight, preferably 10 to 25 parts by weight, and more preferably 15 to 20 parts by weight of fluorine, based on 100 parts by weight of carbon.

[0049] If the content of fluorine is less than 5 parts by weight, the coating of the fluorine-based polymer on the surface of the three-dimensional carbon structure is insufficient. Therefore, there may be a problem that sufficient lithium fluoride is not formed on the outer surface and inside of the three-dimensional carbon structure. In addition, if the content of fluorine exceeds 30 parts by weight, an overreaction may occur between the lithium metal and the fluorine-based polymer when lithium metal is introduced, resulting in a large amount of loss of lithium metal, and therefore the life characteristics of the lithium secondary battery cannot be improved. In addition, when lithium fluoride is excessively generated, the negative electrode may be fragile, which may have a negative impact on the manufacturing process.

[0050] The coating can be applied by conventional methods known in the art. The coating can be applied by, for example, a doctor blade coating method, a dip coating method, a gravure coating method, a slot die coating method, a spin coating method, a comma coating method, a rod coating method, a reverse roll coating method, a screen coating method, a cap coating method, etc. In the present invention, the coating is preferably applied by a dip coating method.

[0051] In the negative electrode for a lithium secondary battery, lithium metal may be introduced into the outer surface and the interior of the three-dimensional carbon structure coated with a fluorine-based polymer by laminating a lithium foil on the three-dimensional carbon structure coated with a fluorine-based polymer.

[0052] The lamination method is not particularly limited as long as it is widely used in the art, but in the present invention, the lithium foil may be preferably laminated by a roll pressing method.

[0053] More specifically, the lithium metal can be introduced into the outer surface and interior of the three-dimensional carbon structure coated with a fluorine-based polymer by laminating lithium foil on both surfaces of the three-dimensional carbon structure coated with a fluorine-based polymer and then rolling them with a roller press.

[0054] If lithium metal is introduced into the outer surface and interior of the three-dimensional carbon structure coated with a fluorine-based polymer, the lithium metal and the fluorine-based polymer can spontaneously react to form lithium fluoride. That is, the negative electrode for a lithium secondary battery of the present invention contains lithium fluoride formed by the reaction of the fluorine-based polymer and the lithium metal, wherein the lithium fluoride can be formed at the interface between the three-dimensional carbon structure coated with a fluorine-based polymer and the lithium metal.

[0055] Lithium fluoride has excellent ionic conductivity, thereby inhibiting the growth of lithium dendrites, and can act as a protective layer to protect the surface of lithium metal, thereby helping to stabilize the lithium metal. Therefore, a lithium secondary battery, preferably a lithium-sulfur battery, comprising the lithium secondary battery negative electrode of the present invention can have improved life characteristics.

[0056] The negative electrode for a lithium secondary battery of the present invention may be a negative electrode for a lithium-sulfur battery.

[0057] Furthermore, the present invention relates to a lithium secondary battery comprising a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the negative electrode is the negative electrode for a lithium secondary battery of the present invention.

[0058] The lithium secondary battery according to the present invention may preferably be a lithium-sulfur battery.

[0059] Since the lithium-sulfur battery uses the negative electrode of the present invention as the negative electrode, the formation of lithium dendrites is suppressed, and thus a lithium-sulfur battery having excellent life characteristics can be provided.

[0060] The positive electrode, separator, and electrolyte of the lithium secondary battery are not particularly limited in the present invention and are as known in the art.

[0061] The positive electrode according to the present invention includes a positive electrode active material formed on a positive electrode current collector.

[0062] The positive electrode current collector supports the positive electrode active material and is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, sintered carbon; copper or stainless steel surface-treated with carbon, nickel, silver, etc.; aluminum-cadmium alloy, etc. can be used as the positive electrode current collector.

[0063] The positive electrode current collector may enhance adhesion to the positive electrode active material by having fine concavoconvexo on its surface, and may be formed into various forms such as a film, sheet, foil, mesh, net, porous body, foam, or non-woven fabric.

[0064] The positive electrode active material may include at least one selected from the group consisting of elemental sulfur (S8) and sulfur compounds. Preferably, the positive electrode active material may include at least one selected from the group consisting of inorganic sulfur, Li2S n (n≥1), disulfide compounds, organic sulfur compounds and carbon-sulfur polymers ((C2S x ) n , x=2.5 to 50, n≥2). Most preferably, the positive electrode active material may include inorganic sulfur.

[0065] Therefore, the lithium secondary battery of the present invention may be a lithium-sulfur battery.

[0066] The sulfur contained in the positive electrode active material is used in combination with a conductive material such as a carbon material, because sulfur itself does not have conductivity. Therefore, the sulfur contained is in the form of a sulfur-carbon composite. Preferably, the positive electrode active material can be a sulfur-carbon composite.

[0067] The sulfur-carbon composite includes a porous carbon material, which not only provides a framework capable of uniformly and stably fixing sulfur, but also compensates for the low electrical conductivity of sulfur, so that the electrochemical reaction can proceed smoothly.

[0068] The porous carbon material can generally be prepared by carbonizing various carbonaceous precursors. The porous carbon material may contain non-uniform pores, the average diameter of the pores being in the range of 1 to 200 nm, and the porosity being in the range of 10% to 90% of the total volume of the porous carbon material. If the average pore diameter is smaller than the above range, the pore size is only at the molecular level, making sulfur impregnation impossible. Conversely, if the average pore diameter exceeds the above range, the mechanical strength of the porous carbon material is weakened, which is not preferred for use in electrode manufacturing processes.

[0069] The porous carbon material is 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 it is commonly used in lithium-sulfur batteries.

[0070] The porous carbon material may have a porous structure or a high specific surface area, and may be any of those conventionally used in the art. For example, the porous carbon material may be, but is not limited to, at least one selected from the group consisting of: graphite; graphene; carbon black such as Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; carbon nanotubes (CNTs) such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs); carbon fibers such as graphite nanofibers (GNFs), carbon nanofibers (CNFs), and activated carbon fibers (ACFs); and graphites such as natural graphite, artificial graphite, and expanded graphite, as well as activated carbon. Preferably, the porous carbon material may be a carbon nanotube.

[0071] The sulfur in the sulfur-carbon composite is located on at least one of the inner and outer surfaces of the porous carbon material. For example, it can be present in an area that accounts for less than 100%, preferably 1% to 95%, and more preferably 40% to 96% of the entire inner and outer surfaces of the porous carbon material. When the sulfur as described above is present on the inner and outer surfaces of the porous carbon material within the above range, the maximum effect can be exhibited in terms of electron transfer area and electrolyte wettability. Specifically, since the sulfur is thinly and uniformly impregnated on the inner and outer surfaces of the porous carbon material within the above range, the electron transfer contact area can be increased during charge and discharge. If the sulfur is located in 100% of the entire inner and outer surfaces of the porous carbon material, the carbon material is completely covered with sulfur, resulting in poor electrolyte wettability and poor contact, making it unable to receive electrons and therefore unable to participate in electrochemical reactions.

[0072] Based on 100 wt % of the sulfur-carbon composite, the sulfur-carbon composite may contain 65 to 90 wt %, preferably 70 to 85 wt %, and more preferably 72 to 80 wt % of sulfur. If the sulfur content is less than the above range, as the content of the porous carbon material in the sulfur-carbon composite increases relatively, the specific surface area increases, and therefore, when manufacturing the positive electrode, the content of the binder increases. This increase in the amount of binder ultimately increases the sheet resistance of the positive electrode and acts as an insulator to prevent electrons from passing through, thereby deteriorating the battery performance. On the contrary, if the sulfur content exceeds the above range, the sulfur that cannot be combined with the porous carbon material will aggregate with each other, or re-leak to the surface of the porous carbon material, making it difficult to receive electrons and unable to participate in the electrochemical reaction, thereby resulting in a loss of battery capacity.

[0073] The present invention has no particular limitation on the preparation method of the sulfur-carbon composite, and methods commonly used in the art can be used. For example, a method of simply mixing sulfur and porous carbon material and then heat-treating them to form a composite can be used.

[0074] In addition to the above components, the positive electrode active material may further include at least one selected from transition metal elements, Group IIIA elements, Group IVA elements, sulfur compounds of these elements, and alloys of these elements with sulfur.

[0075] The transition metal elements 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., the Group IIIA elements may include Al, Ga, In, Tl, etc., and the Group IVA elements may include Ge, Sn, Pb, etc.

[0076] In the positive electrode of the lithium secondary battery of the present invention, the content of the positive electrode active material may be 50 to 95 weight %, based on the total weight of the positive electrode active material layer constituting the positive electrode of the lithium secondary battery as 100 weight %. In terms of the content of the positive electrode active material, the lower limit may be 70 weight % or more or 85 weight % or more, and the upper limit may be 99 weight % or less or 90 weight % or less, based on the total weight of the positive electrode active material layer as 100 weight %. The content of the positive electrode active material can be set by a combination of a lower limit and an upper limit. If the content of the positive electrode active material is less than the above range, it is difficult for the electrode to fully exhibit an electrochemical reaction. On the contrary, if the content of the positive electrode active material exceeds the above range, there is a problem of relatively insufficient binder content, which deteriorates the physical properties of the electrode.

[0077] Furthermore, the positive electrode active material layer may further include a binder and a conductive material in addition to the positive electrode active material.

[0078] The binder may further serve to well adhere the positive electrode active material to the positive electrode current collector.

[0079] The binder holds the positive electrode active material in the positive electrode current collector and organically connects the positive electrode active materials to increase the binding force therebetween, and any binder known in the art may be used.

[0080] For example, the binder can be, but is not limited to, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyacrylic acid metal salt (metal-PAA), polymethacrylic acid (PMA), polymethyl methacrylate (PMMA), polyacrylamide (PAM), polymethacrylamide, polyacrylonitrile (PAN), polymethacrylonitrile, polyimide (PI), chitosan, starch, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, hydroxypropyl cellulose, regenerated cellulose, and various copolymers thereof.

[0081] The content of the binder may be 1 to 10% by weight, based on the total weight of the positive electrode active material layer constituting the positive electrode for a lithium secondary battery being 100% by weight. If the content of the binder is less than the above range, the physical properties of the positive electrode may be deteriorated, and thus the positive electrode active material may be detached. If the content of the binder exceeds the above range, the ratio of the positive electrode active material in the positive electrode is relatively reduced, so that the capacity of the battery may be reduced. Therefore, it is preferred to appropriately determine the binder content within the above range.

[0082] Furthermore, a conductive material may be additionally used to further improve the conductivity of the positive electrode active material.

[0083] The conductive material electrically connects the electrolyte and the positive electrode active material to serve as a path for electrons to move from the current collector to the positive electrode active material, and the conductive material may be used without limitation as long as it has conductivity.

[0084] For example, as the conductive material, graphite such as natural graphite and artificial graphite; carbon black such as Super-P, Danka black, acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal black; carbon derivatives such as carbon nanotubes and fullerenes; conductive fibers such as carbon fibers and metal fibers; fluorocarbons; metal powders such as aluminum and nickel powder; or conductive polymers such as polyaniline, polythiophene, polyacetylene and polypyrrole can be used alone or in combination.

[0085] The content of the conductive material may be 1 to 10% by weight, preferably 4 to 7% by weight, based on the total weight of the positive electrode active material layer constituting the positive electrode. If the content of the conductive material is less than the above range, it is difficult to transfer electrons between the positive electrode active material and the current collector, thereby reducing the voltage and capacity. On the contrary, if the content of the conductive material exceeds the above range, the proportion of the positive electrode active material may decrease, so that the total energy (charge) of the battery may decrease. Therefore, it is preferred to appropriately determine the content of the conductive material within the above range.

[0086] In the present invention, there is no particular limitation on the method for producing the positive electrode, and various methods known to those skilled in the art or various modified methods thereof can be used.

[0087] For example, the positive electrode may be manufactured by preparing a slurry composition for a positive electrode including the above components and then applying it to at least one surface of a positive electrode current collector.

[0088] The positive electrode slurry composition includes the positive electrode active material as described above, and may further include a binder, a conductive material, and a solvent.

[0089] As the solvent, a solvent that can uniformly disperse the positive electrode active material, the conductive material, and the binder is used. As such a solvent, water is the most preferred aqueous solvent. In this case, the water may be distilled water or deionized water, but is not necessarily limited thereto. If necessary, a lower alcohol that can be easily mixed with water may be used. Examples of the lower alcohol include methanol, ethanol, propanol, isopropanol, and butanol, and they are preferably mixed with water for use.

[0090] The solvent may be contained at a concentration level that is beneficial for coating, and the specific content varies depending on the coating method and equipment.

[0091] If necessary, the positive electrode slurry composition may further contain materials commonly used in the related technical field for the purpose of improving its function. For example, a viscosity modifier, a fluidizing agent, a filler, etc. may be mentioned.

[0092] In the present invention, there is no particular limitation on the method of applying the positive electrode slurry composition. For example, methods such as doctor blade, die casting, comma coating, and screen printing can be mentioned. In addition, after forming on a separate substrate, the positive electrode slurry composition can be applied to the positive electrode collector by pressing or laminating.

[0093] After application, a drying process may be performed to remove the solvent. The drying process is performed at a temperature and time level at which the solvent can be fully removed, and the conditions may vary depending on the type of solvent and are therefore not particularly limited in the present invention. For example, a drying method by warm air, hot air or low humidity air, a vacuum drying method, and a drying method by irradiation with (far) infrared radiation or electron beams may be mentioned. The drying speed is adjusted so that the solvent can be removed as quickly as possible within a speed range that does not cause the positive electrode active material layer to rupture due to ordinary stress concentration or within a speed range that the positive electrode active material layer does not peel off from the positive electrode current collector.

[0094] 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 a pressing method, methods such as mold pressing and roll pressing are mentioned.

[0095] The separator may be located between the positive electrode and the negative electrode.

[0096] The separator separates or insulates the positive electrode and the negative electrode from each other and enables lithium ions to be transferred between the positive electrode and the negative electrode, and can be made of a porous non-conductive or insulating material. The separator can be used without particular limitation as long as it is used as a separator in a typical lithium secondary battery. The separator can be an independent component such as a film, or can be a coating added to the positive electrode and / or the negative electrode.

[0097] The separator is preferably one that has excellent electrolyte impregnation ability and low resistance to ion migration in the electrolyte solution.

[0098] The separator may be made of a porous substrate. As the porous substrate, any porous substrate may be used as long as it is a porous substrate commonly used in secondary batteries. The porous polymer film may be used alone or in the form of a laminate. For example, a non-woven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, or a polyolefin-based porous membrane may be used, but is not limited thereto.

[0099] In the present invention, there is no particular limitation on the material of the porous substrate, 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 comprise at least one material selected from the group consisting of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyamides, polyacetals, polycarbonates, polyimides, polyetheretherketones, polyethersulfones, polyphenylene oxides, polyphenylene sulfides, polyethylene naphthalate, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, cellulose, nylon, poly(p-phenylene benzodione), ... azoles) and polyarylates.

[0100] 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 within the above range, if the thickness is too thin and below the above lower limit, the mechanical properties may be degraded, and the separator may be easily damaged during battery use.

[0101] The average size and porosity of the pores in the porous substrate are not particularly limited either, and may be 0.001 μm to 50 μm and 10% to 95%, respectively.

[0102] The electrolyte is a non-aqueous electrolyte containing lithium salt, which is composed of lithium salt and electrolyte. As the electrolyte, non-aqueous organic solvents, organic solid electrolytes and inorganic solid electrolytes are used.

[0103] The lithium salt of the present invention is a material that can be easily dissolved in a non-aqueous organic solvent, and can be, for example, selected from LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiB(Ph)4, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, LiSO3CH3, LiSO3CF3, LiSCN, LiC(CF3SO2)3, LiN(CF3SO2)2, lithium chloroborane, lower aliphatic carboxylic acid lithium and at least one of the group consisting of lithium tetraphenylborate.

[0104] The concentration of the lithium salt may be from 0.2M to 2M, preferably from 0.6M to 2M, more preferably from 0.7 to 1.7M, depending on various factors such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the charging and discharging conditions of the battery, the operating temperature, and other factors known in the field of lithium batteries. If the concentration of the lithium salt is less than 0.2M, the conductivity of the electrolyte may decrease, and thus the performance of the electrolyte may deteriorate. If the concentration of the lithium salt exceeds 2M, the viscosity of the electrolyte may increase, and thus the lithium ion (Li + ) may have reduced mobility.

[0105] The non-aqueous organic solvent should dissolve the lithium salt well. The non-aqueous organic solvent of the present invention may include, for example, aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, 4-methyl-1,3-diethoxyethylene, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate and ethyl propionate. These organic solvents may be used alone or in the form of a mixed solvent of two or more solvents.

[0106] As the organic solid electrolyte, for example, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate polymers, stirred lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polymers containing ion dissociative groups can be used.

[0107] As the inorganic solid electrolyte of the present invention, for example, Li nitrides, halides and sulfates such as Li3N, LiI, Li5NI2, Li3N-LiILiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH, Li3PO4-Li2SSiS2 can be used.

[0108] The positive electrode, separator and electrolyte contained in the lithium secondary battery can be prepared according to conventional components and manufacturing methods, respectively, and the external shape of the lithium secondary battery is not particularly limited, but can be a cylindrical type using a can, a rectangular type, a pouch type or a coin type.

[0109] Modes for carrying out the invention

[0110] The following preferred embodiments are provided to help understand the present invention, but the following embodiments are only used to illustrate the present invention, and it is obvious to those skilled in the art that various changes and modifications can be made within the scope and spirit of the present invention, and such changes and modifications are all within the scope of the appended claims.

[0111] <Manufacturing of negative electrodes for lithium-sulfur batteries>

[0112] Example 1

[0113] Polytetrafluoroethylene (PTFE, product name: Teflon TM PTFE DISP 30) was added to distilled water to prepare an aqueous fluoropolymer solution containing 10 wt% of PTFE based on the total weight of the fluoropolymer aqueous solution.

[0114] Carbon paper was coated with a fluorine-based polymer aqueous solution and dried to prepare a PTFE-coated three-dimensional carbon structure.

[0115] The negative electrode ( Figure 1 At this time, a film having no adhesion to lithium is used for the purpose of demolding the lithium foil on the surface in contact with the calendering roll.

[0116] Heat was generated during the calendering process, which is believed to be due to the exothermic reaction of the spontaneous reaction between PTFE and lithium to form lithium fluoride.

[0117] Example 2

[0118] The negative electrode was manufactured in the same manner as in Example 1, except that a fluoropolymer aqueous solution having a PTFE content of 20 wt % relative to the total weight of the fluoropolymer aqueous solution was used ( Figure 1 ).

[0119] Heat was generated during rolling, which is believed to be due to an exothermic reaction when the PTFE and lithium spontaneously react to form lithium fluoride. Furthermore, within 1 to 2 hours after production, the negative electrode for the lithium secondary battery of Example 2 developed a brown color on the surface, uneven marks formed, and the surface began to roughen. This appears to be due to an excessive reaction between the lithium metal and the PTFE, and confirmed that the electrode was more susceptible to breakage (brittleness).

[0120] Comparative Example 1

[0121] The negative electrode was fabricated by laminating 60 μm thick lithium foil on both surfaces of a 10 μm thick copper current collector ( Figure 2 ).

[0122] Comparative Example 2

[0123] Carbon paper was used as the three-dimensional carbon structure.

[0124] The negative electrode ( Figure 3 At this time, a film having no adhesion to lithium is used for the purpose of demolding the lithium foil on the surface in contact with the calendering roll.

[0125] Experimental Example 1. Measurement of the Fluorine-to-Carbon Content Ratio in a Three-Dimensional Carbon Support Coated with a Fluorine-Based Polymer

[0126] The content of fluorine coated on the three-dimensional carbon structure in the negative electrodes for lithium-sulfur batteries of Examples 1 and 2 was measured.

[0127] The fluorine content was measured using SEM and EDS.

[0128] In the PTFE-coated three-dimensional carbon structure of Example 1, it was confirmed that the content of fluorine (F) was 19 parts by weight relative to 100 parts by weight of carbon (C) (F / C ratio = 0.19).

[0129] Furthermore, in the PTFE-coated three-dimensional carbon structure of Example 2, it was confirmed that the content of fluorine (F) was 33 parts by weight relative to 100 parts by weight of carbon (C) (F / C ratio = 0.33).

[0130] Experimental Example 2. Measurement of the Weight per Unit Area of ​​a PTFE-Coated Three-Dimensional Carbon Structure and a Copper Current Collector

[0131] The weight per unit area of ​​the PTFE-coated three-dimensional carbon structure prepared in Example 1 and the copper current collector of Comparative Example 1 were measured.

[0132] The unit area weight of the PTFE-coated three-dimensional carbon structure prepared in Example 1 is 3.3 mg / cm 2 The unit area weight of the copper current collector of Comparative Example 1 is 9 mg / cm 2 .

[0133] The negative electrode for the lithium-sulfur battery of Example 1 is a negative electrode obtained by introducing lithium metal into the outer surface and interior of a three-dimensional carbon structure coated with a fluorine-based polymer PTFE, and the negative electrode for the lithium-sulfur battery of Comparative Example 1 is a negative electrode obtained by laminating lithium foil on both surfaces of a copper current collector.

[0134] It was confirmed that the three-dimensional carbon structure coated with PTFE of Example 1 had a lower weight per unit area than the copper current collector of Comparative Example 1.

[0135] It can be seen from this that the negative electrode of the present invention has a small loss in energy density per unit weight, thereby reducing the decrease in energy density.

[0136] Experimental Example 3. Surface Analysis of Negative Electrodes for Lithium-Sulfur Batteries

[0137] The surfaces of the negative electrodes for lithium-sulfur batteries prepared in Example 1 and Example 2 were analyzed by XPS to determine whether lithium fluoride was generated.

[0138] As a result, it was confirmed that the fluorine (F) content of Example 1 was 1.5% (atomic percentage), and the fluorine content of Example 2 was 2.4% (atomic percentage) ( Figure 8 ).

[0139] It can be seen from this that in the case of the negative electrode of the present invention, the fluorine-based polymer reacts with lithium to form lithium fluoride (LiF), and the greater the amount of the coated fluorine-based polymer, the greater the amount of lithium fluoride formed.

[0140] Experimental Example 4. Measurement of Lifespan Characteristics of Lithium-Sulfur Batteries

[0141] A sulfur-carbon (CNT) composite (S:C=75:25 (weight ratio)) as a positive electrode active material, a conductive material (VGCF), and a binder (Li-PAA) were mixed at a weight ratio of 87.5:5:7.5 and added to distilled water to prepare a positive electrode slurry.

[0142] The positive electrode slurry was applied to both surfaces of an aluminum current collector, dried at 80°C, and rolled using a roller press to prepare a positive electrode. At this time, the loading capacity was 3.5 mAh / cm 2 .

[0143] 1 M LiTFSI and 1 wt % LiNO 3 were dissolved in an organic solvent obtained by mixing 1,3-dioxolane (DOL) and dimethyl ether (DME) at a volume ratio of 1:1 to prepare an electrolyte.

[0144] A polyethylene porous film having a thickness of 16 μm and a porosity of 68% was used as a separator.

[0145] As the negative electrodes, the negative electrodes produced in Examples 1 and 2 and Comparative Examples 1 and 2 were used, respectively.

[0146] A separator was interposed between the positive electrode and the negative electrode, which were stacked to assemble into a pouch-type battery, and then injected with an electrolyte and sealed to prepare each lithium-sulfur battery.

[0147] Each lithium-sulfur battery was subjected to 0.2C charge and 0.3C discharge cycles (upper / lower limits were 2.5 / 1.8, respectively) in CC mode at a temperature of 25°C to measure the cycle life when 80% of the initial capacity was maintained. The results are shown in Table 1 below.

[0148] Table 1:

[0149] Cycle number (based on 80% capacity retention) Example 1 218 Example 2 139 Comparative Example 1 151 Comparative Example 2 178

[0150] The results in Table 1 show that the lithium-sulfur battery comprising the negative electrode of Example 1, i.e., the negative electrode of the present invention, exhibits the best lifespan characteristics. In the case of the negative electrode of Example 1, lithium metal is incorporated into the exterior and interior of the three-dimensional carbon structure coated with a fluorine-based polymer. Specifically, the three-dimensional carbon structure coated with a fluorine-based polymer of Example 1 was prepared using an aqueous fluorine-based polymer solution containing 10% by weight of the fluorine-based polymer relative to the total weight of the aqueous solution. Furthermore, the fluorine content of the three-dimensional carbon structure coated with a fluorine-based polymer of Example 1 was 19 parts by weight relative to 100 parts by weight of carbon.

[0151] That is, it was found that the fluorine-based polymer spontaneously reacts with lithium metal to form lithium fluoride at the interface between lithium metal and the three-dimensional carbon structure coated with the fluorine-based polymer. Lithium fluoride protects lithium metal due to its high ion conductivity and inhibits the growth of lithium dendrites, thereby improving the life characteristics of lithium-sulfur batteries.

[0152] The negative electrode of Comparative Example 1 is formed by laminating lithium metal on a current collector, and Comparative Example 2 is formed by introducing lithium metal into the outer surface and interior of a three-dimensional carbon structure. Each lithium-sulfur battery containing these does not suppress lithium dendrite growth and therefore shows poor life characteristics.

[0153] The negative electrode of Example 2 is formed by introducing lithium metal onto the outer surface and interior of a three-dimensional carbon structure coated with a fluorine-based polymer. Specifically, the three-dimensional carbon structure coated with a fluorine-based polymer of Example 2 is prepared using an aqueous fluorine-based polymer solution having a fluorine-based polymer content of 20% by weight relative to the total weight of the aqueous fluorine-based polymer solution, and the fluorine content of the three-dimensional carbon structure coated with the fluorine-based polymer of Example 2 is 33 parts by weight relative to 100 parts by weight of carbon. If the fluorine-based polymer content is greater than 20% by weight based on the total weight of the aqueous fluorine-based polymer solution, the lithium metal and the fluorine-based polymer will overreact, resulting in a large loss of metallic lithium. Therefore, the lifespan characteristics of the lithium-sulfur battery containing the negative electrode of Example 2 are not improved.

[0154] It was thus confirmed that if the three-dimensional carbon structure coated with a fluorine-based polymer is prepared using a fluorine-based polymer aqueous solution having a fluorine-based polymer content of 20 weight % or more relative to the total weight of the fluorine-based polymer aqueous solution, then the fluorine content in the three-dimensional carbon structure coated with the fluorine-based polymer exceeds 30 weight parts relative to 100 weight parts of carbon, and therefore the fluorine-based polymer and lithium metal over-react, resulting in increased lithium metal loss and failure to improve the life characteristics of the lithium-sulfur battery.

Claims

1. A negative electrode for a lithium secondary battery, comprising a three-dimensional carbon structure coated with a fluorine-based polymer; and lithium metal introduced into the outer surface and interior of the three-dimensional carbon structure coated with the fluorine-based polymer, wherein the three-dimensional carbon structure coated with the fluorine-based polymer contains 5 to 30 parts by weight of fluorine relative to 100 parts by weight of carbon, Lithium fluoride is formed by the reaction between the fluorine-based polymer and the lithium metal, and the lithium fluoride is formed at the interface between the three-dimensional carbon structure coated with the fluorine-based polymer and the lithium metal. 2 . The negative electrode for a lithium secondary battery according to claim 1 , wherein the three-dimensional carbon structure is a porous carbon structure. 3 . The negative electrode for a lithium secondary battery according to claim 2 , wherein the porous carbon structure comprises at least one selected from the group consisting of carbon paper, carbon felt, and carbon mat. 4 . The negative electrode for a lithium secondary battery according to claim 1 , wherein the three-dimensional carbon structure coated with the fluorine-based polymer is formed by coating the three-dimensional carbon structure with an aqueous solution containing a fluorine-based polymer. 5 . The negative electrode for a lithium secondary battery according to claim 4 , wherein the content of the fluorine-based polymer is 2% by weight or more and less than 20% by weight relative to the total weight of the aqueous solution containing the fluorine-based polymer. 6 . The negative electrode for a lithium secondary battery according to claim 1 , wherein the lithium metal introduced into the outer surface and the interior of the three-dimensional carbon structure coated with a fluorine-based polymer is introduced by laminating a lithium foil onto the three-dimensional carbon structure coated with a fluorine-based polymer.

7. The negative electrode for a lithium secondary battery according to claim 1, wherein the fluorine-based polymer comprises at least one selected from the group consisting of polytetrafluoroethylene, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, perfluoroalkoxyalkane, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, ethylene-tetrafluoroethylene copolymer, tetrafluoroethylene-chlorotrifluoroethylene copolymer and ethylene-chlorotrifluoroethylene copolymer. 8 . The negative electrode for a lithium secondary battery according to claim 1 , wherein the negative electrode for a lithium secondary battery is a negative electrode for a lithium-sulfur battery. 9 . A lithium secondary battery comprising a positive electrode; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte, wherein the negative electrode is the negative electrode according to claim 1 . 10 . The lithium secondary battery according to claim 9 , wherein the lithium secondary battery is a lithium-sulfur battery.

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