Method for producing an anode for a lithium metal secondary battery including a MXene thin film

Abstract

A method for producing an anode for a lithium metal secondary battery includes coating a thin film comprised of Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 on a substrate; providing a lithium metal electrode; and laminating the thin film to a surface of the lithium metal electrode. Coating is accomplished by providing a dispersion of a powder comprising Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2; and coating the dispersion on the substrate by Langmuir-Blodgett scooping (LBS). The method may further include, prior to providing the dispersion, obtaining the powder by etching a MAX phase structure represented by Formula 1, Formula 2 or Formula 3 below:
Nb.sub.2AC(1);
Ti.sub.2AC(2); and
Ti.sub.3AC.sub.2(3), where A is a metal selected from among Group IIIA elements, Group IVA elements, Cd, and combinations thereof. The method may further include, after laminating the thin film, removing the substrate from the thin film.

Claims

1. A method for producing an anode for a lithium metal secondary battery, comprising: coating a thin film comprised of Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 on a substrate; providing a lithium metal electrode; and laminating the thin film to a surface of the lithium metal electrode.

2. The method according to claim 1, wherein coating is accomplished by providing a dispersion of a powder comprising Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2; and coating the dispersion on the substrate by Langmuir-Blodgett scooping (LBS).

3. The method according to claim 2, further comprising, prior to providing the dispersion, obtaining the powder by etching a MAX phase structure represented by Formula 1, Formula 2 or Formula 3 below:
Nb.sub.2AC(1);
Ti.sub.2AC(2); and
Ti.sub.3AC.sub.2(3), where A is a metal selected from the group consisting of Group IIIA elements, Group IVA elements, Cd, and combinations thereof.

4. The method according to claim 3, wherein the MAX phase structure represented by Formula 1 is Nb.sub.2AlC and the etching is performed by mixing the Nb.sub.2AlC with hydrofluoric acid to provide a mixture in amounts so that a weight ratio of the Nb.sub.2AlC to hydrogen fluoride ranges from 1:3.5 to 1:6.0 and allowing the mixture to react for from 2 to 9 days.

5. The method according to claim 3, wherein the MAX phase structure represented by Formula 2 is Ti.sub.2AlC and the etching is performed by mixing the Ti.sub.2AlC with hydrofluoric acid to provide a mixture in amounts so that a weight ratio of the Ti.sub.2AlC to hydrogen fluoride ranges from 1:0.5 to 1:1.5 and allowing the mixture to react for from 12 to 36 hours.

6. The method according to claim 3, wherein the MAX phase structure represented by Formula 3 is Ti.sub.3AlC.sub.2 and the etching is performed by mixing the Ti.sub.3AlC.sub.2 with hydrofluoric acid to provide a mixture in amounts so that a weight ratio of the Ti.sub.3AlC.sub.2 to hydrogen fluoride ranges from 1:3.5 to 1:6.0 and allowing the mixture to react for 2-5 days.

7. The method according to claim 1, wherein laminating the thin film is accomplished by rolling.

8. The method according to claim 7, wherein laminating the thin film by rolling is accomplished by pressing the lithium metal against the thin film to provide a sandwich structure and passing the sandwich structure through a rolling mill.

9. The method according to claim 1, further comprising, after laminating the thin film, removing the substrate from the thin film.

10. The method according to claim 1, wherein laminating takes place in an inert atmosphere comprised of inert gasses.

11. The method according to claim 1, wherein laminating takes place in a dry atmosphere having a relative humidity ranging from 0% to 1%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:

(2) FIG. 1A is a schematic diagram showing formation of a Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film (artificial SEI film) and FIG. 1B shows transfer of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 ultrathin film. Specifically, FIG. 1A shows the formation of an ultrathin film on water and the attachment of the ultrathin film to a solid, and FIG. 1B shows the transfer of the ultrathin film to lithium metal by rolling;

(3) FIG. 2A is a schematic diagram of an anode produced in Example 1-1 in which a Nb.sub.2C thin film was formed on a lithium metal electrode;

(4) FIG. 2B is a schematic diagram of an anode produced in Example 1-2 in which a Ti.sub.2C thin film was formed on a lithium metal electrode;

(5) FIG. 2C is a schematic diagram of an anode produced in Example 1-3 in which a Ti.sub.3C.sub.2 thin film was formed on a lithium metal electrode;

(6) FIG. 3A is a scanning electron microscopy (SEM) image of Nb.sub.2C prepared in Preparative Example 1-1;

(7) FIG. 3B is a scanning electron microscopy (SEM) image of Ti.sub.2C prepared in Preparative Example 1-2;

(8) FIG. 3C is a scanning electron microscopy (SEM) image of Ti.sub.3C.sub.2 prepared in Preparative Example 1-3;

(9) FIG. 4A shows a scanning electron microscopy (SEM) image and EDXS elemental mapping images of Nb.sub.2AlC used in Preparative Example 1-1;

(10) FIG. 4B shows a scanning electron microscopy (SEM) image and EDXS elemental mapping images of Ti.sub.2AlC used in Preparative Example 1-2;

(11) FIG. 4C shows a scanning electron microscopy (SEM) image and EDXS elemental mapping images of Ti.sub.3AlC.sub.2 used in Preparative Example 1-3;

(12) FIG. 5A shows a scanning electron microscopy (SEM) image and EDXS elemental mapping images of Nb.sub.2C particles prepared in Preparative Example 1-1;

(13) FIG. 5B shows a scanning electron microscopy (SEM) image and EDXS elemental mapping images of Ti.sub.2C particles prepared in Preparative Example 1-2;

(14) FIG. 5C shows a scanning electron microscopy (SEM) image and EDXS elemental mapping images of Ti.sub.3C.sub.2 particles prepared in Preparative Example 1-3;

(15) FIG. 6A is an X-ray photoelectron spectrum (XPS) of Nb.sub.2C prepared in Preparative Example 1-1;

(16) FIG. 6B is an X-ray photoelectron spectrum (XPS) of Ti.sub.2C prepared in Preparative Example 1-2;

(17) FIG. 6C is an X-ray photoelectron spectrum (XPS) of Ti.sub.3C.sub.2 prepared in Preparative Example 1-3;

(18) FIG. 7A shows X-ray diffraction (XRD) patterns of Nb.sub.2C products prepared by etching Nb.sub.2AlC with HF for different periods of time in Preparative Example 1-1;

(19) FIG. 7B shows X-ray diffraction (XRD) patterns of Nb.sub.2C products prepared by etching Ti.sub.2AlC with HF for different periods of time in Preparative Example 1-2;

(20) FIG. 7C shows X-ray diffraction (XRD) patterns of Nb.sub.2C products prepared by etching Ti.sub.3AlC.sub.2 with HF for different periods of time in Preparative Example 1-3;

(21) FIG. 8A shows surface scanning electron microscopy (SEM) images of a pristine lithium metal electrode (Pristine Li of Comparative Example 1) and a lithium metal electrode on which a Nb.sub.2C thin film was formed (Nb.sub.2C Li of Example 1-1) after repeated charge/discharge cycles. The rightmost image is an enlargement of the middle image;

(22) FIG. 8B shows surface scanning electron microscopy (SEM) images of (a) a pristine lithium metal electrode (Pristine Li of Comparative Example 1) and (b) a lithium metal electrode on which a Ti.sub.2C thin film was formed (Ti.sub.2C Li of Example 1-2) after repeated charge/discharge cycles. The rightmost image (c) is an enlargement of the middle image (b);

(23) FIG. 8C shows surface scanning electron microscopy (SEM) images of a pristine lithium metal electrode (Pristine Li of Comparative Example 1) and a lithium metal electrode on which a Ti.sub.3C.sub.2 thin film was formed (Ti.sub.3C.sub.2 Li of Example 1-3) after repeated charge/discharge cycles. The rightmost image is an enlargement of the middle image;

(24) FIG. 9A shows cycle characteristics and charge/discharge efficiencies of a lithium metal secondary battery using a pristine lithium metal electrode (Pristine Li of Comparative Example 1) as an anode and a lithium metal electrode on which a Nb.sub.2C thin film was formed (Nb.sub.2C Li of Example 1-1) as an anode;

(25) FIG. 9B shows cycle characteristics and charge/discharge efficiencies of a lithium metal secondary battery using a pristine lithium metal electrode (Pristine Li of Comparative Example 1) as an anode and a lithium metal electrode on which a Nb.sub.2C thin film was formed (Ti.sub.2C Li of Example 1-2) as an anode;

(26) FIG. 9C shows cycle characteristics and charge/discharge efficiencies of a lithium metal secondary battery using a pristine lithium metal electrode (Pristine Li of Comparative Example 1) as an anode and a lithium metal electrode on which a Ti.sub.3C.sub.2 thin film was formed (Ti.sub.3C.sub.2 Li of Example 1-3) as an anode; and

(27) FIG. 10 shows coulombic efficiencies of lithium deposition/dissolution reactions in a lithium metal secondary battery using a copper foil coated with Ti.sub.3C.sub.2 prepared in Preparative Example 1-3 as a cathode and a lithium foil as an anode when charged/discharged at a current density of 1 mA/cm.sup.2 and with a capacity of 1 mAh/cm.sup.2.

DETAILED DESCRIPTION OF THE INVENTION

(28) Several aspects and various embodiments of the present invention will now be described in more detail.

(29) One aspect of the present invention is directed to an anode for a lithium metal secondary battery, including: a lithium metal electrode including lithium metal or a lithium alloy; and a Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film formed on the lithium metal electrode.

(30) During electrochemical cycling of a lithium metal secondary battery, highly active and flexible lithium metal (LiM) tends to form dendrites when the battery is charged due to a local difference in current density on its rough surface. Once formed on the surface of the lithium metal, lithium dendrites penetrate a separator to cause an internal short circuit. This short circuit generates heat, causing the battery to explode. Particularly, high-density lithium metal secondary batteries have 10 times higher energy densities than other lithium ion batteries. Accordingly, one of the most crucial requirements for the commercialization of lithium metal secondary batteries is to develop technologies for improving battery safety while minimizing the danger of battery explosion. Further, repeated charge/discharge cycles lead to an increase in surface area, causing degradation of electrolytes. The continuous destruction and reformation of SEI layers brings about loss of lithium (i.e. low coulombic efficiency).

(31) The present invention is aimed at solving the above-mentioned problems. Specifically, a Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film is formed on a lithium metal electrode to suppress the formation and diffusion of lithium dendrites such that the lithium metal electrode is stabilized and the occurrence of an internal short circuit is prevented. The Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 is a MXene material that exhibits a two-dimensional layered structure, is hydrophilic and electrically conductive, and has high mechanical strength, enabling smooth migration of lithium ions therethrough. Thus, the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 facilitates plating/dissolution of lithium on the lithium metal electrode. The formation and diffusion of dendrites can be suppressed on the stabilized lithium metal electrode.

(32) According to one embodiment of the present invention, the Nb.sub.2C may show first, second, third, and fourth effective XPS peaks at binding energies ranging from 203 to 212 eV, from 282.5 to 287.5 eV, from 527.5 to 535 eV, and from 682.5 to 687.5 eV, respectively, as measured by XPS; the Ti.sub.2C may show first, second, third, and fourth effective XPS peaks at binding energies ranging from 280 to 287.5 eV, from 457.5 to 462.5 eV, from 527.5 to 537.5 eV, and from 682.5 to 690 eV, respectively, as measured by XPS; and the Ti.sub.3C.sub.2 may show first, second, third, and fourth effective XPS peaks at binding energies ranging from 282.5 to 287.5 eV, from 450 to 460 eV, from 527.5 to 537.5 eV, and from 682.5 to 690 eV, respectively, as measured by XPS.

(33) The results of XPS analysis reveal the binding of OH, O, and F as functional groups to the surface ends of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2. Due to their interactions, these polar groups promote the concentration of lithium ions at the edges of the layered structure of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2, that is, where surface energy is high. As a result, smooth migration of lithium ions is permitted, considerably improving the plating/dissolution of lithium.

(34) According to a further embodiment of the present invention, the Nb.sub.2C may show first, second, third, fourth, fifth, and sixth effective X-ray diffraction (XRD) peaks at 2 angles ranging from 5 to 10, from 10 to 15, from 20 to 30, from 30 to 40, from 50 to 55, and from 56 to 65, respectively, as measured by XRD. The results of XRD analysis reveal that the dissolution of Al by hydrofluoric acid allows the Nb.sub.2C to have a layered structure, causing shifts in the first, second, third, fourth, fifth, and sixth effective peaks.

(35) In addition, the Ti.sub.2C may show first, second, and third effective X-ray diffraction (XRD) peaks at 2 angles ranging from 7 to 10, from 23 to 28, and from 48 to 50, respectively, as measured by XRD. The results of XRD analysis reveal that the dissolution of Al by hydrofluoric acid allows the Ti.sub.2C to have a layered structure, causing shifts in the first, second, and third effective peaks.

(36) In addition, the Ti.sub.3C.sub.2 may show first, second, third, fourth, fifth, sixth, and seventh effective X-ray diffraction (XRD) peaks at 2 angles ranging from 5 to 13, from 15 to 20, from 25 to 30, from 31 to 45, from 47 to 50, from 51 to 55, and from 56 to 70, respectively, as measured by XRD. The results of XRD analysis reveal that the dissolution of Al by hydrofluoric acid allows the Ti.sub.3C.sub.2 to have a layered structure, causing shifts in the first, second, third, fourth, fifth, sixth, and seventh effective peaks.

(37) Particularly, the XRD patterns of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 indicate a greatly improved diffusion rate of lithium ions.

(38) A further aspect of the present invention is directed to a lithium metal secondary battery including the anode, an electrolyte, and a cathode.

(39) According to one embodiment of the present invention, the cathode may include at least one compound selected from lithium cobalt oxides, lithium manganese oxides, lithium nickel cobalt aluminum oxides, lithium nickel manganese cobalt oxides, lithium iron phosphate oxide, and sulfur compounds or may be a porous air electrode. The cathode material is not limited but is preferably a lithium nickel manganese oxide.

(40) Another aspect of the present invention is directed to an electric device including the anode wherein the electric device is selected from electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems.

(41) Yet another aspect of the present invention is directed to a method for producing an anode for a lithium metal secondary battery, including: (b) forming a Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film; and (c) transferring the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film to the surface of a lithium metal electrode.

(42) The formation of the layered Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 on the lithium metal electrode enables rapid diffusion of lithium ions to inhibit the formation and diffusion of dendrites on the lithium metal electrode. The layered Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 functions as an artificial SEI layer to effectively prevent side reactions between the lithium metal electrode and the electrolyte.

(43) According to one embodiment of the present invention, in step (b), a dispersion of a Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 powder may be coated on a substrate to form a Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film by a technique selected from Langmuir-Blodgett scooping (LBS), tape casting, vacuum filtration, electrospinning, spin coating, spray coating, and flat rolling. The technique for forming the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film is not limited but is preferably LBS.

(44) Langmuir-Blodgett scooping (LBS) refers to a process for forming a thin film based on self-assembly of film-forming particles (Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 particles in the present invention), diffusion of a water-immiscible solvent (ethanol in the present invention), and the surface tension gradient known as the Marangoni effect. When a suspension of film-forming particles in ethanol or isopropanol (IPA) as a suspension medium is injected into the surface of water, the suspension is rapidly diffused on the water surface while interacting with the water surface, lowering the surface tension of the water. As a result, the particles slip and self-assemble. This self-assembly leads to the formation of a uniform film on the water surface during injection of the suspension.

(45) According to a further embodiment of the present invention, the method may further include (a) etching a MAX phase structure to obtain the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 powder before step (b) wherein the MAX phase structure is represented by Formula 1, 2 or 3:
Nb.sub.2AC(1)
wherein A is a metal selected from Group IIIA elements, Group IVA elements, Cd, and combinations thereof;
Ti.sub.2AC(2)
wherein A is as defined in Formula 1;
Ti.sub.3AC.sub.2(3)
wherein A is as defined in Formula 1.

(46) When the MAX phase structure of Formula 1, 2 or 3 is etched, the element A is removed to allow the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 to have a two-dimensional layered structure.

(47) According to another embodiment of the present invention, the MAX phase structure represented by Formula 1 may be Nb.sub.2AlC and the etching may be performed by mixing the Nb.sub.2AlC with hydrofluoric acid in such amounts that the weight ratio of the Nb.sub.2AlC to hydrogen fluoride is 1:3.5-6.0, preferably 1:4.0-5.5, more preferably 1:4.5-5.0, and allowing the mixture to react for 2-9 days, preferably 3-8 days, more preferably 4-7 days.

(48) Particularly, in the Examples section that follows, it was found that when the above conditions were met, the Nb.sub.2C had the best layered structure.

(49) The MAX phase structure represented by Formula 2 may be Ti.sub.2AlC and the etching may be performed by mixing the Ti.sub.2AlC with hydrofluoric acid in such amounts that the weight ratio of the Ti.sub.2AlC to hydrogen fluoride is 1:0.5-1.5, preferably 1:0.7-1.3, more preferably 1:0.8-1.2, and allowing the mixture to react for 12-36 hours, preferably 18-30 hours, more preferably 22-26 hours.

(50) Particularly, in the Examples section that follows, it was found that when the above conditions were met, the Ti.sub.2C had the best layered structure.

(51) The MAX phase structure represented by Formula 3 may be Ti.sub.3AlC.sub.2 and the etching may be performed by mixing the Ti.sub.3AlC.sub.2 with hydrofluoric acid in such amounts that the weight ratio of the Ti.sub.3AlC.sub.2 to hydrogen fluoride is 1:3.5-6.0, preferably 1:4.0-5.5, more preferably 1:4.5-5.0, and allowing the mixture to react for 2-5 days, preferably 2.5-4.5 days, more preferably 3-4 days.

(52) Particularly, in the Examples section that follows, it was found that when the above conditions were met, the Ti.sub.3C.sub.2 had the best layered structure.

(53) According to an alternative embodiment of the present invention, in step (b), one or more Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin films may be formed on a substrate by transferring a Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film dispersed on the surface of a dispersion medium to the surface of the substrate, drying the thin film, and repeating the above procedure.

(54) According to another embodiment of the present invention, in step (b), the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film may be transferred by immersing the substrate in the dispersion medium and lifting the substrate such that the thin film covers the substrate.

(55) In step (b), a suspension of the material for the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film may be added to the dispersion while lifting the substrate.

(56) According to another embodiment of the present invention, step (b) may be carried out when the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film accounts for 10% to 70% of the surface of the dispersion medium. If the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film accounts for less than 10% of the surface of the dispersion medium, it may not be sufficiently coated on the solid surface. Meanwhile, if the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film accounts for more than 70% of the surface of the dispersion medium, it may not be evenly formed.

(57) According to another embodiment of the present invention, the suspension may have a concentration of 3 to 20% by weight.

(58) According to another embodiment of the present invention, the suspension may be added such that the area of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film on the surface of the dispersion medium is maintained at 10% to 70% of its initial value. If the area of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film is maintained at less than 10% of its initial value, the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film may not be sufficiently coated on the solid surface. Meanwhile, if the area of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film exceeds 70% of its initial value, the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film may not be evenly formed.

(59) According to another embodiment of the present invention, the dispersion medium may be water and the suspension medium of the suspension may be ethanol. The dispersion medium is a polar or non-polar liquid. The use of water is particularly preferred in terms of safety and economic efficiency. Ethanol is particularly preferred as the suspension medium from the viewpoint of safety and economic efficiency.

(60) According to an alternative embodiment of the present invention, in step (c), one or more Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin films formed on the substrate may be transferred to the surface of lithium metal.

(61) According to another embodiment of the present invention, step (c) may be carried out by rolling.

(62) According to another embodiment of the present invention, in step (c), the lithium metal and the substrate may be pressed against each other such that the lithium metal is close to the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film.

(63) According to another embodiment of the present invention, the lithium metal and the substrate may be pressed against each other by sandwiching the lithium metal and the substrate close to each other between a pair of protective films and passing the sandwich structure through a rolling mill.

(64) According to another embodiment of the present invention, roll cylinders of the rolling mill may be arranged at an interval corresponding to 50 to 90% of the total thickness of all layers inserted therebetween and the rolling speed of the rolling mill may be maintained at 0.05 to 0.2 cm/sec.

(65) According to another embodiment of the present invention, step (c) may further include removing the substrate from the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film-transferred lithium metal.

(66) According to another embodiment of the present invention, step (c) may be carried out in a dry atmosphere at a relative humidity of 0% to 1%. If the relative humidity exceeds the upper limit defined above, the lithium metal may be oxidized, impairing the electrochemical properties of the anode.

(67) According to another embodiment of the present invention, step (c) may be carried out in an atmosphere of at least one inert gas selected from argon, nitrogen, helium, and neon to prevent the oxidation and side reactions of the lithium metal.

(68) According to another embodiment of the present invention, the substrate may be a copper foil and the protective films may be polyester films.

(69) Particularly, although not explicitly described in the Examples section that follows, anodes for lithium metal secondary batteries were produced by varying the conditions defined in step (b), the concentration of the suspension, and the kinds of the dispersion medium and the suspension medium, lithium metal secondary batteries fabricated using the anodes were operated at high temperature for 500 hours, and then the cross-sections of the anodes were observed and losses of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin films were investigated by scanning electron microscopy (SEM).

(70) As a result, when the following conditions were met, no voids were formed at the interfaces between the lithium metal and the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin films and no losses of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin films coated on the lithium metal were found even after the lithium metal secondary batteries were operated at high temperature for 500 hours, unlike when other conditions and other numerical ranges were employed: (i) step (b) was carried out when the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film accounted for 10% to 70% of the surface of the dispersion medium; (ii) the suspension had a concentration of 5 to 10% by weight; (iii) the suspension was added such that the area of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film on the surface of the dispersion medium was maintained at 10% to 70% of its initial value; (iv) water was used as the dispersion medium; and (v) ethanol was used as the suspension medium.

(71) When any one of the above conditions was not met, voids were formed in many portions of the interfaces between the lithium metal and the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin films and considerable losses of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin films coated on the lithium metal electrodes were found after the lithium metal secondary batteries were operated at high temperature for 500 hours.

(72) Although not explicitly described in the Examples section that follows, anodes for lithium metal secondary batteries were produced by varying the conditions defined in step (b), the concentration of the suspension, the kinds of the dispersion medium and the suspension medium, and the conditions defined in step (c), lithium metal secondary batteries fabricated using the anodes were operated at a high temperature of 800 C. or above, and the morphologies of the anodes were observed by scanning electron microscopy (SEM).

(73) As a result, when the following conditions were met, no aggregation of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 particles was observed in the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin films coated on the lithium metal even after the lithium metal secondary batteries were operated at high temperature for 800 hours, demonstrating excellent thermal stability of the secondary batteries, unlike when other conditions and other numerical ranges were employed: (i) step (b) was carried out when the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film accounted for 10% to 70% of the surface of the dispersion medium; (ii) the suspension had a concentration of 5 to 10% by weight; (iii) the suspension was added such that the area of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film on the surface of the dispersion medium was maintained at 10% to 70% of its initial value; (iv) water was used as the dispersion medium; (v) ethanol was used as the suspension medium; (vi) step (c) was carried out by rolling; (vii) roll cylinders of the rolling mill were arranged at an interval corresponding to 50 to 90% of the total thickness of all layers inserted therebetween; (vii) the rolling speed of the rolling mill were maintained at 0.05 to 0.2 cm/sec; (ix) step (c) was carried out in a dry atmosphere at a relative humidity of 0% to 1%; (x) a copper foil was used as the substrate; and (xi) polyester films were used as the protective films.

(74) When any one of the above conditions was not met, considerable aggregation of the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 particles was observed in the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin films coated on the lithium metal after the lithium metal secondary batteries were operated at high temperature for 800 hours.

(75) The present invention will be explained in detail with reference with the following examples in conjunction with the accompanying drawings.

PREPARATIVE EXAMPLE 1-1

Preparation of Nb.SUB.2.C

(76) 10 g of commercial Nb.sub.2AlC was mixed with 10 ml of hydrofluoric acid (48% in H.sub.2O). The reaction was allowed to proceed at room temperature for 1-7 days. When the Nb.sub.2AlC was etched for 4-7 days, Nb.sub.2C was optimally induced. The resulting Nb.sub.2C was washed 10 times with deionized water by centrifugation and dried in an oven at 80 C. to obtain a powder.

PREPARATIVE EXAMPLE 1-2

Preparation of Ti.SUB.2.C

(77) 10 g of commercial Ti.sub.2AlC was mixed with 10 ml of hydrofluoric acid (10% in H.sub.2O). The reaction was allowed to proceed at room temperature for 1-7 days. When the Ti.sub.2AlC was etched for 1 day, Ti.sub.2C was optimally induced. The resulting Ti.sub.2C was washed 10 times with deionized water by centrifugation and dried in an oven at 80 C. to obtain a powder.

PREPARATION EXAMPLE 1-3

Preparation of Ti.SUB.3.C.SUB.2

(78) 10 g of commercial Ti.sub.3AlC.sub.2 was mixed with 10 ml of hydrofluoric acid (48% in H.sub.2O). The reaction was allowed to proceed at room temperature for 1-7 days. When the Ti.sub.3AlC.sub.2 was etched for 3-4 days, Ti.sub.3C.sub.2 was optimally induced. The resulting Ti.sub.3C.sub.2 was washed 10 times with deionized water by centrifugation and dried in an oven at 80 C. to obtain a powder.

EXAMPLE 1-1

Production of Lithium Metal Electrode on which Nb.SUB.2.C Thin Film was Formed

(1) Formation of Nb.SUB.2.C Thin Film

(79) The Nb.sub.2C particles (5-10 wt %) prepared in Preparative Example 1-1 were mixed with ethanol, followed by ultrasonic dispersion for 5 min. The resulting suspension was subjected to LBS coating to form a thin film of the Nb.sub.2C particles on a commercial copper foil as a substrate. Specifically, the suspension was added to water in which the substrate was immersed. When 30% of the water surface was covered with a self-assembled film of the Nb.sub.2C particles, the substrate was slowly lifted such that the self-assembled film formed on the water surface was coated on the substrate. The suspension was continuously added at a constant rate such that the self-assembled film was maintained constant on the water surface. The coated substrate was dried for 1 min on a hot plate maintained at 120 C. to remove water. This procedure was repeated 3-5 times.

(2) Transfer of the Nb.SUB.2.C Thin Film to Lithium Metal Electrode

(80) The Nb.sub.2C thin film coated on the copper foil substrate was transferred to the surface of lithium metal using a rolling mill. Specifically, under dry environmental conditions, lithium metal and the Nb.sub.2C film were sandwiched between Mylar films (polyester films) and the sandwich structure was uniformly pressurized in a rolling mill. The interval between rolls of the rolling mill was adjusted to 0.1 mm (corresponding to 70% of the total thickness of all layers inserted between the rolls) and the rolling speed of the rolling mill was maintained at 0.1 cm/sec. Thereafter, the Mylar films were removed, the copper foil attached to the lithium metal was peeled off, and then the electrochemical properties of the Nb.sub.2C thin film-transferred lithium metal were measured. The entire procedure was carried out in a dry atmosphere at an RH of 0-1% and an inert gas atmosphere.

EXAMPLE 1-2

Production of Lithium Metal Electrode on which Ti.SUB.2.C Thin Film was Formed

(81) A Ti.sub.2C thin film-transferred lithium metal electrode was produced in the same manner as in Example 1-1, except that the Ti.sub.2C particles prepared in Preparative Example 1-2 were used instead of the Nb.sub.2C particles prepared in Preparative Example 1-1. The electrochemical properties of the Ti.sub.2C thin film-transferred lithium metal were measured. The entire procedure was carried out in a dry atmosphere at an RH of 0-1% and an inert gas atmosphere.

EXAMPLE 1-3

Production of Lithium Metal Electrode on which Ti.SUB.3.C.SUB.2 .Thin Film was Formed

(82) A Ti.sub.3C.sub.2 thin film-transferred lithium metal electrode was produced in the same manner as in Example 1-1, except that the Ti.sub.3C.sub.2 particles prepared in Preparative Example 1-3 were used instead of the Nb.sub.2C particles prepared in Preparative Example 1-1. The electrochemical properties of the Ti.sub.3C.sub.2 thin film-transferred lithium metal were measured. The entire procedure was carried out in a dry atmosphere at an RH of 0-1% and an inert gas atmosphere.

COMPARATIVE EXAMPLE 1

Pristine Lithium Metal Electrode

(83) A pristine lithium metal electrode (Pristine Li) without a Nb.sub.2C thin film was prepared.

(84) FIGS. 2A, 2B, and 2C are schematic diagrams of the anodes produced in Example 1-1, 1-2, and 1-3 in which the Nb.sub.2C, Ti.sub.2C, and Ti.sub.3C.sub.2 thin films were formed on the lithium metal electrodes, respectively.

(85) As shown in FIGS. 2A, 2B, and 2C, Nb.sub.2C induced from Nb.sub.2AlC, Ti.sub.2C induced from Ti.sub.2AlC, and Ti.sub.3C.sub.2 induced from Ti.sub.3AlC.sub.2 assist in the migration of lithium ions due to their high electrical conductivities and layered structures, which can enhance the electrochemical performance of lithium metal batteries.

(86) FIGS. 3A, 3B, and 3C are scanning electron microscopy (SEM) images of the Nb.sub.2C, Ti.sub.2C, and Ti.sub.3C.sub.2 prepared in Preparative Examples 1-1, 1-2, and 1-3, respectively.

(87) Referring to FIGS. 3A, 3B, and 3C, Nb.sub.2C induced from Nb.sub.2AlC, Ti.sub.2C induced from Ti.sub.2AlC, and Ti.sub.3C.sub.2 induced from Ti.sub.3AlC.sub.2 had well-defined layered structures.

(88) FIGS. 4A, 4B, and 4C show scanning electron microscopy (SEM) images and EDXS elemental mapping images of Nb.sub.2AlC, Ti.sub.2AlC, and Ti.sub.3AlC.sub.2 used in Preparative Examples 1-1, 1-2, and 1-3, respectively.

(89) The EDXS elemental mapping images shown in FIG. 4A reveal that the particles were essentially composed of Nb, Al, and C.

(90) The EDXS elemental mapping images shown in FIGS. 4B and 4C reveal that the particles were essentially composed of Ti, Al, and C.

(91) FIGS. 5A, 5B, and 5C show scanning electron microscopy (SEM) images and EDXS elemental mapping images of the Nb.sub.2C, Ti.sub.2C, and Ti.sub.3C.sub.2 particles prepared in Preparative Examples 1-1, 1-2, and 1-3, respectively.

(92) The elemental maps for Nb, Al, and C shown in FIG. 5A were compared with those shown in FIG. 4A, with the result that Al was dramatically reduced and substantially disappeared.

(93) The elemental maps for Ti, Al, and C shown in FIGS. 5B and 5C were compared with those shown in FIGS. 4B and 4C, respectively, with the result that Al was dramatically reduced and substantially disappeared.

(94) FIGS. 6a, 6b, and 6c are X-ray photoelectron spectra (XPS) of the Nb.sub.2C, Ti.sub.2C, and Ti.sub.3C.sub.2 prepared in Preparative Examples 1-1, 1-2, and 1-3, respectively.

(95) FIG. 6A shows binding energies for Nb and C in the Nb.sub.2C, as clearly expected. FIG. 6A also shows the presence of O and F as surface terminal functional groups resulting from HF etching. The Al atoms were replaced by OH, O, and F during HF etching, which explains the formation of the surface terminal functional groups (Tx) of the Nb.sub.2C. The Nb.sub.2C having the surface terminal functional groups can be represented by Nb.sub.2C(Tx). Due to their interactions, these polar groups further promote the concentration of lithium ions at the edges of the layered structure of the Nb.sub.2C, that is, where surface energy is high. As a consequence, the functional groups assist in the migration and plating/dissolution of lithium ions.

(96) FIG. 6B shows binding energies for Ti and C in the Ti.sub.2C, as clearly expected. FIG. 6B also shows the presence of O and F as surface terminal functional groups resulting from HF etching. The Al atoms were replaced by OH, O, and F during HF etching, which explains the formation of the surface terminal functional groups (Tx) of the Ti.sub.2C. The Ti.sub.2C having the surface terminal functional groups can be represented by Ti.sub.2C(Tx). Due to their interactions, these polar groups further promote the concentration of lithium ions at the edges of the layered structure of the Ti.sub.2C, that is, where surface energy is high. As a consequence, the functional groups assist in the migration and plating/dissolution of lithium ions.

(97) FIG. 6C shows binding energies for Ti and C in the Ti.sub.3C.sub.2, as clearly expected. FIG. 6C also shows the presence of O and F as surface terminal functional groups resulting from HF etching. The Al atoms were replaced by OH, O, and F during HF etching, which explains the formation of the surface terminal functional groups (Tx) of the Ti.sub.3C.sub.2. The Ti.sub.3C.sub.2 having the surface terminal functional groups can be represented by Ti.sub.3C.sub.2(Tx). Due to their interactions, these polar groups further promote the concentration of lithium ions at the edges of the layered structure of the Ti.sub.3C.sub.2, that is, where surface energy is high. As a consequence, the functional groups assist in the migration and plating/dissolution of lithium ions.

(98) FIGS. 7A, 7B, and 7C show X-ray diffraction (XRD) patterns of the Nb.sub.2C, Ti.sub.2C, and Ti.sub.3C.sub.2 products prepared by etching Nb.sub.2AlC, Ti.sub.2AlC, and Ti.sub.3AlC.sub.2 with HF for different periods of time in Preparative Examples 1-1, 1-2, and 1-3, respectively.

(99) Referring to FIG. 7A, the etching time dependent peak shifts and variations indicate the characteristics of layered structures arising from the dissolution of Al in hydrofluoric acid. The Nb.sub.2C particles obtained after 3 days under the etching conditions defined in the present invention had layered structures.

(100) Referring to FIGS. 7B and 7C, the etching time dependent peak shifts and variations indicate the characteristics of layered structures arising from the dissolution of Al in hydrofluoric acid. The Ti.sub.2C and Ti.sub.3C.sub.2 particles obtained after 1 day under the etching conditions defined in the present invention had layered structures.

(101) FIG. 8A shows surface scanning electron microscopy (SEM) images of the pristine lithium metal electrode (Pristine Li of Preparative Example 1) and the lithium metal electrode on which the Nb.sub.2C thin film was formed (Nb.sub.2C Li of Example 1-1) after repeated charge/discharge cycles. The rightmost image of FIG. 8 is an enlargement of the middle image. FIG. 8B shows surface scanning electron microscopy (SEM) images of the pristine lithium metal electrode (Pristine Li of Comparative Example 1) (a) and the lithium metal electrode on which the Ti.sub.2C thin film was formed (Ti.sub.2C Li of Example 1-2) (b) after repeated charge/discharge cycles. The rightmost image (c) is an enlargement of the middle image (b). FIG. 8C shows surface scanning electron microscopy (SEM) images of the pristine lithium metal electrode (Pristine Li of Comparative Example 1) and the lithium metal electrode on which the Ti.sub.3C.sub.2 thin film was formed (Ti.sub.3C.sub.2 Li of Example 1-3) after repeated charge/discharge cycles. The rightmost image is an enlargement of the middle image.

(102) Referring to FIGS. 8a, 8b, and 8c, a number of lithium dendrites were formed in Pristine Li, as expected, but the formation of lithium dendrites was clearly suppressed in the lithium metal electrode on which the Nb.sub.2C, Ti.sub.2C or Ti.sub.3C.sub.2 thin film was formed. The rightmost image shows the formation of a uniform SEI.

(103) FIG. 9A shows cycle characteristics and charge/discharge efficiencies of a lithium metal secondary battery using the pristine lithium metal electrode (Pristine Li of Comparative Example 1) as an anode and a lithium metal secondary battery using the lithium metal electrode on which the Nb.sub.2C thin film was formed (Nb.sub.2C Li of Example 1-1) as an anode. FIG. 9B shows cycle characteristics and charge/discharge efficiencies of a lithium metal secondary battery using the pristine lithium metal electrode (Pristine Li of Comparative Example 1) as an anode and a lithium metal secondary battery using the lithium metal electrode on which the Nb.sub.2C thin film was formed (Ti.sub.2C Li of Example 1-2) as an anode. FIG. 9C shows cycle characteristics and charge/discharge efficiencies of a lithium metal secondary battery using the pristine lithium metal electrode (Pristine Li of Comparative Example 1) as an anode and a lithium metal secondary battery using the lithium metal electrode on which the Ti.sub.3C.sub.2 thin film was formed (Ti.sub.3C.sub.2 Li of Example 1-3) as an anode.

(104) Each of the lithium metal secondary batteries was fabricated by the following procedure. First, a cathode plate was produced using LiNiCoMnO.sub.2 (NCM) [8/1/1] as a three-component system. To this end, the synthesized sample (92%), a conductive material (4%), and a binder (4%) composed of polyvinylpyrrolidone (Mw360,000) (2%), polyethylene oxide (Mw1,000,000) (1%), and sodium carboxymethyl cellulose (Mw250,000) (1%) were mixed in water to prepare a slurry. The slurry was coated on an aluminum foil using a doctor blade, dried in an oven at 80 C. for 1 day, and cut into a circular disc, which was used as a cathode. Pristine Li, Nb.sub.2C Li, Ti.sub.2C Li or Ti.sub.3C.sub.2 Li was used as an anode and Celgard 2500 was used as a separator. 0.6 M LiTFSI, 0.4 M LiF, 0.4 M LiBOB, and 0.05 M LiPF.sub.6 were added to a solution of EC and DMC (4:6 wt %) and 1 wt % of FEC and 2 wt % of VC were added thereto to prepare an electrolyte. The cathode active material NCM was loaded at a concentration of 18 mg/cm.sup.2. The cathode, anode, separator, and electrolyte were assembled into a 2032 coin cell. The battery was subjected to a charge/discharge test at 1C using a Maccor battery tester.

(105) Referring to FIG. 9A, a cycle life of only 100 cycles was achieved for the battery using the pristine lithium metal electrode whereas a cycle life of at least 200 cycles was achieved for the battery using the lithium metal electrode on which the Nb.sub.2C thin film was formed.

(106) Referring to FIG. 9B, a cycle life of only 100 cycles was achieved for the battery using the pristine lithium metal electrode whereas a cycle life of at least 200 cycles was achieved for the battery using the lithium metal electrode on which the Ti.sub.2C thin film was formed.

(107) Referring to FIG. 9C, a cycle life of only 400 cycles was achieved for the battery using the pristine lithium metal electrode whereas a cycle life of at least 530 cycles was achieved for the battery using the lithium metal electrode on which the Ti.sub.3C.sub.2 thin film was formed.

(108) FIG. 10 shows coulombic efficiencies of lithium deposition/dissolution reactions in a lithium metal secondary battery using a copper foil coated with the Ti.sub.3C.sub.2 prepared in Preparative Example 1-3 as a cathode and a lithium foil as an anode when charged/discharged at a current density of 1 mA/cm.sup.2 and with a capacity of 1 mAh/cm.sup.2.

(109) Referring to FIG. 10, the coulombic efficiency of the battery using the copper electrode without Ti.sub.3C.sub.2 was drastically reduced after 30 charge/discharge cycles, whereas that of the battery using the Ti.sub.3C.sub.2-coated copper electrode was maintained at 90% even after 250 charge/discharge cycles.

(110) In conclusion, the formation of the Mxene thin film on the lithium metal electrode in the anode of the present invention enables rapid diffusion and stable deposition of lithium ions to inhibit the formation of dendrites. In addition, the presence of the Mxene thin film prevents side reactions between the lithium metal electrode and the electrolyte in the lithium metal secondary battery of the present invention, achieving good stability and high coulombic efficiency of the lithium metal secondary battery.