Title of invention: negative electrode for lithium secondary battery, method for manufacturing the same, and lithium secondary battery including the same Technical field [One] This application claims the benefit of priority based on Korean Patent Application No. 10-2019-0026807 filed March 08, 2019 and Korean Patent Application No. 10-2020-0027055 filed March 04, 2020. All contents disclosed in the literature are included as part of this specification. [2] The present invention relates to a negative electrode for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the same. Background [3] Until recently, there has been considerable interest in developing a high energy density battery using lithium as a negative electrode. Compared to other electrochemical systems with nickel or cadmium electrodes, lithium intercalated carbon negative electrodes, for example, reducing the energy density of a cell by increasing the weight and volume of the negative electrode in the presence of a non-electroactive material, lithium metal Since silver has low weight and high capacity characteristics, it is attracting very interest as a negative active material for an electrochemical cell. A lithium metal negative electrode, or a negative electrode mainly containing lithium metal, provides an opportunity to construct a battery having a lighter weight and high energy density than a battery such as a lithium-ion, nickel metal hydride or nickel-cadmium battery. These features are highly desirable for batteries for portable electronic devices such as cell phones and laptop computers, where the premium is paid with a low weight. [4] A conventional lithium ion battery has an energy density of 700 wh/l by using graphite for the negative electrode and LCO (Lithium Cobalt Oxide) for the positive electrode. However, as fields requiring a high energy density have recently been expanded, the necessity to increase the energy density of a lithium ion battery has been continuously raised. For example, an increase in energy density is necessary to increase the mileage of an electric vehicle to 500 km or more during a single charge. [5] In order to increase the energy density of lithium ion batteries, the use of lithium electrodes is increasing. However, lithium metal has a high reactivity and is difficult to handle and has a problem that is difficult to handle in a process. [6] When lithium metal is used as a negative electrode of a lithium secondary battery, the lithium metal reacts with an electrolyte, impurities such as water or organic solvents, lithium salts, etc. to form a passivation layer (SEI). Such a passivation layer causes a difference in current density in the local phase to promote the formation of dendritic dendrite by lithium metal during charging, and gradually grows during charging and discharging, causing an internal short circuit between the positive electrode and the negative electrode. In addition, dendrite has a mechanically weak part (bottle neck) and forms inert lithium that loses electrical contact with the current collector during discharging, thereby reducing the capacity of the battery, shortening the cycle life, and the stability of the battery. Adversely affects [7] In order to improve the problems of the lithium metal negative electrode, a lithium metal negative electrode having a protective layer having various compositions or shapes has been developed (Chinese Patent Publication No. 107863488, Korean Patent Publication No. 2018-0012541). [8] However, research results on a protective layer of a lithium metal negative electrode that can improve overall battery performance in a lithium secondary battery are insufficient. [9] Therefore, in a battery using lithium metal as a negative electrode, it is urgent to develop a lithium metal negative electrode that exhibits uniform electrical conductivity on the electrode surface to suppress the growth of lithium dendrites and prevent the occurrence of dead Li in order to improve battery performance. Actually. [10] [Prior technical literature] [11] [Patent Literature] [12] (Patent Document 1) Chinese Published Patent No. 107863488 [13] (Patent Document 2) Korean Patent Publication No. 2018-0012541 Detailed description of the invention Technical challenge [14] As a result of conducting various studies to solve the above problems, the present inventors fabricated a negative electrode by transferring a protective layer including a three-dimensional structure including a void therein to the surface of a lithium metal layer. It was confirmed that the 3D structure includes a metal layer and a lithium nitride layer formed on the surface of the metal layer, so that uniform ionic conductivity and electrical conductivity are induced on the surface of the lithium metal layer by the lithium nitride layer. [15] Accordingly, an object of the present invention is to provide a negative electrode for a lithium secondary battery that exhibits uniform ionic conductivity and electrical conductivity on the surface of a lithium metal layer. [16] Another object of the present invention is to provide a method of manufacturing the negative electrode for a lithium secondary battery. [17] Another object of the present invention is to provide a lithium secondary battery including the negative electrode. Means of solving the task [18] In order to achieve the above object, the present invention, a lithium metal layer; And a protective layer formed on at least one surface of the lithium metal layer, wherein the protective layer includes a three-dimensional structure, wherein the three-dimensional structure includes a metal and lithium nitride, for a lithium secondary battery Provides a cathode. [19] The metal may include one or more lithium-friendly metals selected from the group consisting of Cu, Si, Ge, Zn, and Ti. [20] The thickness of the protective layer may be 1 to 30 μm. [21] The 3D structure may include 50 to 99% by weight of metal and 1 to 50% by weight of lithium nitride. [22] The thickness of the lithium metal layer may be 1 to 700 μm. [23] [24] The present invention also includes the steps of (S1) immersing a metal in an etching solution to form a metal hydroxide having a three-dimensional structure; (S2) forming a metal nitride having a three-dimensional structure by nitration reaction of the metal hydroxide having a three-dimensional structure; And (S3) transferring the metal nitride of the three-dimensional structure onto a lithium metal layer to form a protective layer including a three-dimensional structure including a metal and lithium nitride. to provide. [25] In the step (S1), the etching solution may include at least one alkaline selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, lithium hydroxide, and ammonia. [26] The etching solution may further include at least one persulfate selected from the group consisting of ammonium persulfate, sodium persulfate, and potassium persulfate. [27] In the step (S2), the nitridation reaction may be performed by reacting a nitrogen source gas with the metal hydroxide of the three-dimensional structure in an inert atmosphere. [28] The nitrogen source gas may include at least one selected from the group consisting of ammonia (NH 3 ), nitrogen (N 2 ), and nitrous oxide (N 2 O). [29] In the step (S3), the metal nitride of the three-dimensional structure may be brought into contact with the lithium metal layer and then transferred by pressing. [30] [31] The present invention also provides a lithium secondary battery including a negative electrode. Effects of the Invention [32] According to the present invention, it is possible to manufacture a negative electrode for a lithium secondary battery containing lithium nitride that does not exist naturally and can be produced only through chemical synthesis. Specifically, lithium nitride may be spontaneously formed on the surface of the lithium metal by forming a metal nitride having a three-dimensional structure through the nitridation reaction of the metal hydroxide and then transferring it to the lithium metal layer. A negative electrode for a lithium secondary battery may be prepared including the lithium metal layer and a protective layer on which a three-dimensional structure made of the metal and lithium nitride is formed. Since lithium nitride has excellent lithium ion conductivity, due to the protective layer including lithium nitride formed on the surface of the lithium metal layer, the surface of the lithium metal layer can exhibit uniform ionic conductivity and electrical conductivity, and the lithium metal layer and the electrolyte solution It can suppress side reactions with. [33] In addition, in the negative electrode for a lithium secondary battery according to the present invention, since the lithium nitride forms a three-dimensional structure in the protective layer together with the metal, it prevents the growth of lithium dendrites, and lithium ions are smooth and dense on the surface of the lithium metal layer. It can be deposited into a structure. Accordingly, it is possible to improve the life and safety of the lithium secondary battery. For example, due to the protective layer including the three-dimensional structure made of the metal and lithium nitride, the resistance of the surface of the lithium metal can be reduced, and side reactions can be minimized even after continuous charging and discharging, thereby improving the stability of the interface. [34] In addition, due to the three-dimensional structure included in the protective layer, it is possible to solve the phenomenon of a locally high current on the lithium metal layer. Brief description of the drawing [35] 1 is a schematic diagram showing a longitudinal section of a negative electrode for a lithium secondary battery according to an embodiment of the present invention. [36] 2 is a process chart of manufacturing a negative electrode for a lithium secondary battery of Example 1. [37] 3 is an X-ray photoelectron spectroscopy (XPS) graph for the cathodes of Example 1 and Comparative Example 1. FIG. [38] 4 is a SEM (Scanning Electron Microscopy) photograph of the negative electrode surfaces prepared in Example 1, Comparative Example 1, and Comparative Example 4, respectively. [39] 5A and 5B are photographs showing the form of lithium deposition at the negative electrode when driving the lithium secondary battery including the negative electrode of Example 1 and Comparative Example 1, respectively, showing SEM photographs of the surface and longitudinal section of the negative electrode. [40] 6A, 6B, and 6C are graphs showing measurement results of life characteristics for lithium secondary batteries prepared in Example 1, Comparative Example 1, and Comparative Example 4, respectively. [41] 7A and 7B are graphs showing the results of a performance measurement experiment for a lithium secondary battery including a negative electrode prepared in Example 1 and Comparative Example 1 (FIG. 7a: a lithium secondary battery including an LCO electrode, FIG. 7b: including an LTO electrode) Lithium secondary battery). Best mode for carrying out the invention [42] Hereinafter, the present invention will be described in more detail to aid understanding of the present invention. [43] The terms or words used in the specification and claims should not be construed as being limited to their usual or dictionary meanings, and the inventor may appropriately define the concept of terms in order to describe his own invention in the best way It should be interpreted as a meaning and concept consistent with the technical idea of ​​the present invention based on the principle that there is. [44] The term "three-dimensional structure" as used herein refers to a structure including a void therein, and the cavity is a concept that broadly includes the shape of an empty space such as pores and passages. [45] In the present invention, the "three-dimensional structure" is a framework including a void therein, and means a structure in which the frame is made of metal and lithium nitride. [46] [47] Anode for lithium secondary battery [48] The present invention, a lithium metal layer; And a protective layer formed on at least one surface of the lithium metal layer, wherein the protective layer includes a three-dimensional structure, and the three-dimensional structure includes a metal and lithium nitride (Li 3 N). It relates to a negative electrode for a lithium secondary battery. [49] [50] 1 is a schematic diagram showing a longitudinal section of a negative electrode for a lithium secondary battery according to an embodiment of the present invention. [51] Referring to FIG. 1, the negative electrode 1 for a lithium secondary battery includes a lithium metal layer 10; And a protective layer 20 formed on at least one surface of the lithium metal layer 10. In addition, the lithium metal layer 10 may be formed on at least one surface of a current collector (not shown). [52] [53] In the negative electrode for a lithium secondary battery according to the present invention, the three-dimensional structure formed on the protective layer may include metal and lithium nitride, and specifically, 50 to 99% by weight of the metal and 1 to 50% by weight of lithium nitride It can be. [54] [55] In the present invention, the metal may serve to maintain the shape of the three-dimensional structure while exhibiting electrical conductivity. [56] The metal may include at least one lithium-friendly metal selected from the group consisting of copper, Cu, Si, Ge, Zn, and Ti, and preferably includes Cu. When a lithium-friendly metal is used as the metal, it may be advantageous to form a three-dimensional structure including lithium nitride, the resistance of the lithium metal surface may be reduced, and side reactions may be minimized even after continuous charging and discharging, thereby improving the stability of the interface. . [57] [58] The metal may be included in an amount of 50% to 99% by weight based on the total weight of the 3D structure. Specifically, the content of the metal may be 50% by weight or more, 70% by weight or more, 90% by weight or more, and 99% by weight or less and 98% by weight or less based on the total weight of the 3D structure. If the content of the metal is less than 50% by weight, the durability of the 3D structure may decrease, and the electrical conductivity at the surface of the lithium metal layer may decrease. If it is more than 99% by weight, the content of lithium nitride contained in the 3D structure is Since it is relatively reduced, the lithium ion conductivity may decrease. [59] [60] In the present invention, the lithium nitride is suitable as a protective layer material for protecting lithium metal due to its high lithium ion conductivity. When lithium nitride is used as the material for the protective layer of the lithium metal, the electrical conductivity at the interface between the lithium metal and the electrolyte solution may be lowered and ionic conductivity may be increased. [61] The lithium nitride may be included in an amount of 1% to 50% by weight based on the total weight of the 3D structure. Specifically, the content of the lithium nitride may be 1% by weight or more, 2% by weight or more, 50% by weight or less, 30% by weight or less, and 10% by weight or less based on the total weight of the 3D structure. If the content of the lithium nitride is less than 1% by weight, the lithium ion conductivity in the negative electrode may decrease, and if it exceeds 50% by weight, the content of the metal contained in the 3D structure is relatively reduced. Conductivity may decrease. [62] [63] In the present invention, the protective layer may include the three-dimensional structure. [64] The thickness of the protective layer may be 1 μm to 30 μm. Specifically, the thickness of the protective layer may be 1 µm or more, 2 µm or more, 30 µm or less, 10 µm or less, and 5 µm or less. If the thickness of the protective layer is less than 1 µm, the ability to protect the lithium metal layer from moisture and outside air may be degraded, and if it is more than 30 µm, the protective layer itself acts as a resistance and battery performance may be degraded. [65] [66] In the present invention, the thickness of the lithium metal layer may be 1 μm to 700 μm. Specifically, the thickness of the lithium metal layer may be 1 µm or more, 5 µm or more, 50 µm or more, 100 µm or more, and 700 µm or less, 600 µm or less, and 550 µm or less. If the thickness of the lithium metal layer is less than 1 μm, the battery capacity may be lowered, and if it exceeds 700 μm, the lithium dendrite growth inhibiting effect may be insignificant. [67] [68] In the negative electrode for a lithium secondary battery according to the present invention, the current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery, for example, copper, stainless steel, aluminum, nickel, titanium, and calcined carbon. It may be selected from. In addition, the copper or stainless steel may be surface-treated with carbon, nickel, titanium, silver, or the like. In addition, the current collector may be an aluminum-cadmium alloy or the like. In addition, the current collector may be in various forms such as a film, sheet, foil, net, porous body, foam, and nonwoven fabric having fine irregularities formed on the surface. [69] [70] Method of manufacturing negative electrode for lithium secondary battery [71] The present invention also includes the steps of (S1) immersing a metal in an etching solution to form a metal hydroxide having a three-dimensional structure; (S2) forming a metal nitride having a three-dimensional structure by nitration reaction of the metal hydroxide having a three-dimensional structure; And (S3) transferring the metal nitride of the three-dimensional structure onto a lithium metal layer to form a protective layer including a three-dimensional structure including metal and lithium nitride; About. [72] [73] In the present invention, in the step (S1), a metal hydroxide having a three-dimensional structure may be formed by immersing the metal in the etching solution. [74] The metal may include a lithium-friendly metal selected from the group consisting of Cu, Si, Ge, Zn, and Ti, but is not limited thereto. For example, the metal reacts with the etching solution for metal capable of etching the metal to form a metal hydroxide having a three-dimensional structure, and may form a metal nitride by a nitridation reaction. Any metal having a property capable of forming lithium nitride by reaction of the metal nitride and lithium is not limited thereto. Preferably, the metal may be Cu. [75] [76] In addition, the metal etching solution may grow into a three-dimensional structure while etching a material containing the metal as well as the metal to form a metal hydroxide having a three-dimensional structure. For example, the metal may be etched by the etching solution to grow in the form of nanowires (NW) or nanorods (NR) to form a metal hydroxide having a three-dimensional structure. [77] The metal etching solution may include alkaline, or may be a solution containing alkaline and persulfate. Preferably, the metal etching solution may include alkaline and persulfate, and in this case, it may be advantageous in that the time for forming the metal hydroxide is shortened. [78] In addition, the concentration of the metal etching solution may be 1 M to 10 M. Specifically, the concentration of the metal etching solution may be 1 M or more, 1.5 M or more, 2 M or more, and 10 M or less, 8 M or less, and 5 M or less. If the concentration of the etching solution for metal is less than 1 M, it may take a long time to synthesize a metal hydrate of a three-dimensional structure, and if it is more than 10 M, it may be difficult to synthesize a metal hydrate of a three-dimensional structure. [79] In addition, the metal etching solution may preferably be an aqueous solution using water as a solvent. [80] The alkaline may be one or more selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, lithium hydroxide, and ammonia, and preferably sodium hydroxide. The persulfate may be at least one selected from the group consisting of ammonium persulfate (APS), sodium persulfate, and potassium persulfate, preferably ammonium persulfate. [81] [82] In the present invention, in the step (S2), the metal hydroxide having a three-dimensional structure may be subjected to a nitration reaction to form a metal nitride having a three-dimensional structure. [83] The nitriding reaction may be performed by reacting a nitrogen source gas with the metal hydroxide of the three-dimensional structure in an inert atmosphere. [84] The inert atmosphere may be formed by at least one inert gas selected from the group consisting of nitrogen, argon, helium, neon, and xenon, and preferably, the inert gas may be nitrogen or argon. [85] The nitrogen source gas may be at least one selected from the group consisting of ammonia (NH 3 ), nitrogen (N 2 ) and nitrous oxide (N 2 O), and preferably ammonia. [86] When the nitrogen source gas is supplied to the metal hydroxide having a three-dimensional structure, a nitriding reaction proceeds to form a metal nitride having a three-dimensional structure. [87] [88] In the present invention, in the step (S3), the metal nitride of the three-dimensional structure may be transferred onto the lithium metal layer to form a protective layer including a three-dimensional structure including a metal and lithium nitride. [89] [90] In the present invention, in the step (S3), the metal nitride layer having the three-dimensional structure may be transferred onto the lithium metal layer. [91] The transfer may be performed by bringing the metal nitride layer of the three-dimensional structure into contact with the lithium metal layer, and then applying mechanical energy by pressing. [92] During the transfer, a portion of the lithium metal included in the lithium metal layer and the metal nitride of the three-dimensional structure may react to form a three-dimensional structure including a metal and lithium nitride on the lithium metal layer. [93] In other words, a protective layer is formed on the lithium metal layer, and the protective layer may include a three-dimensional structure including the metal and lithium nitride. [94] [95] Lithium secondary battery [96] The present invention also relates to a lithium secondary battery comprising the negative electrode as described above. [97] The lithium secondary battery according to the present invention may include a positive electrode, a negative electrode, a separator and an electrolyte interposed therebetween. [98] In the lithium secondary battery according to the present invention, the negative electrode is as described above. [99] In the lithium secondary battery according to the present invention, the positive electrode may include a positive electrode current collector and a positive electrode active material layer having a positive electrode active material formed on the positive electrode current collector. In addition, the positive active material layer may further include at least one of a conductive material and a binder. [100] The positive electrode active material may preferably be a lithium-containing transition metal oxide, for example, LiCoO 2 , LiNiO 2 , LiMnO 2 , LiMn 2 O 4 , Li(Ni a Co b Mn c )O 2 (0