Lithium metal patterning and electrochemical device using the same

Abstract

A lithium metal is physically pressed to a silicon wafer having a uniform intaglio or embossed pattern formed thereon in advance, or liquid lithium is applied to the silicon wafer and may then be cooled in order to form a uniform pattern on the surface of the lithium metal.

Claims

1. A method of patterning a surface of a lithium metal, the method comprising: forming a uniform intaglio or embossed pattern having a size of 0.001 to 900 ?m on a silicon wafer, wherein the size is a length or a width; physically pressing a lithium metal to the silicon wafer having the uniform intaglio or embossed pattern formed thereon or applying liquid lithium to the silicon wafer and cooling the liquid lithium in order to form a uniform pattern on a surface of the lithium metal; and separating the lithium metal having the uniform intaglio or embossed pattern formed thereon from the silicon wafer.

2. The method according to claim 1, wherein a horizontal shape of the uniform intaglio or embossed pattern is polygonal, circular, or oval.

3. The method according to claim 1, wherein a vertical sectional shape of the uniform intaglio or embossed pattern is polygonal, circular, oval, or slit-shaped.

4. The method according to claim 1, wherein a height or depth of the uniform intaglio or embossed pattern is 1/100 to 10 times a size of the uniform intaglio or embossed pattern, wherein the size is a length or a width.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a view showing an example of a conventional method of treating the surface of a lithium metal using micro needles (a) and illustrating a cause by which non-uniform pressure is generated in the case in which the micro needles are used (b);

(2) FIG. 2 is an electron microscope photograph showing a patterned lithium metal according to the present invention;

(3) FIG. 3 is another electron microscope photograph showing a patterned lithium metal according to the present invention;

(4) FIG. 4 is a graph showing a charging capacity and a discharging capacity measured during charging and discharging processes according to an Example and a Comparative Example; and

(5) FIG. 5 is a graph showing charging and discharging efficiencies during the charging and discharging processes according to the Example and the Comparative Example.

BEST MODE

(6) In the present application, it should be understood that the terms comprises, has, or includes, etc. specify the presence of features, integers, steps, operations, components, parts, or combinations thereof described in the specification, but do not preclude the presence or addition of one or more other features, integers, steps, operations, components, parts, or combinations thereof.

(7) It should be understood that when a component is referred to as being connected to or coupled to another component, it may be directly connected to or coupled to another component, or intervening components may be present therebetween. In contrast, it should be understood that when a component is referred to as being directly connected to or directly coupled to another component, there are no intervening components present. Other terms that describe the relationship between components, such as between and directly between or adjacent to and directly adjacent to, are to be interpreted in the same manner.

(8) In addition, all terms including technical or scientific terms have the same meanings as those generally understood by a person having ordinary skill in the art to which the present invention pertains, unless defined otherwise. Generally used terms, such as terms defined in a dictionary, should be interpreted as coinciding with the meanings of the related art from the context. Unless obviously defined in the present application, such terms are not to be interpreted as having ideal or excessively formal meanings.

(9) The present invention provides a method of patterning the surface of a lithium metal including the following steps.

(10) 1) a step of forming a uniform intaglio or embossed pattern having a size of 0.001 to 900 ?M on a silicon wafer, 2) physically pressing a lithium metal to the silicon wafer having the uniform intaglio or embossed pattern formed thereon or applying liquid lithium to the silicon wafer and cooling the liquid lithium in order to form a uniform intaglio or embossed pattern on the surface of the lithium metal, and 3) separating the lithium metal having the uniform intaglio or embossed pattern formed thereon from the silicon wafer.

(11) Step 1), i.e. the step of forming the uniform intaglio or embossed pattern on the silicon wafer, may be achieved by etching in a conventional semiconductor process, and therefore a detailed description thereof will be omitted.

(12) At step 2), a lithium metal foil is placed on the silicon wafer having the intaglio or embossed pattern formed thereon, and uniform physical pressure is applied to the lithium metal foil using a press having a large area. At this time, the physical pressure may be changed depending on the thickness of the lithium metal foil and the height of the intaglio or embossed pattern of the silicon wafer. After the physical pressure is removed, the lithium metal foil is separated from the silicon wafer, whereby a patterned lithium metal is finally obtained.

(13) Various patterns may be formed on the silicon wafer. A pattern having a larger area may be formed than in the case of using conventional roller-shaped micro needles. The most basic pattern may include quadrangles or hexagons (honeycomb shapes) that are repeatedly arranged. In addition, polygons, such as triangles, circles or ovals, or various geometrical lattice patterns may be formed.

(14) In general, the vertical surface of the pattern may be rectangular. In addition, a polygonal shape, a circular shape, an oval shape, or a slit shape may be used. The height or depth of the pattern may be 1/100 to 10 times, preferably 1/50 to 1/10 times, the size of the pattern.

(15) It is possible to manufacture a secondary battery using the patterned lithium metal according to the present invention as a negative electrode. Here, the materials that are generally used for a lithium secondary battery may be used as a current collector, a positive electrode, a separator, and an electrolytic solution, which correspond to the negative electrode.

Example

(16) Hereinafter, the present invention will be described in detail with reference to the following Example and Experimental Examples; however, the present invention is not limited by the Example and the Experimental Examples. The Example may be modified into various other forms, and the scope of the present invention should not be interpreted as being limited by the Example, which will be described in detail. The Example is provided in order to more completely explain the prevent invention to a person who has average knowledge in the art to which the present invention pertains.

(17) <Lithium Metal Patterning>

(18) A lithium metal (having a length of 150 ?m and a width of 150 ?m) was placed on a micro-patterned silicon wafer having a length of 100 ?m, a width of 100 ?m, and a height of 32 ?m, and then uniform pressure was applied to the lithium metal. Subsequently, the pressed lithium metal was separated from the silicon wafer. In this way, a micro-patterned lithium metal foil having a length of 100 ?m, a width of 100 ?m, and a height of 32 ?m was manufactured.

(19) <Manufacture of a Lithium Secondary Battery>

(20) 96 weight % of LiCoO.sub.2 as a positive electrode active material, 2 weight % of Denka black (a conductive agent), and 2 weight % of polyvinylidene fluoride (PVDF) (a binder) were added to N-methyl-2-pyrrolidone (NMP) in order to manufacture a positive electrode material slurry. The manufactured positive electrode material slurry was coated on one surface of an aluminum current collector such that the positive electrode material slurry had a thickness of 65 ?m. The positive electrode material slurry was dried, and the aluminum current collector was rolled. Subsequently, the aluminum current collector was punched so as to have a predetermined size, whereby a positive electrode was manufactured.

(21) The patterned lithium metal foil, manufactured as described above, was used as a counter electrode. A polyolefin-based separator was interposed between the positive electrode and the counter electrode, and an electrolytic solution, in which 1M LiPF6 was dissolved in a solvent obtained by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 50:50, was injected into the electrode assembly in order to manufacture a coin-type half battery.

(22) <Charging and Discharging>

(23) The coin-type half battery, manufactured as described above, was charged and discharged using an electrochemical charging and discharging device. Charging was performed until the voltage of the coin-type half battery became 4.4 V vs. Li/Li.sup.+, and discharging was performed until the voltage of the coin-type half battery became 3.0 V vs. Li/Li.sup.+. At this time, the current density was 0.5 C-rate.

Comparative Example

(24) A coin-type half battery was manufactured in the same manner as in the Example except that a non-patterned lithium metal foil was used as a counter electrode, in place of the patterned lithium metal foil according to the Example, and the coin-type half battery was charged and discharged under the same conditions as in charging and discharging according to the Example.

Experimental Example 1: Observation of the Shape of the Surface of a Lithium Metal

(25) An electron microscope (SEM) photograph of the patterned lithium metal manufactured according to the Example is shown in FIG. 1. It can be seen that a micro pattern having a distance between respective lattices of 100 ?m is uniformly formed.

Experimental Example 2: Comparison in Electrochemical Charging and Discharging Performance

(26) A charging capacity and a discharging capacity were measured during charging and discharging processes according to the Example and the Comparative Example. The results are shown in FIG. 2. The charging and discharging efficiencies at that time are shown in FIG. 3.

(27) Referring to FIG. 2, the cycle performance of the Example and the cycle performance of the Comparative Example are similar to each other in the early cycles; however, the cycle performance of the Example and the cycle performance of the Comparative Example are very different from each other after 50 cycles. It can be seen that the Example exhibits better performance than the Comparative Example and that the difference in the performance between the Example and the Comparative Example becomes greater as the number of cycles is increased.

(28) Referring to FIG. 3, it can be seen that the Example exhibits higher charging and discharging efficiency than the Comparative Example even in the early cycles.

(29) As described above, it can be seen that, in the case in which the patterned lithium metal is used, it is possible to remarkably improve charging and discharging capacity and efficiency of the battery.

(30) The patterned lithium metal has a wider surface area than an untreated lithium metal. Since the current density of the Example is lower than the current density of the Comparative Example even when charging and discharging are performed at the same current density, more stable charging and discharging can be performed in the Example than in the Comparative Example.

(31) A lithium metal secondary battery is charged and discharged as lithium is deposited to and is separated from the surface of a lithium metal. If the lithium metal secondary battery is charged and discharged at high current density, the lithium metal grows abnormally. Separation of lithium from such a portion of the lithium metal reduces the reversibility of a cell, and seriously affects the safety of the cell.

(32) The present invention has an effect in that the patterned lithium metal has an increased surface area, whereby the current density of the lithium metal is reduced and thus it is possible to restrain abnormal growth of the lithium metal. Due to these characteristics, the overall performance of the battery is improved, which becomes increasingly notable as the number of cycles is increased.

(33) The present invention has other effects in that the patterned lithium metal can be manufactured so as to have a wider area and to be more uniform than by conventional patterning using micro needles, in that a difference in pressure due to rolling does not occur, and in that large-area patterning is possible, whereby the present invention is applicable to an actual commercial process.

INDUSTRIAL APPLICABILITY

(34) As is apparent from the above description, according to the present invention, a lithium metal may be physically pressed to a silicon wafer having a uniform intaglio or embossed pattern formed thereon in advance, or liquid lithium may be applied to the silicon wafer and may then be cooled in order to form a uniform pattern on the surface of the lithium metal, whereby it is possible to manufacture a large number of patterned lithium metal foils at once. In addition, it is possible to perform patterning so as to have various shapes or to have a large area, and it is possible to minimize the formation of a non-uniform pattern due to a difference in pressing pressure. The patterned lithium metal exhibits higher reversibility than a conventional lithium metal, whereby the lifespan of a battery may be greatly increased.