KR101755121B1 - Lithium metal electrode for lithium secondary battery with safe protective layer and lithium secondary battery comprising the same - Google Patents

Abstract

The lithium metal electrode having a stable protective layer according to the present invention includes a lithium dendrite absorbing material contained in a polymer protective layer and intercalates and absorbs free lithium ions or lithium dendrites generated by charge and discharge, Short circuit is prevented and charge / discharge cycle life characteristics are improved. In addition, since the lithium dendrite absorbent material itself is relatively low in reactivity, the battery stability is improved.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a lithium metal electrode having a stable protective layer, and a lithium secondary battery comprising the same. BACKGROUND OF THE INVENTION [0002]

The present invention relates to a lithium metal electrode having a stable protective layer and a lithium secondary battery including the same, and more particularly, to an electrode having a polymer protective layer on which a lithium dendrite absorbing material is distributed on a lithium metal electrode.

Various devices that require batteries ranging from mobile phones to wireless home appliances and electric vehicles are being developed. With the development of these devices, the demand for secondary batteries is also increasing. Particularly, along with the tendency of miniaturization of electronic products, secondary batteries are also becoming lighter and smaller.

Recently, a lithium metal secondary battery (LMB) has come into the spotlight in accordance with this trend. The lithium metal has a low redox potential (-3.045 V versus the standard hydrogen electrode) and a large weight energy density (3,860 mAhg ^-1 ), which is expected as a cathode material for high capacity secondary batteries.

However, when lithium metal is used as a cathode material, its safety is poor and its life is short. This problem is known to be caused by the short circuit of the battery and the dead lithium due to the growth of the dendritic lithium produced during the charging and discharging of the battery, and many studies have been conducted to solve this problem have.

In the previous research reports, the search for new electrolytes and additives was the main method for suppressing the growth of lithium dendrite. In addition, methods for controlling the concentration, current density, and temperature of lithium salt have been proposed The problem has not been solved.

An object of the present invention is to provide a lithium metal electrode which can inhibit the growth of lithium dendrites and controls the reactivity of the lithium metal and thus has high utilization.

Another object of the present invention is to provide a lithium secondary battery which uses lithium metal as a negative electrode and which has high energy efficiency and excellent safety and lifetime characteristics by repetition of charge / discharge cycles.

An electrode for a lithium secondary battery according to an embodiment of the present invention includes a lithium metal plate; And a polymeric protective layer formed on the lithium metal plate, wherein the polymeric protective layer contains any one of the following materials: a) a carbonaceous material, b) a silicon-based material, and mixtures thereof .

a) a carbonaceous material selected from a crystalline carbon, an amorphous carbon, a carbon composite, and a mixture thereof; And

b) a silicon-containing material selected from silicon (Si), silicon monoxide and mixtures thereof.

The lithium dendrite absorbent material contained in the polymer protective film absorbs and absorbs free lithium particles or lithium dendrites generated by charging and discharging to prevent internal short-circuiting of the battery, . In addition, since the lithium dendrite absorbent material itself is relatively low in reactivity, the battery stability is improved.

In one example, the lithium dendrite absorbent material may be graphite or silicon.

In one example, the thickness of the polymeric protective layer may be 0.1 to 50 탆.

In one example, the size of the lithium dendrite-absorbing material may be 0.01 to 20 [mu] m.

In one example, the polymeric protective layer comprises a polyvinylidene fluoride-hexafluoropropylene copolymer, tetrafluoroethylene, polyvinylidene fluoride, tetrafluoroethylene-hexafluoropropylene copolymer, and mixtures thereof, ≪ / RTI > complexes or copolymers.

In one example, the lithium dendrite absorbent material may be graphite or silicon.

In one example, the thickness of the polymeric protective layer may be 0.1 to 50 탆.

In one example, the size of the lithium dendrite-absorbing material may be 0.01 to 20 [mu] m.

In one example, the polymeric protective layer may be formed by adding a lithium dendritic water absorbing material together with at least one compound selected from the group consisting of aluminum (Al), magnesium (Mg), tin (Sn), manganese dioxide (MnO ₂ ), vanadium pentoxide (V ₂ O ₅ ) Any one selected filler may be further included.

A lithium secondary battery according to an embodiment of the present invention includes a cathode, an anode, and a separator interposed between the anode and the cathode, and uses the lithium metal electrode having the above-described stable protective layer as the cathode.

The lithium dendrite absorbent material contained in the polymer protective film for a lithium secondary battery of the present invention intercalates and absorbs free lithium particles or lithium dendrites generated by charging and discharging to prevent occurrence of internal short circuit of the battery, Discharge cycle life characteristics can be improved.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The singular expressions include plural expressions unless the context clearly dictates otherwise. In the present invention, terms such as "comprises" or "having" are intended to specify that there are stated features, numbers, steps, operations, elements, parts or combinations thereof, and that one or more other features But do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, parts, or combinations thereof.

A lithium metal electrode according to an embodiment of the present invention includes a lithium metal plate and a polymer protective layer formed on the lithium metal plate. In the polymer protective layer, a substance capable of absorbing lithium dendrite is distributed. Hereinafter, each element will be described in detail.

Lithium metal plate

The lithium metal plate may be a lithium metal plate or a metal plate on which a lithium metal thin film is formed on the current collector. The current collector may be any one selected from the group consisting of copper, aluminum, stainless steel, titanium, silver, palladium, nickel, alloys thereof, and combinations thereof. The method of forming the lithium metal thin film is not particularly limited, and a known metal thin film forming method such as a lamination method and a sputtering method can be used. Also, a case where a metal lithium thin film is formed on the metal plate by initial charging after assembling the battery without the lithium thin film in the current collector is also included in the lithium metal plate of the present invention.

The width of the lithium metal plate may be adjusted according to the shape of the electrode to facilitate the production of the electrode. The thickness of the lithium metal plate may be 30 to 500 탆.

Polymer protective layer

The polymer protective layer relatively reduces the reactivity of the lithium metal plate and prevents the lithium metal electrode from being directly exposed to the electrolyte, thereby preventing the formation of an uneven passivation layer. The polymer layer is preferably formed of a material having ionic conductivity and stable in the environment of the battery, and examples thereof include polyvinylidene fluoride-hexafluoropropylene copolymer, tetrafluoroethylene, polyvinylidene fluoride, tetrafluoroethylene, Ethylene-hexafluoropropylene copolymer, a mixture thereof, a complex or a copolymer thereof.

The polymer protective layer may be formed on one side or both sides of the lithium metal sheet and is preferably applied to the entire surface of the electrode with a uniform thickness. The method of forming the polymer protective layer is not particularly limited and may be carried out by various known coating methods. For example, there are methods such as spin coating, doctor blade coating, dip coating, gravure coating, slit die coating, and screen coating. But is not limited thereto.

If the thickness of the polymer protective layer is less than 0.1 탆, it is difficult to exhibit a sufficient protective function of the lithium metal plate and the ion conductivity and electron conductivity are insufficient, resulting in a reduction in battery capacity. On the contrary, when the thickness exceeds 50 탆, Therefore, it is preferable to keep the thickness in the range of 0.1 to 50 mu m.

Lithium dendrite absorbent material

The lithium dendritic water absorbent material is absorbed by reacting with inert lithium or lithium dendrites which are not involved in charging and discharging on the negative electrode to form a lithium intercalated material. As a result, the internal short circuit of the battery is prevented, and the cycle life characteristics at the time of charging and discharging are improved.

It is preferable that the lithium dendritic water absorbent material is any one selected from the group consisting of a) a carbonaceous material, b) a siliconaceous material, and mixtures thereof.

The carbonaceous material may be any one selected from a crystalline carbon, an amorphous carbon, a carbon composite, and a mixture thereof, and the siliconaceous material may be any one selected from silicon (Si), silicon monoxide, Lt; / RTI >

Preferable examples of the carbonaceous material include natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber and the like. Such a carbonaceous material is preferably applicable to the present invention because the diffusion rate of lithium ions is fast and the reaction rate with lithium dendrites is fast.

The shape of the lithium dendritic water absorbent material is not particularly limited, such as spherical shape, plate shape, fiber shape, or amorphous shape.

When the lithium dendrite absorbent material is brought into contact with and agglomerates, a conductive network is formed, so that the conductive network is charged first before the cathode is charged. As a result, the amount of dendrite absorption decreases and the cycle characteristics of the battery may be deteriorated. Therefore, it is preferable that the lithium dendrite-absorbing material is uniformly distributed, so that the application amount is preferably 1 to 5 mg / cm ² per unit area.

The particle diameter of the lithium dendritic water absorbent material is not particularly limited, but if it exceeds 20 m, the uniformity of the electrode surface is lowered and the adhesive strength is lowered. When the particle diameter is less than 0.01 탆, aggregation occurs to form a conductive network So that it is preferable to maintain the thickness in the range of 0.01 to 20 mu m.

According to an embodiment of the present invention, the lithium dendrite absorbent material may further include a filler having a predetermined strength without causing a chemical change in the battery.

The filler may be, for example, aluminum (Al), magnesium (Mg), tin (Sn), manganese dioxide (MnO _2), vanadium pentoxide (V ₂ O ₅₎ and any one selected from a mixture thereof. The fillers can not only reinforce the strength but also form an alloy with lithium to partially assist the lithium dendrite absorption function.

Another aspect of the present invention relates to a lithium secondary battery including the above-described lithium metal electrode having a stable protective layer in a negative electrode.

The lithium secondary battery is not particularly limited and may be classified into a lithium ion battery, a lithium ion polymer battery, and a lithium polymer battery depending on the type of the separator and the electrolyte used. The lithium secondary battery may be classified into a cylindrical shape, a square shape, a coin shape, And can be divided into a bulk type and a thin film type depending on the size, and all of them are included in the present invention.

In one example, the lithium secondary battery can be manufactured by disposing a separator between a cathode and an anode to prepare an electrode assembly, placing the electrode assembly in a case, and injecting a non-aqueous electrolyte containing a lithium salt. Here, as the cathode, the electrode according to the present invention described above can be preferably used.

The separator is made of an insulating thin film having high ion permeability and mechanical strength. The pore diameter of the separator is generally 0.01 to 10 mu m, and the thickness is generally 5 to 300 mu m. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like is used. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

 The electrolyte is composed of a nonaqueous electrolyte and lithium. As the non-aqueous electrolyte, a non-aqueous electrolyte, a solid electrolyte, an inorganic solid electrolyte and the like are used.

Examples of the nonaqueous electrolytic solution include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, Such as tetrahydrofuran, tetrahydrofuran (THF), 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylform Amide, dioxolan, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolan derivatives, sulfolane, methylsulfolane, 1,3-dimethyl- -Ammonic organic solvents such as imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl pyrophosphate and ethyl propionate can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, Polymers containing ionic dissociation groups, and the like can be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) ₂ NLi, may be used, such as chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate.

For the purpose of improving charge / discharge characteristics, flame retardancy, etc., non-aqueous electrolytes may be used in the form of, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. are added It is possible. In some cases, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further added to impart nonflammability, or a carbon dioxide gas may be further added to improve high-temperature storage characteristics.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

( Example  One)

A 20 m thick lithium metal plate was used as the cathode. A polymer protective layer made of a polyvinylidene fluoride hexafluoropropylene copolymer (thickness: 20 mu m) was formed on the lithium metal plate.

In the polymer protective layer, graphite having a particle size of 5 占 퐉 was evenly distributed as a lithium dendrite-absorbing material. Graphite was included in an amount of 10 wt% based on the total weight of the polymer protective layer.

The LCO-based cathode active material, Super-P and PVdF were mixed at a weight ratio of 95: 2.5: 2.5 and then applied to an aluminum current collector to form a cathode active material layer.

A lithium secondary battery was fabricated by preparing an electrode assembly between the prepared positive electrode and negative electrode through a porous polyethylene separator, placing the electrode assembly inside the case, and then injecting an electrolyte.

The electrolyte was prepared by dissolving 1 M LiPF ₆ in an organic solvent composed of EC: EMC (mixing volume ratio = 1: 1).

( Example  2)

Except that 10 wt% of silicon having a particle diameter of 5 탆 was used as a lithium dendrite water absorbing material.

( Example  3)

Except that 5 wt% of graphite having a particle diameter of 5 탆 and 5 wt% of silicon having a particle diameter of 5 탆 were simultaneously used as a lithium dendrite water absorbing material.

( Example  4)

The procedure of Example 1 was repeated except that 10 wt% of silicon having a particle diameter of 10 탆 was used to increase the size of the lithium dendritic water absorbing material.

( Example  5)

The same procedure as in Example 1 was carried out except that the size of the lithium dendritic water absorbing material was reduced and 10 wt% of silicon having a particle diameter of 2 탆 was used.

( Example  6)

And a polymer protective layer made of a polyvinylidene fluoride hexafluoropropylene copolymer having a thickness of 30 탆 was formed by increasing the thickness of the polymer protective layer.

( Example  7)

And a polymer protective layer made of a polyvinylidene fluoride hexafluoropropylene copolymer having a thickness of 10 탆 was formed by reducing the thickness of the polymer protective layer.

( Comparative Example  One)

The procedure of Example 1 was repeated except that the polymer protective layer was not formed.

( Comparative Example  2)

The procedure of Example 1 was repeated except that the dendritic water absorbing material was not included in the polymer protective layer.

[ Experimental Example : Measurement of performance of manufactured lithium secondary battery]

Each of the lithium secondary batteries according to Examples and Comparative Examples was charged with C-rate 1C and DOD (depth of discharge) of 10% discharged from the anode to the cathode, and the cycle of charging was repeated 15 times. The cycle ends, and the efficiency is calculated by the number of cycles. The results are shown in Table 1 below.

Lithium charge / discharge efficiency (%) Example 1 75 Example 2 86 Example 3 81 Example 4 72 Example 5 79 Example 6 61 Example 7 75 Comparative Example 1 63 Comparative Example 2 68

As shown in Table 1, the difference in the lithium dendrite absorption effect between small amounts of graphite and silicon was confirmed through Example 1 using graphite as a polymer protective layer and Example 2 using silicon. In the case of using graphite and silicon, excellent charging / discharging efficiency of 75% or more was exhibited.

Example 3 using graphite and silicon at the same time also demonstrated excellent lithium dendrite absorption effect even when mixed.

As a result of examining the difference in the effect of absorbing lithium dendrite according to the size of silicon through Examples 2, 4 and 5, it was found that lithium absorption did not occur to the inside of silicon when silicon size was increased, And the charge / discharge efficiency of lithium was decreased.

As a result of examining Examples 1, 6 and 7, it was possible to observe the lithium charging / discharging efficiency according to the thickness of the polymer protective layer. As a result, when the polymer protective layer is thick, the cell resistance increases and the efficiency decreases. It was confirmed that the dendrite growth was not sufficiently performed and the lithium charging / discharging efficiency was inferior.

In the case of Comparative Example 1 in which there is no protective layer and in Comparative Example 2 in which the protective layer is provided but not the lithium dendrite absorbing material, the lithium charge / discharge efficiencies are respectively 63% and 68% I could.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, Of the right.

Claims (7)

Lithium metal plate; And
And a polymer protective layer formed on the lithium metal plate,
Wherein at least one lithium dendrite absorbing material selected from graphite, silicon, and mixtures thereof is distributed in the polymer protective layer,
The polymer protective layer may be any one selected from polyvinylidene fluoride-hexafluoropropylene copolymer, tetrafluoroethylene, tetrafluoroethylene-hexafluoropropylene copolymer, and mixtures, complexes and copolymers thereof One
A lithium metal electrode having a stable protective layer. delete delete The method according to claim 1,
Wherein the polymer protective layer has a thickness of 0.1 to 50 占 퐉. The method according to claim 1,
Characterized in that the size of the lithium dendrite-absorbing material is 0.01 to 20 占 퐉. The method according to claim 1,
With lithium dendrites absorbent material in the polymer protective layer of aluminum (Al), magnesium (Mg), tin (Sn), manganese dioxide (MnO _2), vanadium pentoxide (V ₂ O ₅₎ and any one selected from a mixture thereof A lithium metal electrode having a stable protective layer, characterized by further comprising a filler. And a separator interposed between the positive electrode and the negative electrode, wherein the negative electrode is a lithium metal electrode having a stable protective layer according to any one of claims 1, 4, 5, and 6, In lithium secondary battery.