Electroplating solution for lithium metal, and method for manufacturing lithium metal electrode by using same

Abstract

An electroplating solution for lithium metal, and a method for preparing a lithium metal electrode using the same, and in particular, while preparing a lithium metal electrode using electroplating, a lithium metal electrode having enhanced surface properties may be prepared by electroplating using a plating solution including a lithium nitrogen oxide and a metal nitrogen oxide, and, by using such a lithium metal electrode in a battery, lifetime properties of the battery may be enhanced.

Claims

1. A method for preparing a lithium metal electrode by electroplating, the method comprising: providing a cathode; providing an anode selected from the group consisting of a lithium ingot and a lithium transition metal oxide; providing an electroplating solution between the anode and the cathode, wherein the electroplating solution comprises: an ether-based solvent; a lithium salt; a lithium nitrogen oxide; and an additive represented by the following Chemical Formula 1:
MNO.sub.x<Chemical Formula 1> wherein, in Chemical Formula 1, M is Cs, Rb, K, Ba, Sr, Ca, Na or Mg, and x is 2 or 3; and applying a current between the anode and the cathode to electroplate lithium onto the cathode and provide an additional source of lithium ions into the electroplating solution from the anode.

2. The method of claim 1, wherein a concentration of the lithium salt in the electroplating solution is 1 M to 7 M.

3. The method of claim 1, wherein a content of the lithium nitrogen oxide in the electroplating solution is 1% by weight to 5% by weight based upon a total weight of the electroplating solution.

4. The method of claim 1, wherein Li.sup.+ of the lithium nitrogen oxide and M.sup.+ of the additive represented by Chemical Formula 1 have a concentration ratio ([Li.sup.+]/[M.sup.+]) of between 10 to 40.

5. The method of claim 1, wherein the ether-based solvent is one or more types selected from the group consisting of tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl ether and dibutyl ether.

6. The method of claim 1, wherein the lithium salt is one or more types selected from the group consisting of LiFSi, LiPF.sub.6, LiCl, LiBr, LiI, LiClO.sub.4, LiBF.sub.4, LiB.sub.10Cl.sub.10, LiCF.sub.3SO.sub.3, LiCF.sub.3CO.sub.2, LiAsF.sub.6, LiSbF.sub.6, LiPF.sub.6, LiAICl.sub.4, CH.sub.3SO.sub.3Li, CF.sub.3SO.sub.3Li, (CF.sub.3SO.sub.2).sub.2NLi, chloroborane lithium and lithium tetraphenylborate.

7. The method of claim 1, wherein the lithium nitrogen oxide is one or more types selected from the group consisting of lithium nitrate (LiNO.sub.3) and lithium nitrite (LiNO.sub.2).

8. The method of claim 1, wherein the additive is one or more types selected from the group consisting of potassium nitrate (KNOB), cesium nitrate (CsNO.sub.3), magnesium nitrate (MgNO.sub.3), barium nitrate (BaNO.sub.3), potassium nitrite (KNO.sub.2) and cesium nitrite (CsNO.sub.2).

9. The method of claim 1, wherein the lithium nitrogen oxide and the additive are lithium nitrate (LiNO.sub.3) and cesium nitrate (CsNO.sub.3), respectively.

10. The method of claim 1, wherein the cathode is a current collector selected from the group consisting of Cu, Al, Ni, Fe, stainless steel and Ti.

11. The method of claim 1, wherein the cathode has a three-dimensional structure form.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a mimetic diagram of a lithium half-cell capable of carrying out electroplating according to one embodiment of the present invention.

(2) FIG. 2 is a mimetic diagram illustrating a lithium source according to one embodiment of the present invention.

(3) FIG. 3 shows scanning electron microscope (SEM) images of surfaces of lithium metal electrodes each prepared in Examples 1 to 4 and Comparative Examples 1 to 4.

BEST MODE

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

(5) 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.

(6) Electroplating Solution for Lithium Metal

(7) The present invention relates to an electroplating solution for lithium metal, which comprises an ether-based solvent; a lithium salt; a lithium nitrogen oxide; and an additive represented by the following Chemical Formula 1.
MNO.sub.X<Chemical Formula 1>

(8) In Chemical Formula 1, M is Cs, Rb, K, Ba, Sr, Ca, Na or Mg, and x is 2 or 3.

(9) The electroplating solution of the present invention is used for electroplating lithium metal, and particularly, may be used for, among electroplating, electroplating using one or more types selected from the group consisting of a lithium salt (Li Salt), lithium ingot (Li ingot) and a transition metal oxide as a lithium metal source, but is not limited thereto as long as it is a compound capable of providing lithium ions (FIG. 2).

(10) In the present invention, the lithium salt may be one or more types selected from the group consisting of LiPF.sub.6, LiClO.sub.4, LiAsF.sub.6, LiBF.sub.4, LiSbF.sub.6, LiAlO.sub.4, LiAlCl.sub.4, LiCF.sub.3SO.sub.3, LiC.sub.4F9SO.sub.3, LiN(C.sub.2F.sub.5SO.sub.3).sub.2, LiN(C.sub.2F.sub.5SO.sub.2).sub.2, LiN(CF.sub.3SO.sub.2) LiN(CaF.sub.2a+1SO.sub.2) (CbF.sub.2b+1SO.sub.2) (herein, a and b are natural numbers, and preferably 1a20 and 1b20), LiCl, LiI and LiB(C.sub.2O.sub.4).sub.2.

(11) The transition metal oxide may be one or more types selected from the group consisting of LiMO.sub.2 (M is Co, Ni or Mn), Li.sub.1+1Mn.sub.2xO.sub.4.sup.+ (0x0.3) and LiNi.sub.1xM.sub.xO.sub.2 (M is Co, Mn, Al, Cu, Fe, Mg, B or Ga, and 0.01x0.3). For example, the lithium metal oxide may be LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2O.sub.4, Li(Ni.sub.aMn.sub.bCo.sub.c)O.sub.2 (a+b+c=1), LiNi.sub.0.5Mn.sub.1.5O.sub.4 or LiNi.sub.0.5Mn.sub.0.5O.sub.2.

(12) In the present invention, the ether-based solvent is a non-aqueous solvent for forming the ether-based plating solution, and may be one or more types selected from the group consisting of tetrahydrofuran (THF), 2-methyltetrahydrofuran (MTHF), dimethyl ether (DME) and dibutyl ether (DBE). Particularly, using dimethyl ether (DME) may be advantageous in electroplating lithium metal on a current collector.

(13) In the present invention, the lithium salt may be one or more types selected from the group consisting of one or more selected from the group consisting of LiFSI, LiPF.sub.6, LiCl, LiBr, LiI, LiClO.sub.4, LiBF.sub.4, LiB.sub.10Cl.sub.10, LiPF.sub.6, LiCF.sub.3SO.sub.3, LiCF.sub.3CO.sub.2, LiAsF.sub.6, LiSbF.sub.6, LiPF.sub.6, LiAlCl.sub.4, CH.sub.3SO.sub.3Li, CF.sub.3SO.sub.3Li, (CF.sub.3SO.sub.2).sub.2NLi, chloroborane lithium and lithium tetraphenylborate, and particularly, using LiFSI may be advantageous in electroplating lithium metal on a current collector.

(14) In addition, the concentration of the lithium salt may be properly controlled depending on a composition of the electroplating solution, and for example, may be from 1.0 M to 7.0 M and preferably from 1 M to 4 M. When the lithium salt is in less than 1.0 M, conductivity of the electroplating solution is not favorable, and full high-rate discharge properties and lifetime properties may decline, and when the concentration is greater than 7.0 M, low-temperature discharge properties and high-rate discharge properties are not favorable, and usage properties as an actual plating solution may decline.

(15) In the present invention, the electroplating solution may form a stable film on a lithium metal electrode by the lithium nitrogen oxide and the additive represented by Chemical Formula 1 having NO bonds in the molecules, and accordingly, stability of the lithium metal electrode and the electroplating solution is more improved as side reactions between the lithium metal and the electroplating solution are suppressed, and as a result, a battery lifetime may be greatly enhanced.

(16) For example, the lithium nitrogen oxide may be one or more types selected from the group consisting of lithium nitrate (LiNO.sub.3) and lithium nitrite (LiNO.sub.2), and the additive may be one or more types selected from the group consisting of potassium nitrate (KNO.sub.3), cesium nitrate (CsNO.sub.3), magnesium nitrate (MgNO.sub.3), barium nitrate (BaNO.sub.3), potassium nitrite (KNO.sub.2) and cesium nitrite (CsNO.sub.2).

(17) The content of the lithium nitrogen oxide included may be from 1% by weight to 5% by weight based on the total weight of the electroplating solution. When the lithium nitrogen oxide content is less than 1% by weight, a product (Li.sub.xNO.sub.y) amount is excessively small leading to a problem of insufficient thickness as a protective layer, and when the content is greater than 5% by weight, lithium, an active material, is excessively consumed when producing a protective layer leading to a problem of efficiency decline and the like.

(18) In the electroplating solution, the amounts of the lithium nitrogen oxide and the additive represented by Chemical Formula 1 used may be defined by a concentration ratio of the lithium nitrogen oxide-derived Li.sup.+ and the additive represented by Chemical Formula 1-derived M.sup.+ ([Li.sup.+]/[M.sup.+]).

(19) The concentration ratio ([Li.sup.+]/[M.sup.+]) may be 10 or greater. When the concentration ratio is less than the above-mentioned range, M+ is not present in an ionic state and is reduced, which makes surface smoothness difficult due to an insignificant lithium dendrite suppression effect. The concentration ratio ([Li.sup.+]/[M.sup.+]) is preferably from 10 to 40.

(20) Particularly, the electroplating solution comprising lithium nitrate (LiNO.sub.3) and cesium nitrate (CsNO.sub.3) as the lithium nitrogen oxide and the additive, respectively, may be advantageous in smoothing the lithium metal electrode surface.

(21) When comprising cesium nitrate (CsNO.sub.3) as the additive, the concentration thereof in the electroplating solution may be 0.1 M or less per 1 M Li.sup.+. For example, reduction potential of the Cs.sup.+ ions varies depending on the concentration, and when the Cs.sup.+ concentration calculated by the Nernst equation is greater than 0.1 M per 1 M Li.sup.+, the Cs.sup.+ ions have higher reduction potential than the Li.sup.+ ions and are reduced before the Li.sup.+ ions. The Cs.sup.+ needs to be present in an ionic state to perform a role of suppressing dendrite as a leveler, and therefore, the Cs.sup.+ concentration may be 0.1 M or less per 1 M Li.sup.+ ions, and preferably, the Cs.sup.+ concentration may be from 0.03 M to 0.07 M per 1 M Li.sup.+.

(22) Method for Preparing Lithium Metal Electrode

(23) The present invention also relates to a method for preparing a lithium metal electrode using electroplating, and relates to a method for preparing a lithium metal electrode having a surface of the prepared lithium metal electrode controlled depending on a concentration of a plating solution used for the electroplating.

(24) According to one embodiment of the present invention, electroplating may be carried out using a lithium half-cell.

(25) FIG. 1 is a mimetic diagram of a half-cell (1) capable of carrying out electroplating according to one embodiment of the present invention.

(26) When referring to FIG. 1, a lithium metal electrode may be prepared using a Cu current collector (10) as a negative electrode, a lithium metal source (20) as a positive electrode, and an ether-based plating solution (30), and by electroplating lithium metal (40) on the Cu current collector (10).

(27) Herein, a specific condition of the electroplating may use a current with a C-rate of 0.01 C to 0.5 C and current density of 0.1 mAh/cm.sup.2 to 5 mAh/cm.sup.2. When the electroplating condition is outside such a condition, surface properties of a lithium metal electrode formed when electroplating lithium metal may decline. In other words, problems such that the lithium metal electrode surface is not smoothly electroplated, or the electroplated thickness becomes thick may occur.

(28) The ether-based plating solution (30) is the same as the electroplating solution for lithium metal described above.

(29) The lithium metal source (20) is also the same as described above.

(30) In the present invention, the current collector capable of electroplating lithium metal may be selected from the group consisting of Cu, Al, Ni, Fe, steel use stainless (SUS) and Ti, and the current collector may have a three-dimensional structure form.

(31) Such a current collector has not been able to be used in existing rolling processes used for lithium metal, and has an advantage of using more diverse current collectors obtained by electroplating using an ether-based plating solution.

(32) A lithium metal electrode prepared using the electroplating method as described above may have a smoother surface due to reduced surface roughness. When using a lithium metal electrode having a smooth surface, battery driving safety may be enhanced by preventing lithium growth occurring during charge and discharge growing in a needle shape, a cause of internal short circuit.

(33) In addition, the thickness of the plated lithium metal is readily controlled by directly electroplating lithium metal on the current collector. Accordingly, the lithium metal may be electroplated to a small thickness that has not been able to be prepared using existing rolling processes, and as a result, a lithium metal electrode having a thickness of 20 m or less, a smallest thickness produced by rolling, may be prepared.

(34) In addition, the shape of lithium dendrite formed on the lithium metal electrode surface may also be controlled depending on a composition of the ether-based plating solution used for electroplating. When the lithium dendrite has a needle shape, the lithium dendrite readily falls off from an electrode losing electrical conductivity, which increases the possibility of becoming dead lithium causing a decrease in the efficiency. In addition, a needle shape penetrates a separator and causes a short-circuit, which may cause problems such as a fire due to excessive heat.

(35) As above, by controlling surface properties of the lithium metal electrode such as roughness, the degree of smoothness, a thickness or lithium dendrite shape through varying a composition of the ether-based plating solution, lifetime properties of a battery using the lithium metal electrode may be enhanced.

(36) Hereinafter, preferred examples will be provided in order to illuminate the present invention, however, the following examples are for illustrative purposes only, and it will be obvious to those skilled in the art that various changes and modifications may be made within the category and technological ideas of the present invention, and such changes and modifications also belong to the scope of the attached claims.

(37) In the following Examples 1 to 4 and Comparative Examples 1 to 4, lithium metal was plated on a Cu current collector by electroplating, wherein a composition of a plating solution used for the electroplating varied as described in the following Table 1 when preparing a lithium metal electrode.

(38) TABLE-US-00001 TABLE 1 Plating Solution Concen- tration Lit- Lithium Ratio.sup.note1) hium Nitrogen ([Li.sup.+]/ Solvent Salt Oxide Additive [M.sup.+]) Example 1 DME LiFSI LiNO.sub.3 CsNO.sub.3 20 (3M) (2% by (0.15M) weight) Example 2 DME LiFSI LiNO.sub.3 CsNO.sub.3 20 (1M) (2% by (0.15M) weight) Example 3 DME LiFSI LiNO.sub.3 CsNO.sub.3 60 (3M) (2% by (0.05M) weight) Example 4 DME LiFSI LiNO.sub.3 CsNO.sub.3 10 (3M) (2% by (0.3M) weight) Comparative EC:DEC:DMC LiPF.sub.6 VC (2% Example 1 (1M) by weight) Comparative DME LiFSI LiNO.sub.3 Example 2 (1M) (2% by weight) Comparative DME LiFSI LiNO.sub.3 Example 3 (3M) (2% by weight) Comparative DME LiFSI CsNO.sub.3 Example 4 (3M) (0.15M) .sup.note1)Concentration ratio ([Li.sup.+]/[M.sup.+]) means a concentration ratio of Li.sup.+ of the lithium nitrogen oxide and Cs.sup.+ of the additive

Example 1

(39) A lithium metal electrode was prepared by plating lithium metal on a Cu current collector through electroplating.

(40) Herein, as for the electroplating solution, a plating solution prepared by dissolving LiFSI, a lithium salt, in dimethyl ether (DME), an ether-based solvent, in 3 M, adding LiNO.sub.3, a lithium nitrogen oxide, in 2% by weight based on the total plating solution weight, and employing the LiNO.sub.3-derived Li.sup.+ and CsNO.sub.3-derived Cs.sup.+, an additive, to have a concentration ratio ([Li.sup.+]/[Cs.sup.+]) of 20 was used (Table 1).

(41) In addition, electroplating was carried out using a lithium half-cell employing the Cu current collector as a negative electrode, and including a positive electrode including LiCoO.sub.2 as a lithium source, a polyethylene separator provided between the positive electrode and the negative electrode, and the electroplating solution, through flowing a current with a C-rate of 0.2 C (0.95 mA) and current density of 3 mA/cm.sup.2.

Example 2

(42) A lithium metal electrode was prepared in the same manner as in Example 1, except that electroplating was carried out by dissolving LiFSI, the lithium salt, in 1 M.

Example 3

(43) A lithium electrode was prepared in the same manner as in Example 1, except that a plating solution prepared so that the LiNO.sub.3-derived Li.sup.+ and the CsNO.sub.3-derived Cs.sup.+, the additive, had a concentration ratio ([Li.sup.+]/[Cs.sup.+]) of 60 was used.

Example 4

(44) A lithium electrode was prepared in the same manner as in Example 1, except that a plating solution prepared so that the LiNO.sub.3-derived Li.sup.+ and the CsNO.sub.3-derived Cs.sup.+, the additive, had a concentration ratio ([Li.sup.+]/[Cs.sup.+]) of 10 was used.

Comparative Example 1

(45) A lithium metal electrode was prepared in the same manner as in Example 1 except that, as for the electroplating solution, a plating solution prepared by, as described in Table 1, dissolving LiPF6, a lithium salt, in EC:DEC:DMC (25:50:25 v/v), a carbonate-based solvent, in 1 M, and then dissolving vinylene carbonate (VC) in 2% by weight based on the total plating solution weight was used. Herein, EC is ethylene carbonate, DEC is diethylene carbonate and DMC is dimethylene carbonate.

Comparative Example 2

(46) A lithium metal electrode was prepared in the same manner as in Example 1 except that, as for the electroplating solution, a plating solution prepared by, as described in Table 1, dissolving LiFSI, a lithium salt, in dimethyl ether (DME), an ether-based solvent, in 1 M, and adding LiNO.sub.3, a lithium nitrogen oxide, in 2% by weight without using the additive was used.

Comparative Example 3

(47) A lithium metal electrode was prepared in the same manner as in Example 1 except that, as for the electroplating solution, a plating solution prepared by, as described in Table 1, dissolving LiFSI, a lithium salt, in dimethyl ether (DME), an ether-based solvent, in 2 M, and adding LiNO.sub.3, a lithium nitrogen oxide, in 2% by weight without using the additive was used.

Comparative Example 4

(48) A lithium metal electrode was prepared in the same manner as in Example 1 except that, as for the electroplating solution, a plating solution prepared by, as described in Table 1, dissolving LiFSI, a lithium salt, in dimethyl ether (DME), an ether-based solvent, in 3 M, and adding CsNO.sub.3, an additive, in 0.15 M without using LiNO.sub.3, a lithium nitrogen oxide, was used.

Experimental Example 1: Comparison of Surface Properties of Lithium Metal Electrodes

(49) Surface properties of the lithium metal electrodes each prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were observed.

(50) FIG. 3 shows scanning electron microscope (SEM) images of surfaces of the lithium metal electrodes each prepared in Examples 1 to 4 and Comparative Examples 1 to 4.

(51) When referring to FIG. 3, it was seen that the surface of the lithium metal electrode prepared in Example 1 was relatively smoothly formed compared to the surfaces of the lithium metal electrodes prepared in Comparative Examples 1 to 4.

(52) From this, it was seen that surface properties of the lithium metal electrode electroplated using the electroplating solution comprising both LiNO.sub.3 and CsNO.sub.3.

(53) In addition, when referring to Examples 1 to 4, it was seen that, even when using a plating solution comprising both LiNO.sub.3 and CsNO.sub.3, surface properties of the lithium metal electrode varied depending on the concentration ratio ([Li.sup.+]/[Cs.sup.+]) of the LiNO.sub.3-derived Li.sup.+ and CsNO.sub.3-derived Cs.sup.+, an additive.

(54) For example, it was seen that surface properties of the lithium metal electrode somewhat declined when the concentration ratio ([Li.sup.+]/[Cs.sup.+]) was relatively high as in Example 3.

(55) In addition, among Comparative Examples 1 to 4, Comparative Example 1 using a carbonate-based solvent had a least favorable degree of surface smoothness, and lithium dendrite was observed in a needle shape.

(56) When lithium dendrite has a needle shape, the lithium dendrite readily falls off from an electrode losing electrical conductivity, which increases possibility of being dead lithium causing a decrease in the efficiency. In addition, a needle-shaped lithium dendrite penetrates a separator and causes a short-circuit, which may cause problems such as fires due to excessive heat. Accordingly, when dendrite smoothly grows, possibility of the dendrite falling off from an electrode and losing functions as an active material is reduced increasing lithium metal efficiency, and safety may be greatly enhanced by preventing a short-circuit occurring when destroying a separator.

(57) Hereinbefore, the present invention has been described with reference to limited embodiments 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 and the range of equivalents of the attached claims of the present invention.

REFERENCE NUMERAL

(58) 1: Lithium Half-Cell 10: Current Collector 20: Lithium Source 30: Ether-Based Plating Solution 40: Lithium Metal