TECHNICAL FIELD

The present application claims the benefits of Korean Patent Application No. 10-2017-0133772 filed on Oct. 16, 2017 and Korean Patent Application No. 10-2018-0122935 filed on Oct. 16, 2018, all the contents of which are incorporated herein by reference.

The present invention relates to a lithium electrode containing a lithium metal layer capable of improving the energy density of a battery by having a uniform thin film shape and a lithium secondary battery comprising the lithium electrode.

BACKGROUND ART

Until a recent date, there has been considerable interest in developing batteries with high energy densities using lithium as a negative electrode. For example, as compared to other electrochemical systems with a lithium inserted carbon negative electrode and a nickel or cadmium electrode that reduce the energy density of the battery by increasing the weight and volume of the negative electrode due to the presence of the non-electroactive material, since lithium metal has low weight and high capacity characteristics, lithium metal has attracted much attention as a negative electrode active material for electrochemical batteries. Lithium metal negative electrode, or negative electrodes, which mainly comprise lithium metal, provide the opportunity to construct a battery that is lighter and has a higher energy density than the battery such as a lithium-ion, nickel metal hydride or nickel-cadmium battery. These features are highly desirable for batteries for portable electronic devices, such as cell phones and lap-top computers, where premiums are paid with low weighted value.

Conventional lithium ion batteries have an energy density of 700 wh/l by using graphite as a negative electrode and using lithium cobalt oxide (LCO) as a positive electrode. However, in recent years, the fields requiring high energy density are expanding, and thus there is a continuing need to increase the energy density of a lithium ion battery. For example, even in order to increase the mileage of an electric car by a single charge to more than 500 km, an increase in energy density is required.

In order to increase the energy density of the lithium ion battery, the use of lithium electrode is increasing. However, there is a problem that lithium metal is difficult to handle in process because it is highly reactive and difficult to handle.

An existing lithium electrode with lithium metal as a raw material was manufactured by depositing lithium directly on the Cu foil corresponding to the current collector. This method of directly depositing lithium metal on the current collector has an advantage that the process is simple and efficient, but it is not easy to manufacture a real lithium electrode due to the following two reasons.

First, the Cu foil has a thickness of 10 μm, and the thickness is gradually getting thinner in order to increase the volume energy density (the energy per unit volume of the battery). In recent years, the Cu foil having a thickness of 6 μm has also been used for mass production. However, the Cu foil of such a thin film is difficult to handle, and as the thickness of the Cu foil becomes thin, there is a great risk of fracture, and there is a high possibility that the Cu foil will cause folding or wrinkling during driving.

Second, in order to manufacture a pouch-type cell, since lithium must be deposited on both sides of the Cu foil, the process should be proceeded in the order of first depositing lithium on one side of the Cu foil and then depositing lithium on the opposite side in the process. However, in these cases, although the deposition conditions of the two evaporators are the same, when considering the sensitivity of the deposition process, it is difficult to equalize the morphologies of lithium deposited on both sides of the Cu foil and battery performance. In addition, since the first deposited lithium has to undergo another heating step at the time of the second deposition, lithium on both sides cannot be said to be deposited under the same conditions.

Korean Patent Registration No. 2011-0017214 relates to a lithium metal capacitor of an energy storage device and describes that a lithium thin film can be formed on both sides of a current collector. However, since the lithium thin film is formed by deposition, there are problems that the morphologies of lithium deposited on both sides of the current collector cannot be the same and the battery performance will be adversely affected as mentioned above.

Therefore, there is a continuing need to develop a technique for manufacturing lithium electrodes having a thin and uniform thickness, which can be manufactured by a method for preventing the degradation phenomenon of the physical properties of lithium by the process conditions of the lithium electrode and can minimize the formation of the oxide layers by protecting lithium from moisture and outside air during the manufacturing process.

PRIOR ART DOCUMENT

(Patent Document 1) Korean Patent Laid-open Publication No. 2011-0017214, “Lithium metal capacitor of Energy storage device and manufacturing method therefor”.(Patent Document 2) Japanese Patent Registration No. 6005938, “Electrode composition and method”.

DISCLOSURE

Technical Problem

As a result of various studies to solve the above problems, one embodiment of the present invention relates to a lithium electrode having a thin and uniform thickness which can be manufactured by first forming, when manufacturing a lithium electrode, a protective layer capable of protecting lithium metal on the substrate, and depositing lithium metal on the protective layer and then transferring it to the Cu current collector, and the energy density of the lithium secondary battery using the lithium electrode thus prepared is improved.

Therefore, one embodiment of the present invention is to provide a lithium electrode having a thin and uniform thickness by minimizing the formation of an oxide layer due to the production by the transfer process.

Another embodiment of the present invention is to provide a lithium secondary battery comprising the lithium electrode having the thin and uniform thickness.

Technical Solution

In order to achieve the above objects, the present invention provides a lithium electrode comprising a current collector; a lithium metal layer on at least one side of the current collector; and a protective layer on the lithium metal layer.

The lithium electrode may further comprise an SiO coating layer between the current collector and the lithium metal layer.

The SiO coating layer may be a mixed coating layer including SiO and graphite.

The SiO and graphite may be mixed in a weight ratio of 1:1 to 1:9.

The current collector may be selected from the group consisting of copper, aluminum, nickel, titanium, sintered carbon, aluminum-cadmium allow, and stainless steel. The stainless steel may be surface treated with carbon, nickel, titanium, or silver.

The thickness of the lithium metal layer may be 1 to 50 μm.

The protective layer may comprise at least one selected from the group consisting of poly vinylidene fluoride (PVDF), poly vinylidene fluoride-hexafluoroethylne copolymer (PVDF-HFP copolymer), cyclo olefin polymer, cyclo olefin copolymer and styrene butadiene rubber-carboxymethyl cellulose (SBR-CMC).

The lithium electrode may further comprise a substrate formed on the protective layer.

The substrate may have a release layer formed on at least one side thereof.

The release layer may comprise at least one selected from the group consisting of Si, melamine, and fluorine. The release layer may comprise at least one selected from the group consisting of a silicone-based resin, a melamine-based resin and a fluorine-based resin.

The substrate may comprise at least one selected from the group consisting of polyethylene terephthalate (PET), polyimide (PI), poly(methylmethacrylate) (PMMA), cellulose tri-acetate (TAC), polypropylene, polyethylene and polycarbonate.

The present invention also provides a lithium electrode comprising a current collector; a lithiated SiO coating layer formed on at least one side of the current collector; and a protective layer formed on the lithiated SiO coating layer.

Advantageous Effects

The lithium electrode according to the present invention has the protective layer formed on the lithium metal layer and thus may protect lithium from outside air and moisture.

Also, the present invention may manufacture the lithium electrode in which the current collector, the lithium metal layer, and the lithium metal protective layer are sequentially stacked by depositing lithium metal on the lithium metal protective layer and then transferring lithium metal onto the current collector and thus may prevent deterioration of the physical properties of lithium metal due to deposition process conditions.

Also, the lithium electrode according to the present invention may have a thin and uniform thickness by protecting the lithium metal from exposure to external environments such as moisture or outside air during the manufacturing process by the protective layer and thus minimizing the formation of an oxide layer (native layer) on the surface of lithium metal.

Also, since a method of forming the lithium metal layer on the current collector by transfer without directly depositing lithium metal on the current collector is used, it is possible to compensate for the problem of the current collector that is liable to break during the deposition process and thus it is possible to manufacture lithium electrodes using various types of current collectors.

