LITHIUM METAL ANODE, METHOD FOR MANUFACTURING SAME, AND LITHIUM SECONDARY BATTERY USING SAME

Abstract

The present invention relates to a lithium metal anode, a method for manufacturing same, and a lithium secondary battery using same. The lithium metal anode include a current collector having a flat structure and an anode active material layer including a lithium metal disposed on the current collector, wherein the lithium metal has a concave-convex structure and no dendrite is present on the surface of the lithium metal.

Claims

1. A lithium metal anode, comprising a current collector having a flat structure, and an anode active material layer including a lithium metal disposed on the current collector, and the lithium metal has a concavo-convex structure, wherein no dendrite is present on the surface of the lithium metal, provided that the dendrite refers to a particle in which a ratio of the longest length of the particle in one direction and a length in the direction perpendicular to the longest length of the particle is greater than or equal to 3.

2. The lithium metal anode of claim 1, wherein based on the planar projection area of the lithium metal, it has pores of 5% to 30% with respect to the entire area.

3. The lithium metal anode of claim 1, wherein the depth of the concavo-convex in the thickness direction is 20 to 100% based on the total thickness of the lithium metal.

4. The lithium metal anode of claim 3, wherein in the lithium metal, the depth of the at least one concavo-convex is 20 to 50% based on the total thickness of the lithium metal.

5. The lithium metal anode of claim 1, wherein in the lithium metal, an interval of the concavo-convex is 5 to 100 μm.

6. The lithium metal anode of claim 1, wherein the thickness of the lithium metal is 1 to 100 μm.

7. The lithium metal anode of claim 1, wherein a film is further included on the surface of the anode active material layer.

8. The lithium metal anode of claim 7, wherein the film is a Li—N—C—H—O-based ionic compound, a Li—P—C—H—O-based ionic compound, LiF, or a combination thereof.

9. A method of manufacturing lithium metal anode, comprising preparing a current collector and a lithium source opposed thereto, which are impregnated in an electrolyte solution; and applying a current to the current collector and the lithium source to electroplate lithium metal on the surface of the current collector, wherein in applying of the current to the current collector and the lithium source to electroplate lithium metal on the surface of the current collector, the current application is performed through two steps, and a current density at second current application is higher than a current density at first current application.

10. The method of claim 9, wherein the current density at the first current application is 0.2 to 0.8 mA/cm.sup.2.

11. The method of claim 9, wherein the current density at the second current application is 1 to 4 mA/cm.sup.2.

12. The method of claim 9, wherein the second current application time is longer than the first current application time.

13. The method of claim 12, wherein the first current application time is 1 to 4 hours.

14. The method of claim 12, wherein the second current application time is 1 to 24 hours.

15. A lithium secondary battery, comprising a cathode; an anode; and an electrolyte between the cathode and anode, and the anode is the anode of claim 1.

Description

DESCRIPTION OF THE DRAWINGS

[0061] FIG. 1 is a schematic view of an apparatus for a lithium metal anode according to an embodiment of the present invention.

[0062] FIG. 2 shows shapes of the microstructures of the surfaces of lithium metals controlled by Examples 1, 2, and 3.

[0063] FIG. 3 shows a surface and cross-sectional microstructure of a lithium metal having a thickness of 100 μm formed according to Example 4.

[0064] FIG. 4 is a photograph of a surface of a lithium metal formed according to Comparative Example 1.

[0065] FIG. 5 is a photograph of a surface of a lithium metal formed according to Comparative Example 2.

[0066] FIG. 6 is an image analysis photograph of the surface of Example 2 in order to analyze a porosity.

[0067] FIG. 7 shows a method of measuring the depths of the concavo-convex generated by the electroplating process, that is, distances in the thickness direction from the surface of electroplated lithium.

[0068] FIGS. 8 and 9 show a method of measuring the intervals of the concavo-convex generated by the electroplating process.

[0069] FIG. 10 shows a scatter plot and a box plot showing the measured values for the intervals of the concavo-convex.

[0070] FIG. 11 shows evaluation results of charge/discharge performances of lithium secondary battery cells to which a lithium metal anode manufactured according to Examples 1, 2, 3 and 4 and Comparative Examples 1 and 2 is applied.

MODE FOR INVENTION

[0071] Hereinafter, embodiments of the present invention are described in detail. However, these embodiments are exemplary, the present invention is not limited thereto and the present invention is defined by the scope of claims.

[0072] In forming a thin film lithium metal anode to realize a high energy density, it is formed to structurally have a three-dimensional concavo-convex structure, so that lithium may be charged in an empty space during the charge to minimize a volume change of the thin film lithium metal anode during the charge and discharge process.

[0073] Accordingly, since stress generation due to the volume change during the charge and discharge process is also suppressed, the lithium anode material may be eventually prevented from cracks due to the charge and discharge.

[0074] In addition, the three-dimensional concavo-convex structure is formed with a fine size within a few micrometers to maximize a surface area of the lithium anode and thus lower a current density actually applied during the charge and discharge, resultantly suppressing formation of the lithium dendrite at high currents.

[0075] Furthermore, a conventional three-dimensional structure was obtained by separately forming a concavo-convex in a Cu foil or Cu in the form of mesh or foam as a current collector, but the present invention uses the flat Cu foil as it is without a separate process of the current collector and thus may not only be economically competitive but also prevent the dendrite formation due to an empty space and thus achieve excellent charge and discharge performance of a battery.

[0076] During the electroplating, a three-dimensional lithium metal may be formed on one surface of the anode current collector, if necessary, and a three-dimensional lithium metal may be also formed on both surfaces of the anode current collector at the same time.

[0077] In order to control the thickness of the electroplated lithium, the size of the particles, and the porosity, the magnitude of the electroplating current, the application time, and the current application method may be adjusted to obtain a desired three-dimensional structure.

[0078] In addition, an electroplating electrolyte composition and additives may be adjusted to suppress the dendrite formation caused by applying high currents, when the lithium is formed in an electrochemical method. This microstructural control with respect to the lithium metal may make it possible to form the lithium metal with a pore structure but no dendrite, which is not obtained in the conventional method, on the flat current collector.

[0079] When this obtained lithium metal and current collector is used as an anode to implement a battery, the three-dimensional pore structure may secure a free space for accommodating the lithium growth during the charge and discharge and thus solve the short circuit problem with a cathode due to the dendrite growth. In addition, since the change in the volume of the anode material may be accommodated, even though the anode material, lithium metal, repeatedly contracts and expands during the charge and discharge, the resulting mechanical stress may be resolved and thus generation of cracks in the anode material may be prevented, thereby improving battery cycle-life.

[0080] Hereinafter, examples of the present invention and comparative examples are described. However, the following examples are only examples of the present invention, and the present invention is not limited to the following examples.

EXAMPLES AND COMPARATIVE EXAMPLES

(Example 1) Manufacture of Porous Lithium Metal Using Electroplating Process (Adjustment of Electroplating Profile)

[0081] A thin film of lithium metal was prepared using an electroplating process in the same manner as in FIG. 1.

[0082] A plating solution was prepared by adding 40% of LiFSI and 10% of lithium nitrate based on 100% weight of the plating solution and then, 19% of FEC, a fluorine-based compound, based on the 100% weight of the plating solution thereto. A 500 μm-thick lithium metal plate with purity of 99.9% as a lithium source was pressed onto a copper current collecting plate (a Cu plate), wherein the copper current collector was a 10 μm-thick Cu thin plate (a Cu foil) with both flat surfaces.

[0083] In a state that the lithium source and the current collector were electrically insulated in the plating solution, a lithium supply device was used to apply a current to the lithium source and the current collector as each (+) and (−) electrode and thus form a lithium metal layer on the surface of the current collector.

[0084] The method of applying the current was divided into two steps, wherein in the first step, the current was applied at a current density of 0.8 mA/cm.sup.2 for 1 hour, and in the second step, the current was applied at a current density of 4 mA/cm.sup.2 for 1 hour. An about 20 μm-thick lithium thin film was formed by the method as described above.

(Example 2) Porosity Control Through Adjustment of Electroplating Profile

[0085] A thin film of lithium metal was prepared in the same manner as in Example 1.

[0086] At this time, it was attempted to change the pore structure of the lithium metal and the resulting concavo-convex structure by adjusting the magnitude and time of the applied current.

[0087] The method of applying the current was divided into two steps, wherein in the first step, the current was applied with a current density of 0.4 mA/cm.sup.2 for 2 hours, and in the second step, the current was applied at a current density of 2 mA/cm.sup.2 for 1 hour. An about 20 μm-thick lithium thin film was formed by the method as described above.

(Example 3) Porosity Control Through Adjustment of Electroplating Profile

[0088] A thin film of lithium metal was prepared in the same manner as in Example 1.

[0089] At this time, it was attempted to change the pore structure of the lithium metal and the resulting concavo-convex structure by adjusting the magnitude and time of the applied current.

[0090] The method of applying the current was divided into two steps, wherein in the first step, the current was applied with a current density of 0.2 mA/cm.sup.2 for 4 hours, and in the second step, the current was applied at a current density of 1 mA/cm.sup.2 for 4 hours. An about 20 μm-thick lithium thin film was formed by the method as described above.

(Example 4) Thickness Control Through Adjustment of Electroplating Profile

[0091] A thin film of lithium metal was prepared in the same manner as in Example 1.

[0092] At this time, it was attempted to change pores, concavo-convex structure, and thickness of the manufactured lithium metal by adjusting the magnitude and time of the applied current.

[0093] The method of applying the current was divided into two steps, wherein in the first step, the current was applied with a current density of 0.2 mA/cm.sup.2 for 1 hour, and in the second step, the current was applied at a current density of 1 mA/cm.sup.2 for 24 hours. In the aforementioned method, an about 100 μm-thick lithium thin film was formed.

(Comparative Example 1) Porosity and Thickness Effect Through Adjustment of Electroplating Profile

[0094] A thin film of lithium metal was prepared in the same manner as in Example 1.

[0095] At this time, it was attempted to change pores, concavo-convex structure, and thickness of the manufactured lithium metal by adjusting the magnitude and time of the applied current.

[0096] The method of applying the current was performed at a current density of 2 mA/cm.sup.2 for 0.6 hours, forming an about 5 μm-thick lithium thin film.

(Comparative Example 2) Porosity Effect Through Adjustment of Electroplating Profile

[0097] A thin film of lithium metal was prepared in the same manner as in Example 1.

[0098] At this time, it was attempted to change the pore structure of the lithium metal and the resulting concavo-convex structure by adjusting the magnitude and time of the applied current.

[0099] The method of applying the current was performed at a current density of 0.2 mA/cm.sup.2 for 24 hours, forming an about 20 μm-thick lithium thin film.

[0100] Table 1 shows electroplating conditions of the examples and the comparative examples and the results thereof.

TABLE-US-00001 TABLE 1 First current Second current Lithium application application metal Current Time Current Time thickness (mA/cm.sup.2) (h) (mA/cm.sup.2) (h) (μm) Example 1 0.8 1 4 1 20 Example 2 0.4 2 2 2 20 Example 3 0.2 4 1 4 20 Example 4 0.2 1 1 24 100 Comparative 2.0 0.6 — — 5 Example 1 Comparative 0.2 24 — — 20 Example 2

EXPERIMENTAL EXAMPLES

[0101] FIG. 2 shows shapes of the microstructures of the surfaces of the lithium metals controlled in Examples 1, 2, and 3. In the lithium metal thin films having the same thickness of about 20 μm, the microstructures with different porosity and the resulting different concavo-convex shapes were formed.

[0102] FIG. 3 shows the surface and cross-sectional microstructures of the an 100 μm-thick lithium metal foil formed according to Example 4.

[0103] Even though the lithium metal became thicker, a three-dimensional porous structure was formed with no dendrite formation by adjusting the current and time factors of the electroplating process.

[0104] On the other hand, FIG. 4 shows Comparative Example 1, a case of having an extremely high pore fraction, since lithium metal was insufficiently formed by the electroplating and thus not formed on a large portion out of the entire area. In addition, at the edges of particles, lots of dendrites were formed.

[0105] FIG. 5 shows a structure in which lithium was completely electroplated over the entire area, so that no pores appear due to the microstructure, and thus no concavo-convex on the surface are also formed.

[0106] FIG. 6 is an image analysis photograph of the surface of Example 2 in order to analyze a porosity.

[0107] Specifically, in order to compare pores formed by the electroplating and the resulting concavo-convex states, each surface microstructure photograph shown in FIG. 6 was image-analyzed to measure a pore fraction in an entire area.

[0108] The values are shown in Table 2.

TABLE-US-00002 TABLE 2 Pore fraction (%) Example 1 21.3 Example 2 17.3 Example 3 6.9 Example 4 14.4 Comparative Example 1 45.8 Comparative Example 2 0.3 * For the examples and the comparative examples produced through the electroplating process, the pore fraction based on the planar projection area is shown.

[0109] FIG. 7 shows a method of measuring the depths of the concavo-convex generated by the electroplating process, that is, distances in the thickness direction from the surface of electroplated lithium. In a photograph showing the cross-section of the electroplated lithium, when a total thickness of the electroplated lithium was assumed as 100%, a flat surface state was marked as 0%, a state that a concave portion of the concavo-convex reached a half of the entire lithium was marked as 50%, and a state that the concave portion reached the copper current collector was marked as 100%.

[0110] Table 3 shows minimum values and maximum values among the concavo-convex depths excluding flat surface values with respect to the examples and the comparative examples obtained by the electroplating.

[0111] In Examples 1 to 4, minimum depths formed by pores were in a range of 20% to 50% of the total lithium thickness, and maximum depths of the concavo-convex were 100% where the concave portion of the concavo-convex reached the copper current collector.

[0112] In Comparative Example 1, a maximum depth of the concavo-convex was 100%, but since a portion where lithium metal was not formed was too large, a minimum depth of the concavo-convex was immeasurable in the other portion excluding the flat surface.

[0113] In Comparative Example 2, since a cross-section was very dense, the number of concavo-convex formed by the pores was not only very small, but also a maximum depth of the concavo-convex was about 10% or less.

TABLE-US-00003 TABLE 3 Depth of concavo-convex Lithium metal Minimum Maximum thickness (μm) (%) (%) Example 1 20 50 100 Example 2 20 40 100 Example 3 20 20 100 Example 4 100 30 100 Comparative Example 1 5 — 100 Comparative Example 2 20  0 10

[0114] FIGS. 8 and 9 show a method of measuring the intervals of the concavo-convex generated by the electroplating process. The intervals of the concavo-convex indicate lengths defined as d1, d2, d3, etc. in the schematic cross-sectional view of FIG. 8 and were obtained by measuring an interparticle distance between the pores in the surface microstructure photograph of FIG. 9.

[0115] Table 4 summarizes minimum, maximum, average, and median values of the intervals of the concavo-convex with respect to the examples and the comparative examples by the electroplating process. In addition, FIG. 10 shows a scatter plot and a box plot showing the measured values for the intervals of the concavo-convex.

[0116] In a conventional three-dimensional structure formed by applying a physical force, lithium metal with a thickness of hundreds of micrometers was transformed to have wide intervals of the concavo-convex with a width of about tens to hundreds of micrometers, but in Examples 1 to 4 according to the present invention, a lithium thin film with a thickness of 20 μm to 100 μm had fine intervals of concavo-convex with a width of about 10 μm to 100 μm. This well serves a fundamental goal of forming the three-dimensional structure, that is, securing a micro space and expanding a surface area.

[0117] When Examples 1, 2, and 3 having the same thickness are compared, a pore area gradually decreased from Example 1 to Example 3, but the intervals of the concavo-convex gradually increased. On the other hand, in Comparative Example 2, the intervals of the concavo-convex were immeasurable due to too high density.

TABLE-US-00004 TABLE 4 Interval of concavo-convex (μm) Minimum Maximum Average Median Example 1 7 60 32 31 Example 2 17 105 50 47 Example 3 12 119 74 73 Example 4 11 97 36 33 Comparative Example 1 16 75 33 33 Comparative Example 2 — — — —

[0118] FIG. 11 shows evaluation results of charge/discharge performances of lithium secondary battery cells to which a lithium metal anode manufactured according to Examples 1, 2, 3 and 4 and Comparative Examples 1 and 2 is applied.

[0119] In addition, Table 5 shows the results of evaluating charge and discharge cycle-life based on 80% of the initial discharge capacity. Referring to FIG. 11 and Table 5, in the case of the embodiment having pores and concavo-convex structures, charging and discharging performance is greatly improved. The reason is that the structure formed by the present invention had an effect of suppressing formation of dendritic phases of lithium during the charge and discharge process and also, generation of lithium electrode cracks due to the charge and discharge stress.

[0120] On the other hand, Example 4 exhibited very excellent charge and discharge performance, because the lithium metal thickness of Example 4 was 100 μm, which was thicker than 20 μm of the lithium metal thickness of Examples 1, 2, and 3, bringing about an excessive lithium supply effect in addition to the effect of the present invention.

TABLE-US-00005 TABLE 5 Charge and discharge cycle-life (times) Example 1 42 Example 2 41 Example 3 38 Example 4 88 Comparative Example 1 7 Comparative Example 2 26

[0121] When a lithium metal having a porous three-dimensional concavo-convex structure according to the present invention was used as an anode to configure a lithium secondary battery cell, lithium was charged in an empty space during the charge process and thus minimized a volume change during the charge and discharge process and suppressed stress generation due to the volume change, resultantly preventing generation of cracks of the lithium anode during the charge and discharge.

[0122] In addition, the three-dimensional structure expanded a surface area of the lithium anode and reduced actual current density applied thereto during the charge and discharge and thus had the effect of suppressing lithium dendrite during the charge at a high current.

[0123] In particular, the present invention may, unlike a conventional method of using a separate preliminary process for forming the three-dimensional structure, used a flat current collector and thus achieve an excellent economical effect due to the process simplification.

[0124] The present invention may be embodied in many different forms, and should not be construed as being limited to the disclosed embodiments. In addition, it will be understood by those skilled in the art that various changes in form and details may be made thereto without departing from the technical spirit and essential features of the present invention. Therefore, the aforementioned embodiments should be understood to be exemplary but not limiting the present invention in any way.