Lithium electrode and lithium secondary battery including same

Abstract

A lithium electrode and a lithium secondary battery including the same. The lithium electrode has a surface oxide layer with a controlled thickness and surface roughness. The lithium electrode may be used as a negative electrode of a lithium secondary battery, for example, a lithium-sulfur secondary battery. A lithium-sulfur battery including the lithium electrode has an enhanced lifetime due to suppression of side reactions with polysulfide.

Claims

1. A lithium electrode, comprising: a current collector; a lithium metal layer; and an oxide layer, wherein the oxide layer is a native layer, and wherein the oxide layer is present on a surface of the lithium metal layer; wherein a surface of the oxide layer has surface properties measured by laser confocal microscopy and defined by the following Sa (arithmetic mean height of surface), Sz (maximum height roughness of surface), Sp (roughness by a number of peaks) and Sdr (degree of interfacial increase): (i) Sa≥1 μm; (ii) Sz≥14 μm; (iii) Sp≥1000 mm.sup.−1; and (iv) Sdr≥0.5, wherein, the Sa is an arithmetic mean height of the surface of the oxide layer, wherein the Sz is a maximum height roughness of the surface of the oxide layer, which is a distance between a highest point and a lowest point on the surface of the oxide layer, wherein the Sp is the number of peaks, and wherein the Sdr is a degree of an interfacial increase, wherein the oxide layer consists of a first oxide layer, which consists of Li.sub.2O; a second oxide layer, which consists of Li.sub.2O and LiOH; and a third oxide layer, which consists of Li.sub.2O, LiOH and Li.sub.2CO.sub.3, wherein the first oxide layer has a thickness of 10 nm to 50 nm, the second oxide layer has a thickness of 1 nm to 10 nm, and the third oxide layer has a thickness of 1 nm to 5 nm, and wherein the oxide layer is formed under vacuum by flowing Ar/CO.sub.2 mixture gas over the lithium metal layer, and the oxide layer is rolled and brushed.

2. The lithium electrode of claim 1, wherein the oxide layer has surface properties defined by 1 μm≤Sa≤2 μm, 15 μm≤Sz≤20 μm, 1000 mm.sup.−1≤Sp<1500 mm.sup.−1 and 0.5≤Sdr≤1.0.

3. The lithium electrode of claim 1, wherein the oxide layer has a thickness of 50 nm or less.

4. The lithium electrode of claim 3, wherein the oxide layer has a thickness of 10 nm to 50 nm.

5. A lithium secondary battery comprising the lithium electrode of claim 1.

6. The lithium secondary battery of claim 5, which is a lithium-sulfur secondary battery.

7. The lithium secondary battery of claim 6, wherein the lithium-sulfur secondary battery comprises the lithium electrode as a negative electrode, and comprises a positive electrode, which comprises a mixture of sulfur and polyacrylonitrile (S-PAN).

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a sectional diagram of a lithium electrode including a surface oxide layer according to the present invention.

(2) FIG. 2 is a sectional diagram of a lithium electrode prepared according to Example 1 of the present invention having surface properties controlled.

(3) FIG. 3 is a sectional diagram of Comparative Example 2 of the present invention having surface properties not controlled.

(4) FIG. 4 is a graph measuring a cycle lifetime when using lithium electrodes of Example 1, and Comparative Examples 1 and 2 as a negative electrode of a lithium-sulfur secondary battery.

BEST MODE

(5) Hereinafter, the present invention will be described in more detail in order to illuminate the present invention.

(6) Terms or words used in the present specification and the claims are not to be interpreted limitedly to common or dictionary meanings, and shall be interpreted as meanings and concepts corresponding to technological ideas of the present disclosure based on a principle in which the inventors may suitably define the concepts of terms in order to describe the invention in the best possible way.

(7) Lithium Electrode

(8) A lithium electrode prepared using common methods has a surface oxide layer of a few hundred nm produced on the surface.

(9) Such a surface oxide layer is produced by the lithium electrode being exposed to moisture, oxygen and carbon dioxide present under the preparation environment, and since a stable solid electrolyte interphase (SEI) layer may not be formed by the surface oxide layer formed on the lithium electrode, formation of the surface oxide layer needs to be prevented or the form needs to be changed in order to increase lithium efficiency.

(10) In view of the above, the present invention relates to a lithium electrode having surface properties controlled, and, in a lithium electrode including a surface oxide layer, provides a lithium electrode having surface roughness of the surface oxide layer controlled. (i) Sa≥1 μm; (ii) Sz≥14 μm; (iii) Sp≥1000 mm.sup.−1; and (iv) Sdr≥0.5,

(11) The surface roughness may be defined by Sa (arithmetic mean height of surface), Sz (maximum height roughness of surface), Sp (roughness by the number of peaks) and Sdr (degree of interfacial increase).

(12) Sa is an arithmetic mean height of a surface, which is a mean of absolute values of differences between each point with respect to an average surface of a surface. As the value decreases, the surface roughness becomes lower. The value is generally used when evaluating surface roughness.

(13) In the present invention, Sa may be Sa≥1 μm, and preferably 1 μm≤Sa≤2 μm. Satisfying the above-mentioned range is advantageous in forming a stable SEI, and when the value is outside the above-mentioned range, SEI formation may be difficult.

(14) Sz is maximum height roughness of a surface, and is a second common roughness parameter. It represents a distance between a highest point and a lowest point on the surface.

(15) In the present invention, Sz may be Sz≥14 μm, and preferably 15 μm≤Sz≤20 μm. Satisfying the above-mentioned range is advantageous in forming a stable SEI, and when the value is outside the above-mentioned range, SEI formation may be difficult.

(16) Sp is roughness by the number of steep peaks, and shows how steep the peaks are. This value being higher means more steep peaks on the surface.

(17) In the present invention, Sp may be Sp≥1000 mm.sup.−1, and preferably 1000 mm.sup.−1≤Sp≤1500 mm.sup.−1. Satisfying the above-mentioned range is advantageous in forming a stable SEI, and when the value is outside the above-mentioned range, SEI formation may be difficult.

(18) Sdr is a developed area ratio of an interface, and represents how much increased the developed area (surface area of measured shape) with respect to an area when looking at the measured area perpendicularly from the above.

(19) In the present invention, Sdr may be Sdr≥0.5, and preferably 0.5≤Sdr≤1.0. Satisfying the above-mentioned range is advantageous in forming a stable SEI, and when the value is outside the above-mentioned range, SEI formation may be difficult.

(20) By the surface oxide layer satisfying Sa, Sz, Sp and Sdr ranges as described above in the lithium electrode according to the present invention, optimal surface roughness capable of forming a stable SEI through side reactions with a liquid electrolyte is obtained, which helps with initial stable SEI formation by improving reactivity between the lithium and the liquid electrolyte.

(21) In addition, the present invention provides a lithium electrode having a thickness controlled as well as surface roughness as surface properties of the lithium electrode.

(22) The surface oxide layer may have a thickness of 50 nm or less and preferably 10 nm to 50 nm.

(23) The thickness of the surface oxide layer concerns lithium electrode reactivity, and when the thickness is greater than 50 nm, an effect of improving a lifetime property of a battery may be insignificant since a stable SEI layer is not formed.

(24) The lithium electrode according to the present invention may be formed on one surface of a current collector, and the surface oxide layer may be formed on the opposite surface of the lithium electrode.

(25) In addition, the surface oxide layer may include one or more types selected from the group consisting of Li.sub.2O, LiOH and Li.sub.2CO.sub.3.

(26) Structures of such a surface lithium electrode and a surface oxide layer will be described in more detail with reference to the drawing.

(27) FIG. 1 is a sectional diagram of a lithium electrode including a surface oxide layer according to the present invention.

(28) When referring to FIG. 1, the surface oxide layer (120) is formed on one surface of the lithium electrode (100) not adjoining a current collector, and the surface oxide layer (120) of the lithium electrode (100) includes a first oxide layer (121) including Li.sub.2O; a second oxide layer (122) including Li.sub.2O and LiOH; and a third oxide layer (123) including Li.sub.2O, LiOH and Li.sub.2CO.sub.3.

(29) The first oxide layer (121) to the third oxide layer (123) are layers arbitrarily divided according to the oxide composition distribution rather than having their critical planes present. Depths of Li.sub.2O, LiOH and Li.sub.2CO.sub.3 formed from an outermost surface of the lithium electrode (100) each vary, and depths thereof are in order of Li.sub.2O>LiOH>Li.sub.2CO.sub.3.

(30) More specifically, the distance from the outermost surface to a spot where Li.sub.2O presents is defined as the first oxide layer (121), and the thickness may be from 10 nm to 50 nm, and preferably from 8 nm to 30 nm.

(31) In addition, the distance from the outermost surface layer to a spot where LiOH presents is defined as the second oxide layer (122), and the thickness may be from 1 nm to 10 nm, and preferably from 3 nm to 10 nm.

(32) In addition, the distance from the outermost surface layer to a spot where Li.sub.2CO.sub.3 presents is defined as the third oxide layer (123), and the thickness may be from 1 nm to 5 nm, and preferably from 0.5 nm to 1 nm.

(33) The lithium metal layer (110) is a layer remaining after forming the surface oxide layer (120) in the lithium electrode (100), and means a metal layer including a lithium metal element. Materials of the lithium metal layer may include lithium alloys, lithium metal, oxides of lithium alloys or lithium oxides. As nonlimiting examples, the negative electrode may be a thin film of lithium metal, or an alloy of lithium and one or more types of metals selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Ba, Ra, Al and Sn. Herein, the lithium metal layer (110) may be partly changed by oxygen or moisture besides the surface oxide layer (120), or may include impurities.

(34) The thickness of the lithium electrode (100) including the lithium metal layer (110) and the surface oxide layer (120) may be from 0.01 μm to 100 μm, preferably from 0.05 μm to 75 μm and more preferably from 0.1 μm to 50 μm. When the thickness is less than 0.01 μm, it is difficult to satisfy a cycle property due to insufficient lithium efficiency, and the thickness being greater than 100 μm causes a problem of reducing energy density with an increase in the lithium thickness.

(35) Lithium Secondary Battery

(36) The present invention also relates to a lithium secondary battery including a lithium electrode having surface properties controlled by a thickness and surface roughness of a surface oxide layer as described above.

(37) The surface property-controlled lithium electrode may be used as a negative electrode of a lithium-sulfur secondary battery.

(38) Herein, a positive electrode of the lithium-sulfur secondary battery may include a mixture of sulfur (S) and polyacrylonitrile (PAN), and specifically, a mixture (S-PAN) obtained through heat treatment.

(39) When using the S-PAN compared to a S/C composite generally normally used as a positive electrode material of a lithium-sulfur secondary battery, an effect of drastically reducing an amount of charge and discharge polysulfide elution of a lithium-sulfur secondary battery is obtained.

(40) This is a phenomenon obtained by sulfur elements or short-chain sulfurs covalently bonding to a carbonized polymer backbone in an evenly dispersed form, and thereby suppressing polysulfide production from a positive electrode including S-PAN during discharge.

(41) In view of the above, when using the surface property-controlled lithium metal as a negative electrode of a lithium-sulfur secondary battery, side reactions with polysulfide partly produced from a S-PAN positive electrode in which polysulfide production is suppressed are also reduced, and an effect of improving a lifetime property of a battery may be maximized.

(42) Such a surface-controlled lithium electrode may be prepared using a method for preparing a lithium electrode as follows.

(43) Lithium is deposited on a current collector through a high temperature vacuum deposition method using a lithium source. Herein, the lithium source may be lithium ingot, and the current collector may be copper foil. Herein, the high temperature vacuum deposition method may be performed under a condition of 500° C. to 700° C. and 10.sup.−7 torr to 10.sup.−3 torr, and under such a condition, lithium deposition may be efficiently achieved.

(44) A lithium electrode may be prepared by performing deposition so as not to form a surface oxide layer in the deposited lithium layer, then transferring the result from a vacuum chamber into a glove box in which Ar/CO.sub.2 mixture gas is distributed, and storing the result for a certain period of time.

(45) As described above, surface roughness of the oxide layer may increase enough to produce side reactions with a liquid electrolyte using only a deposition process, however, in order to further increase surface roughness, a rolling and brushing method may also be used.

MODE FOR INVENTION

(46) The embodiments are obvious to those skilled in the art, and it is obvious that such modifications and changes also belong to the scope of the attached claims.

Example 1

(47) A surface property-controlled lithium electrode as illustrated in FIG. 2 was prepared.

(48) In a vacuum chamber, using copper foil as a current collector, lithium was deposited on a surface of the copper foil using a high temperature vacuum deposition method under 600° C. and 10.sup.−5 torr, and the result was stored in a glove box filled with Ar/CO.sub.2 mixture gas to prepare a lithium electrode having an oxide layer of 50 nm.

Example 2

(49) A lithium electrode was prepared in the same manner as in Example 1, except that a rolling and brushing process was further performed on the lithium electrode to increase surface roughness.

Comparative Example 1

(50) A lithium electrode was prepared in the same manner as in Example 1, except for being stored for 4 days in a dry room, the thickness of the oxide layer was increased to approximately 100 nm, and surface roughness was naturally formed accordingly.

Comparative Example 2

(51) A lithium electrode having surface properties not controlled as illustrated in FIG. 3 was prepared.

Experimental Example 1: Measurement of Thickness and Surface Roughness of Surface Oxide Layer

(52) For the lithium electrodes each prepared in Examples 1 and 2 and Comparative Examples 1 and 2, a thickness and surface roughness of the surface oxide layer were measured, and the results are described in Table 1.

(53) Herein, the thickness of the surface oxide layer was measured through an X-ray photoelectron spectroscopy (XPS) depth-profile, and the surface roughness of the surface oxide layer was measured using laser confocal microscope equipment.

(54) TABLE-US-00001 TABLE 1 Surface Properties of Oxide Layer Surface Roughness Thickness Sa (μm) Sz (μm) Sp (mm.sup.−1) Sdr Example 1  50 nm 1.2 ± 0.03 14.1 ± 0.09 1255.5 ± 107.13 0.8 ± 0.01 Comparative 100 nm 1.2 ± 0.03 13.3 ± 0.16 1100.8 ± 100.03 0.8 ± 0.03 Example 1 Comparative 100 nm 0.9 ± 0.02 10.2 ± 0.57  967.9 ± 166.61 0.4 ± 0.08 Example 2

(55) Based on the result, it was seen that, as described in Table 1, Example 1 had a reduced surface oxide layer thickness and increased surface roughness compared to Comparative Examples 1 and 2.

Experimental Example 2: Measurement of Non-Reversible Capacity

(56) In a lithium-sulfur secondary battery including a positive electrode including S-PAN as a positive electrode material, each of the lithium electrodes prepared in Examples 1 and 2 and Comparative Examples 1 and 2 was used as a negative electrode, and battery capacity was evaluated while charging and discharging in a voltage range of 4.3 V to 2.5 V. The results are described in Table 2.

(57) TABLE-US-00002 TABLE 2 Second Third Charging Discharging Capacity Capacity Efficiency (mAh/g) (mAh/g) (%) Example 1 1470 1465 99.7 Comparative 1504 1495 99.4 Example 1 Comparative 1488 1370 92 Example 2

Experimental Example 3: Measurement of Cycle Lifetime of Lithium-Sulfur Secondary Battery

(58) In a lithium-sulfur secondary battery including a positive electrode including S-PAN as a positive electrode material, each of the lithium electrodes prepared in Examples 1 and 2 and Comparative Examples 1 and 2 was used as a negative electrode, and a cycle lifetime was measured. A condition of charge and discharge was as follows. Charge and discharge driving: 0.1 C 2.5 times.fwdarw.[0.2 C 3 times.fwdarw.0.3 C/0.5 C 10 times]n

(59) The number of cycles when discharging capacity reached 80% compared to initial capacity of the battery was measured while repeating cycles under the above-mentioned condition.

(60) FIG. 4 is a graph measuring a cycle lifetime when using the lithium electrodes of Example 1, and Comparative Examples 1 and 2 as a negative electrode of the lithium-sulfur secondary battery.

(61) When referring to FIG. 4, it was seen that, a lifetime property of the lithium electrode was significantly higher in Example 1 having a reduced surface oxide layer thickness and increased surface roughness compared to in Comparative Examples 1 and 2.

(62) Accordingly, it was seen that surface roughness and thickness of the surface oxide layer were closely related to a battery lifetime property, and it was identified that a battery lifetime property was enhanced when surface roughness increases or thickness of the surface oxide layer decreases.

(63) Hereinbefore, the present invention has been described with limited examples and drawings, however, the present invention is not limited thereto, and various modifications and changes may be made by those skilled in the art within technological ideas of the present invention and the range of equivalents of the attached claims to describe below. [Reference Numeral] 100: Lithium Electrode 110: Lithium Metal Layer 120: Surface Oxide Layer 121: First Oxide Layer 122: Second Oxide Layer 123: Third Oxide Layer