KR20200076076A - Negative electrode comprising magnesium protective layer, method for manufacturing and lithium secondary battery comprising thereof - Google Patents

Abstract

The present invention relates to a negative electrode for a lithium secondary battery including a magnesium protective layer formed on at least one surface of a lithium metal layer, a manufacturing method thereof, and a lithium secondary battery including the same.

Description

A negative electrode comprising a magnesium protective layer, a method for manufacturing the same, and a lithium secondary battery comprising the same

The present invention relates to a negative electrode for a lithium secondary battery comprising a magnesium protective layer, a manufacturing method thereof, and a lithium secondary battery comprising the same.

Recently, interest in energy storage technology has been increasing. Mobile phones, camcorders and notebook PCs, and furthermore, the field of application to the energy of electric vehicles is expanding, and efforts for research and development of electrochemical devices are gradually becoming concrete.

Electrochemical devices are the areas that are receiving the most attention in this aspect, and among them, the development of secondary batteries that can be charged and discharged has become a focus of attention, and recently, in order to improve capacity density and energy efficiency in developing such batteries. Research and development on the design of new electrodes and batteries is in progress.

Among the currently applied secondary batteries, lithium secondary batteries developed in the early 1990s have the advantage of higher operating voltage and higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries using aqueous electrolyte solutions. Is in the limelight.

The lithium secondary battery has a structure in which an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode is stacked or wound, and this electrode assembly is built into a battery case and a non-aqueous electrolyte is injected therein. do. The lithium secondary battery produces electrical energy by oxidation and reduction reactions when lithium ions are inserted/detached from the positive electrode and the negative electrode.

Usually, lithium metal, carbon, and the like are used as the negative electrode of a lithium secondary battery, and lithium oxide, a transition metal oxide, a metal chalcogen compound, and a conductive polymer are used as the active material. Lithium secondary batteries using lithium metal as a negative electrode mostly attach lithium foil to a copper current collector or use the lithium metal sheet itself as an electrode. Lithium metal has received great attention as a high-capacity negative electrode material because of its low potential and large capacity.

When lithium metal is used as the negative electrode, electron density non-uniformity may occur on the surface of the lithium metal due to various factors when driving the battery. Accordingly, a lithium dendrite in the form of a tree branch is formed on the surface of the electrode, and a protrusion is formed or grown on the surface of the electrode, resulting in a very rough electrode surface. Such lithium dendrites cause damage to the separator and short circuit of the battery in severe cases along with deterioration of the performance of the battery. As a result, the temperature in the battery rises, and there is a risk of battery explosion and fire.

In order to solve this, studies have been conducted to introduce a polymer protective layer or an inorganic solid protective layer to the lithium metal layer, to increase the salt concentration of the electrolyte, or to apply an appropriate additive. However, the effect of suppressing lithium dendrites in these studies is insignificant. Therefore, it may be an effective alternative to solve the problem by applying a protective material of the lithium metal anode.

KR 2018-0053481

As described above, lithium dendrites of lithium secondary batteries are precipitated on the surface of the negative electrode, thereby causing the cell to expand in volume. As a result, the present inventor has conducted various studies and found out that the problem caused by lithium dendrites can be solved by applying a magnesium protective layer to the lithium metal layer, thereby completing the present invention.

Accordingly, an object of the present invention is to provide a negative electrode for a lithium secondary battery comprising a magnesium protective layer.

In addition, an object of the present invention is to provide a lithium secondary battery including the negative electrode.

In order to achieve the above object,

The present invention is a lithium metal layer; And

It provides a negative electrode for a lithium secondary battery including a magnesium protective layer formed on at least one surface of the lithium metal layer.

In addition, the present invention provides a negative electrode manufacturing method for a lithium secondary battery comprising the step of forming a magnesium protective layer on at least one side of the lithium metal layer.

In addition, the present invention is an anode; The negative electrode of the present invention; And a separator interposed between the anode and the cathode. And it provides a lithium secondary battery comprising an electrolyte.

The negative electrode for a lithium secondary battery of the present invention can suppress the growth of lithium dendrites, thereby obtaining a negative electrode surface having a uniform shape. Therefore, it is possible to prevent an electrolyte shortage due to an increase in electrode volume and an increase in cathode porosity caused by uneven growth of lithium dendrites, and to prevent short circuits of the battery, thereby improving stability and lifespan characteristics. Can provide.

1 is an image taken with a shape measurement laser microscope (KEYENCE) on the surface of the negative electrode for a lithium secondary battery prepared in Example 1.
2 is a photograph observing the surface of the negative electrode after the initial discharge of the lithium-sulfur battery of Example 1.
3 is a photograph observing the surface of the negative electrode after the initial discharge of the lithium-sulfur battery of Comparative Example.
FIG. 4 is an image taken by a shape measurement laser microscope (KEYENCE) on the surface of the negative electrode after the initial discharge of the lithium-sulfur battery of Example 1.
5 is an image taken by a shape measurement laser microscope (KEYENCE) on the surface of the negative electrode after the initial discharge of the lithium-sulfur battery of Comparative Example 1.
6 is an initial discharge capacity graph of the lithium-sulfur batteries of Example 1 and Comparative Example 1.
7 is an initial discharge capacity graph of the lithium-sulfur batteries of Example 2 and Comparative Example 2.
8 is an initial discharge capacity graph of the lithium-sulfur batteries of Example 3 and Comparative Example 2.
9 is a graph of initial discharge capacity of the lithium-sulfur batteries of Comparative Examples 2 to 5.
10 is a graph of the life characteristics of the lithium-sulfur batteries of Example 1 and Comparative Example 1.
11 is a graph of the life characteristics of the lithium-sulfur batteries of Example 2 and Comparative Example 2.
12 is a graph of the life characteristics of the lithium-sulfur batteries of Example 3 and Comparative Example 2.
13 is a graph of the life characteristics of the lithium-sulfur batteries of Comparative Examples 2 to 5.

Hereinafter, the present invention will be described in more detail.

The main cause of the early deterioration of the lithium secondary battery is a decrease in efficiency of the lithium anode. When lithium metal is used as the negative electrode, not only the reaction is not uniform due to the non-uniform surface oxide layer, and lithium dendrites grow during charging (Li plating), so that dead lithium is easily generated, Lithium, which can participate in the reaction, is consumed and the efficiency of the lithium anode is reduced. In addition, as the lithium dendrite continues to grow, the volume of the negative electrode increases, a phenomenon in which an electrolyte is insufficient due to an increase in the negative electrode porosity occurs, and when the lithium dendrite grows through a separator, a short circuit of the battery may occur. There is a problem that arises.

Accordingly, the present invention was to provide a negative electrode for a lithium secondary battery that can prevent the growth of lithium dendrites and improve the life characteristics of the battery.

Cathode for lithium secondary battery

The present invention is a lithium metal layer; And

It relates to a negative electrode for a lithium secondary battery comprising a magnesium protective layer formed on at least one surface of the lithium metal layer.

The lithium metal layer may be a lithium metal or a lithium alloy. At this time, the lithium alloy contains an element capable of alloying with lithium, specifically lithium and Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, It may be an alloy with one or more selected from the group consisting of Sr, Sb, Pb, In, Zn, Ba, Ra, Ge and Al.

The lithium metal layer may be in the form of a sheet or foil, and in some cases, a lithium metal or lithium alloy is deposited or coated on a current collector by a dry process, or a metal or alloy on a particle is deposited or coated by a wet process or the like. Can be in the form of

At this time, the method of forming the lithium metal layer is not particularly limited, and a known method of forming a metal thin film, a lamination method, a sputtering method, or the like can be used. In addition, the case where the 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 layer of the present invention.

The lithium metal layer may be adjusted in width according to the shape of the electrode to facilitate electrode manufacturing. The thickness of the lithium metal layer may be 1 to 500 μm, preferably 10 to 350 μm, and more specifically 50 to 200 μm.

In addition, the lithium metal layer may further include a current collector on one surface not in contact with the magnesium protective layer. Specifically, the lithium metal layer may be a negative electrode, and a negative electrode current collector may be used as the current collector.

The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and copper, aluminum, stainless steel, zinc, titanium, silver, palladium, nickel, iron, chromium, alloys thereof, and these It can be selected from the group consisting of. The stainless steel may be surface-treated with carbon, nickel, titanium, or silver, and an aluminum-cadmium alloy may be used as the alloy. In addition, calcined carbon, a non-conductive polymer surface-treated with a conductive material, or a conductive polymer, etc. You can also use Generally, a thin copper plate is used as the negative electrode current collector.

In addition, various forms such as a film/sheet, foil, net, porous body, foam, non-woven fabric with fine irregularities formed on the surface may be used.

In addition, the negative electrode current collector is applied in a thickness range of 3 to 500 μm. If the thickness of the negative electrode current collector is less than 3 μm, the current collecting effect is deteriorated. On the other hand, when the thickness exceeds 500 μm, when the cells are assembled by folding, there is a problem in that workability is deteriorated.

In general, the following problems exist when lithium metal is used as a negative electrode.

First, when lithium is used as a negative electrode, it reacts with electrolyte, water, impurities in the battery, and lithium salt to form a passivation layer, and this layer causes a local current density difference to form dendritic lithium dendrites. In addition, the lithium dendrites thus formed may grow and cause a direct internal short circuit between the anode and the pores of the separator, resulting in a battery explosion.

Second, lithium is a soft metal and has poor mechanical strength, which makes handling very poor for use without additional surface treatment.

Therefore, in the present invention, the mechanical strength can be reinforced by forming the magnesium protective layer on the surface of the lithium metal layer, and particularly, the formation of lithium dendrites can be effectively prevented. Accordingly, life characteristics of a battery including the negative electrode for a lithium secondary battery of the present invention can be improved.

The magnesium protective layer of the present invention is formed on at least one surface of the lithium metal layer described above, and thus a magnesium protective layer is formed on at least one surface of the lithium metal layer, thereby lowering the surface diffusion energy barrier of lithium and reducing lithium dendrites. By suppressing growth, it is possible to obtain a cathode having a uniform surface.

In addition, the magnesium protective layer can effectively suppress the growth of lithium dendrites compared to protective layers such as zinc, nickel, and manganese, thereby improving the life characteristics of the battery.

The magnesium protective layer may be a form in which magnesium particles are deposited on a lithium metal layer or a form in which a magnesium salt forms an alloy with a lithium surface.

The magnesium protective layer may be formed of magnesium or a magnesium alloy. At this time, the magnesium alloy contains elements capable of alloying with magnesium, specifically magnesium and Si, Sn, C, Pt, Ir, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, It may be an alloy with one or more selected from the group consisting of Sr, Sb, Pb, In, Ba, Ra, Ge and Al.

In addition, the thickness of the magnesium protective layer may be 2nm to 5μm, preferably 10nm to 1μm. If the thickness of the magnesium protective layer is less than 2 nm, it cannot function as a protective layer, and thus the growth of lithium dendrites cannot be suppressed. If it exceeds 5 μm, the interfacial resistance increases, and battery characteristics may deteriorate.

That is, the negative electrode for a lithium secondary battery of the present invention can suppress the formation of lithium dendrites due to the magnesium protective layer formed on at least one surface of the lithium metal layer, thereby solving the problems caused by lithium dendrites, and The life characteristics can be improved.

Method for manufacturing a negative electrode for a lithium secondary battery

In addition, the present invention relates to a method of manufacturing a negative electrode for a lithium secondary battery comprising the step of forming a magnesium protective layer on at least one surface of the lithium metal layer.

The method of forming the magnesium protective layer can be used without limitation the methods commonly used in the art. Specifically, for example, evaporation (Evaporation), chemical vapor deposition (CVD), physical vapor deposition (PVD: Physical Vapor Deposition), atomic layer deposition (ALD: Atomic Layer Deposition), arc discharge (acro discharge) ) And dip coating (dip coating) may be formed of a magnesium protective layer by at least one method selected from the group consisting of, preferably by at least one method selected from the group consisting of physical vapor deposition and dip coating method A magnesium protective layer can be formed.

The physical vapor deposition method may be a sputtering method, and the sputtering method may be specifically DC sputtering, RF sputtering, ion beam sputtering, bias sputtering, and magnetron. It may be one or more sputtering methods selected from the group consisting of sputtering (Magnetron Sputtering).

When the magnesium protective layer is formed by the sputtering method, a magnesium protective layer having a uniform thickness can be formed without causing any change on the lithium metal layer. In addition, the magnesium protective layer prepared by using this is deposited with excellent adhesion without defects such as pores and cracks with the lithium metal layer, so no additional thin film such as an adhesive or additional process such as heat treatment is required.

In addition, the magnesium protective layer of the present invention can be formed not only by the physical vapor deposition method, but also by a chemical method. In the present invention, a dip coating method is most preferable among the chemical methods.

The coating solution of the dip coating method includes a magnesium salt, preferably Mg(TFSI) ₂ , MgCl ₂ , MgF ₂ , MgI ₂ , MgBr ₂ , MgO, MgCO ₃ , Mg(OH) ₂ , MgSO ₄ , Mg ₃ N ₂ , Mg(ClO ₄ ), C ₆ H ₆ MgO ₇ , Mg(NO3) ₂ , Mg ₃ (PO ₄ ) ₂ , [CH ₃ (CH ₂ ) ₁₆ CO ₂ ] ₂ Mg, (CH ₃ COO) It may include one or more selected from the group consisting of ₂ Mg, (CF ₃ SO ₃ ) ₂ Mg, MgB ₂ , Mg(OCH ₃ ) ₂ .

The concentration of the magnesium salt in the coating solution may be 5mM to 1M, and preferably 5 to 50mM. If the concentration of the magnesium salt is less than 5mM, the reaction of forming the magnesium protective layer on the lithium surface may not occur sufficiently, and if it exceeds 1M, the reaction may occur suddenly, and the surface may be difficult to form a uniform protective layer.

Further, the solvent of the coating solution is not particularly limited as long as it has a stable tablet for lithium and can dissolve the magnesium salt, but an ether-based solvent is preferred, for example, dimethyl ether, diethyl ether, di Propyl ether, methylethyl ether, methylpropyl ether, ethylpropyl ether, dimethoxyethane, diethoxyethane, methoxyethoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol methylethyl ether, tri Ethylene glycol dimethyl ether, triethylene glycol diethyl ether, triethylene glycol methylethyl ether, tetraethylene glycol dimethyl ether, tetraethylene glycol diethyl ether, tetraethylene glycol methylethyl ether, 1,3-dioxolane, polyethylene glycol dimethyl ether , Polyethylene glycol diethyl ether, polyethylene glycol methyl ethyl ether, one or more selected from the group may be used, but is not limited thereto.

The dip coating method is to form a magnesium protective layer by supporting a lithium metal layer on the coating solution, and then brush polishing a lithium metal layer in the coating solution after loading to remove a surface native layer of the lithium metal layer. At the same time, the magnesium protective layer may be formed.

The thickness of the magnesium protective layer formed by the above method may be 2 nm to 5 μm, and preferably 10 nm to 1 μm. If the thickness of the magnesium protective layer is less than 2 nm, it cannot function as a protective layer, and thus the growth of lithium dendrites cannot be suppressed. If it exceeds 5 μm, the interfacial resistance increases, and battery characteristics may deteriorate.

Lithium secondary battery

In addition, the present invention is an anode; cathode; And a separator interposed between the anode and the cathode. And it relates to a lithium secondary battery comprising an electrolyte.

The negative electrode may be the negative electrode for a lithium secondary battery of the present invention described above, and the negative electrode may be a magnesium protective layer formed on a surface facing the positive electrode.

As described above, the negative electrode for a lithium secondary battery of the present invention includes a magnesium protective layer on at least one surface of the lithium metal layer, thereby suppressing the formation of lithium dendrites to obtain a negative electrode surface having a uniform shape, thereby causing the lithium secondary battery It can improve the life characteristics of. Therefore, the lithium secondary battery of the present invention can exhibit improved life characteristics.

The positive electrode according to the present invention can be prepared in the form of a positive electrode by forming a positive electrode current collector on a positive electrode active material, a composition comprising a conductive material and a binder.

The positive electrode active material may vary depending on the use of the lithium secondary battery, and a specific composition uses a known material. For example, any one lithium transition metal oxide selected from the group consisting of lithium cobalt oxide, lithium manganese oxide, lithium copper oxide, lithium nickel oxide and lithium manganese composite oxide, lithium-nickel-manganese-cobalt oxide. And, more specifically, lithium manganese oxides such as Li _{1 + x} Mn _{2 - x} O ₄ (where x is 0 to 0.33), LiMnO ₃ , LiMn ₂ O ₃ , and LiMnO ₂ ; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , LiFe ₃ O ₄ , V ₂ O ₅ and Cu ₂ V ₂ O ₇ ; Lithium nickel oxide represented by LiNi _{1 - x} M _x O ₂ (where M=Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x=0.01 to 0.3); LiMn _{2 - x} M _x O ₂ (where M=Co, Ni, Fe, Cr, Zn or Ta, x=0.01 to 0.1) or Li ₂ Mn ₃ MO ₈ (where M=Fe, Co, Ni , Cu or Zn), Li(Ni _a Co _b Mn _c )O 2 (here, 0<a<1, 0<b<1, 0<c<1, a+b+ lithium-nickel-manganese-cobalt oxide represented by c=1), Fe ₂ (MoO ₄ ) ₃ ; Elemental sulfur, disulfide compounds, organosulfur compounds and carbon-sulfur polymers ((C ₂ S _x ) _n : x= 2.5 to 50, n≧2 ); Graphite-based materials; Carbon black-based materials such as Super-P, Denka Black, Acetylene Black, Ketjen Black, Channel Black, Furnace Black, Lamp Black, Summer Black, Carbon Black; Carbon derivatives such as fullerene; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder, and nickel powder; And conductive polymers such as polyaniline, polythiophene, polyacetylene, and polypyrrole; A catalyst such as Pt or Ru is supported on the porous carbon support, and the like is not limited thereto. Preferably, a carbon-sulfur polymer may be used.

Therefore, the lithium secondary battery of the present invention may preferably be a lithium-sulfur battery.

The conductive material is a component for further improving the conductivity of the positive electrode active material, non-limiting examples, graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder, and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used.

The positive electrode may further include a binder for bonding the positive electrode active material and the conductive material and bonding to the current collector. The binder may include a thermoplastic resin or a thermosetting resin. For example, polyethylene, polypropylene, polytetrafluoro ethylene (PTFE), polyvinylidene fluoride (PVDF), styrene-butadiene rubber, tetrafluoroethylene-perfluoro alkylvinyl ether copolymer, vinylidene fluoride-hexa Fluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer, polychlorotrifluoroethylene, vinylidene fluoride-pentafluorofluoropropylene copolymer, propylene-tetrafluoro Roethylene copolymer, ethylene-chlorotrifluoroethylene copolymer, vinylidene fluoride-hexafluoropropylene-tetra fluoroethylene copolymer, vinylidene fluoride-perfluoromethylvinyl ether-tetrafluoro ethylene copolymer, ethylene -Acrylic acid copolymers and the like can be used alone or in combination, but are not necessarily limited to these and can be used as long as it can be used as a binder in the art.

The positive electrode current collector is the same as described above in the negative electrode current collector, and in general, an aluminum thin plate may be used as the positive electrode current collector.

The positive electrode as described above may be manufactured according to a conventional method, specifically, a positive electrode active material layer-forming composition prepared by mixing a positive electrode active material, a conductive material, and a binder on an organic solvent is coated and dried on a current collector, and optionally In order to improve the electrode density, compression molding may be performed on the current collector. At this time, as the organic solvent, a positive electrode active material, a binder, and a conductive material can be uniformly dispersed, and it is preferable to use one that is easily evaporated. Specific examples include acetonitrile, methanol, ethanol, tetrahydrofuran, water, and isopropyl alcohol.

The positive electrode composition may be coated on a positive electrode current collector using a conventional method known in the art, for example, dipping, spray, roll court, or gravure printing. Various methods such as a bar court method, a die coating method, a comma coating method, or a mixing method thereof can be used.

The positive electrode and the positive electrode composition that have undergone such a coating process are then evaporated from a solvent or dispersion medium through a drying process, denseness of the coating film, and adhesion between the coating film and the current collector. At this time, drying is performed according to a conventional method, and is not particularly limited.

A conventional separator may be interposed between the anode and the cathode. The separator is a physical separator having a function of physically separating an electrode, and can be used without particular limitation as long as it is used as a normal separator. In particular, it is preferable that it has low resistance to ion movement of the electrolyte and has excellent electrolyte moisture absorption ability.

In addition, the separator enables the transport of lithium ions between the positive electrode and the negative electrode while separating or insulating the positive electrode and the negative electrode from each other. The separator may be made of a porous, non-conductive or insulating material. The separator may be an independent member such as a film, or may be a coating layer added to the anode and/or cathode.

Examples of the polyolefin-based porous membrane that can be used as the separator include polyolefin-based polymers such as high-density polyethylene, linear low-density polyethylene, low-density polyethylene, and ultra-high molecular weight polyethylene, polypropylene, polybutylene, and polypentene, respectively. And a film formed of a mixed polymer.

Examples of the non-woven fabric that can be used as the separator, polyphenylene oxide (polyphenyleneoxide), polyimide (polyimide), polyamide (polyamide), polycarbonate (polycarbonate), polyethylene terephthalate (polyethyleneterephthalate), polyethylene naphthalate (polyethylenenaphthalate) , Polybutyleneterephthalate, polyphenylenesulfide, polyacetal, polyethersulfone, polyetheretherketone, polyester, etc. individually or Non-woven fabrics formed of polymers mixed with them are possible, and these non-woven fabrics are in the form of fibers forming a porous web, and include a spunbond or meltblown form composed of long fibers.

The thickness of the separator is not particularly limited, but is preferably in the range of 1 to 100 μm, more preferably in the range of 5 to 50 μm. When the thickness of the separator is less than 1 μm, mechanical properties cannot be maintained. When the thickness of the separator exceeds 100 μm, the separator acts as a resistance layer, thereby deteriorating the performance of the battery.

The pore size and porosity of the separator are not particularly limited, but the pore size is preferably 0.1 to 50 μm, and the porosity is preferably 10 to 95%. When the pore size of the separator is less than 0.1 μm or the porosity is less than 10%, the separator acts as a resistive layer. When the pore size exceeds 50 μm or the porosity exceeds 95%, mechanical properties cannot be maintained.

The electrolyte of the lithium secondary battery is a lithium salt-containing electrolyte, which may be an aqueous or non-aqueous electrolyte, and is specifically a non-aqueous electrolyte composed of an organic solvent electrolyte and a lithium salt. In addition, an organic solid electrolyte or an inorganic solid electrolyte may be included, but is not limited thereto.

The lithium salt may be used without limitation those commonly used in lithium secondary battery electrolyte. For example is the above lithium salt anion ^{F -, Cl -, Br -} , I -, NO 3 -, N (CN) 2 -, BF 4 -, ClO 4 -, PF 6 -, (CF 3) 2 PF _{^{4 -, (CF 3) 3}} PF 3 -, (CF 3) 4 PF 2 -, (CF 3) 5 PF -, (CF 3) 6 P -, CF 3 SO 3 -, CF 3 CF 2 SO 3 - _{, (CF 3 SO 2) 2} N -, (FSO 2) 2 N -, CF 3 CF 2 (CF 3) 2 CO -, (CF 3 SO 2) 2 CH -, (SF 5) 3 C -, ( CF ₃ SO _{2) 3} C ^- from the group consisting of ^{_{-, CF 3 (CF 2)}} 7 SO 3 -, CF 3 CO 2 -, CH 3 CO 2 -, SCN - , and _{(CF 3 CF 2 SO 2)} 2 N It can include any one selected or two or more of them.

As the organic solvent included in the non-aqueous electrolyte, those commonly used in the electrolyte for lithium secondary batteries can be used without limitation, for example, ether, ester, amide, linear carbonate, cyclic carbonate, etc., alone or in combination of two or more. It can be used by mixing. Among them, a carbonate compound which is representatively a cyclic carbonate, a linear carbonate, or a mixture thereof may be included.

Specific examples of the cyclic carbonate compound are ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, vinylethylene carbonate, and any one selected from the group consisting of halides, or mixtures of two or more of them. Examples of these halides include, but are not limited to, fluoroethylene carbonate (FEC).

In addition, specific examples of the linear carbonate compound is any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC), methylpropyl carbonate and ethylpropyl carbonate Or a mixture of two or more of them may be used, but is not limited thereto.

In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are high-viscosity organic solvents and have higher dielectric constants, so that lithium salts in the electrolyte can be better dissociated, such as dimethyl carbonate and diethyl carbonate. When a low-viscosity, low-permittivity linear carbonate is mixed and used in an appropriate ratio, an electrolyte having a higher electrical conductivity can be produced.

In addition, as the ether of the organic solvent, any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methylethyl ether, methylpropyl ether and ethylpropyl ether, or a mixture of two or more of them may be used. , But is not limited thereto.

And among the organic solvents, esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ -Any one or a mixture of two or more selected from the group consisting of valerolactone and ε-caprolactone may be used, but is not limited thereto.

The injection of the non-aqueous electrolyte may be performed at an appropriate stage during the manufacturing process of the electrochemical device, depending on the manufacturing process of the final product and the required physical properties. That is, it can be applied before the assembly of the electrochemical device or at the final stage of the assembly of the electrochemical device.

The lithium secondary battery according to the present invention is capable of lamination, stacking, and folding processes of a separator and an electrode in addition to the general process of winding. In addition, the battery case may be cylindrical, prismatic, pouch-type, or coin-type.

At this time, the lithium secondary battery can be classified into various batteries such as lithium-sulfur batteries, lithium-air batteries, lithium-oxide batteries, and lithium solid-state batteries, depending on the type of anode material and separator used. It can be classified into coin type, pouch type, etc. and can be divided into bulk type and thin film type according to size. The structure and manufacturing method of these batteries are well known in the art, so detailed descriptions thereof are omitted.

Hereinafter, a preferred embodiment is provided to help the understanding of the present invention, but the following examples are merely illustrative of the present invention, and it is apparent to those skilled in the art that various changes and modifications are possible within the scope and technical scope of the present invention. It is no wonder that such variations and modifications fall within the scope of the appended claims.

<Production of lithium-sulfur batteries>

Example 1.

A lithium metal having a thickness of 40 μm was prepared. A negative electrode was prepared by depositing magnesium metal on the lithium metal for 5 minutes using a DC sputter (manufactured by LG Chemical) under the conditions of 10 mTorr, Sputter power 200 W, and Sample rotation speed 5 rpm.

As a result of observation by photographing and observing the surface of the prepared negative electrode with a shape measurement laser microscope (KEYENCE), it was confirmed that magnesium particles were deposited on the surface of the lithium foil (FIG. 1).

A conductive carbon and sulfur were mixed in a weight ratio of 30:70 to prepare a sulfur-carbon composite through a ball mill process. The sulfur-carbon composite was used as a positive electrode active material.

Sulfur-carbon composite: Conductive material: Binder = After preparing a slurry in a weight ratio of 90:5:5 and applying it to a current collector of 20 μm thick aluminum foil, dried in an oven at 80° C. for 30 minutes, and then rolled to roll the anode. It was prepared. At this time, carbon black was used as the conductive material, and styrene-butadiene rubber and carboxymethyl cellulose were used as the binder.

After interposing a polyethylene separator between the positive electrode and the negative electrode, a lithium-sulfur battery of Example 1 was prepared using a mixed solution of dimethoxyethane:dioxolane (1:1 volume ratio) in which LiFSI at a concentration of 1M was dissolved as an electrolyte. Did.

Example 2.

A coating solution was prepared by dissolving ₁₀ mM Mg(TFSI) ₂ in a 1,2-dimethoxyethane (DME) solution. After supporting a lithium foil having a thickness of 40 μm in the coating solution, a lithium foil was brush-polished in the coating solution to remove the surface oxide film, and at the same time, a magnesium protective layer was formed to prepare a negative electrode.

Thereafter, the lithium-sulfur battery of Example 2 was manufactured in the same manner as in Example 1.

Example 3.

Mg(TFSI) ₂ The lithium-sulfur battery of Example 3 was manufactured in the same manner as in Example 2, except that MgI ₂ was used instead.

Comparative Example 1.

A lithium-sulfur battery of Comparative Example 1 was prepared in the same manner as in Example 1, except that a lithium foil having a thickness of 40 μm was used as a negative electrode.

Comparative Example 2.

A lithium foil having a thickness of 40 μm was removed from the surface oxide film in a dry room to prepare a negative electrode. Then, the lithium-sulfur battery of Comparative Example 2 was prepared in the same manner as in Example 1.

Comparative Example 3.

Mg(TFSI) ₂ The lithium-sulfur battery of Comparative Example 3 was manufactured in the same manner as in Example 2, except that Zn(TFSI) ₂ was used instead.

Comparative Example 4.

Mg(TFSI) ₂ The lithium-sulfur battery of Comparative Example 3 was manufactured in the same manner as in Example 2, except that Ni(TFSI) ₂ was used instead.

Comparative Example 5.

Mg(TFSI) ₂ The lithium-sulfur battery of Comparative Example 3 was prepared in the same manner as in Example 2, except that Mn(TFSI) ₂ was used instead.

Experimental Example 1. Measurement of characteristics of lithium-sulfur battery

The initial discharge profile and life characteristics of the lithium-sulfur batteries prepared in Examples 1 to 3 and Comparative Examples 1 to 5 were measured.

1-1. Observation of surface shape of cathode after initial discharge

The initial discharge profile was measured for the initial capacity under 0.1C/0.1C charge/discharge conditions.

As a result of observing the negative electrode surface after the initial discharge of the lithium-sulfur batteries of Example 1 and Comparative Example 1, the negative electrode surface of the lithium-sulfur battery of Example 1 with a magnesium protective layer exhibited a uniform shape (FIG. 2). . However, the negative electrode surface of the lithium-sulfur battery of Comparative Example 1 without the magnesium protective layer was observed unevenly (Fig. 3).

In addition, as a result of measuring the roughness of the surface image of the negative electrode after the initial discharge of the lithium-sulfur batteries of Example 1 and Comparative Example 1, the discharge shape of the negative electrode of the lithium-sulfur battery of Example 1 is the lithium-sulfur battery of Comparative Example 1 It was confirmed that it was formed evenly than the discharge shape of the negative electrode of FIGS. 4 and 5.

1-2. Initial discharge profile measurement

The initial discharge profile was measured for the initial capacity under 0.1C/0.1C charge/discharge conditions.

From the initial discharge profile results of the lithium-sulfur batteries of Examples 1 to 3 and Comparative Examples 1 and 2, it was confirmed that the lithium-sulfur batteries of Examples 1 to 3 and Comparative Examples 1 and 2 did not generate overvoltage ( 6 to 8). Particularly, although the negative electrode of Examples 1 to 3 was formed with a magnesium protective layer on the lithium metal layer, it was confirmed that the overvoltage caused by this did not occur.

On the other hand, the lithium-sulfur batteries of Comparative Examples 3 to 5 including the negative electrode each having a zinc, nickel, and manganese protective layer formed on the surface of the lithium metal layer, it was confirmed that overvoltage occurred at the distal end during the initial discharge (FIG. 9).

From this, it can be seen that the lithium-sulfur battery including the negative electrode having the magnesium protective layer formed on the surface of the lithium metal layer does not generate an overvoltage during initial discharge.

1-3. Life characteristics measurement

Life characteristics are 0.1C/0.1C charging/discharging during the initial 3 cycles, then 0.2C/0.2C charging/discharging during the 3 cycles, and then charging/discharging at 0.3C/0.5C to repeat 100 cycles of charging/discharging. The life characteristics were measured.

The lithium-sulfur battery of Example 1 was found to have improved life characteristics by 100% compared to the lithium-sulfur battery of Comparative Example 1 (FIG. 10).

In addition, it was confirmed that the lithium-sulfur batteries of Examples 2 and 3 had 80% improved life characteristics compared to the lithium-sulfur batteries of Comparative Example 2 (FIGS. 11 and 12 ).

That is, the lithium-sulfur batteries of Examples 1 to 3 including the negative electrode on which the magnesium protective layer was formed on the surface of the lithium metal layer showed better life characteristics than the lithium-sulfur batteries of Comparative Examples 1 and 2 not containing the magnesium protective layer. .

The lithium-sulfur batteries of Comparative Examples 3 to 5 each include a negative electrode having a zinc, nickel, and manganese protective layer formed on the surface of the lithium metal layer, and have poorer life characteristics than the lithium-sulfur batteries of Comparative Example 2 without a protective layer. The results were shown (FIG. 13). Accordingly, it was found that even if the protective layer was included, when the protective layer made of a metal other than magnesium was included, the lifespan characteristics of the lithium-sulfur battery could not be improved, but the lifespan characteristics were poor.

The negative electrode of the present invention includes a magnesium protective layer on the surface of the lithium metal layer, and even if the magnesium protective layer is included, overvoltage of the battery does not occur, and the formation of lithium dendrites can be suppressed by including the magnesium protective layer. The life characteristics can be improved.

In addition, the magnesium protective layer may be formed on the surface of the lithium metal layer by a physical or chemical method, without generating an overvoltage of the battery regardless of the method of forming the magnesium protective layer, and the life characteristics of the battery may be improved. In particular, when the magnesium protective layer is formed by a dip coating method, which is a chemical method, the magnesium protective layer can be formed regardless of the type of the magnesium salt, and the above effects can be exhibited.

Claims (12)

Lithium metal layer; And
A negative electrode for a lithium secondary battery including a magnesium protective layer formed on at least one surface of the lithium metal layer. The negative electrode for a lithium secondary battery according to claim 1, wherein the magnesium protective layer is formed of magnesium or a magnesium alloy. The negative electrode for a lithium secondary battery according to claim 1, wherein the magnesium protective layer has a thickness of 2 nm to 5 μm. The negative electrode for a lithium secondary battery according to claim 1, wherein the lithium metal layer has a thickness of 1 to 500 μm. The negative electrode for a lithium secondary battery according to claim 1, wherein the lithium metal layer further includes a current collector on one surface not in contact with the magnesium protective layer. A method of manufacturing a negative electrode for a lithium secondary battery, comprising forming a magnesium protective layer on at least one surface of the lithium metal layer. The lithium of claim 6, wherein the magnesium protective layer is formed by at least one method selected from the group consisting of vapor deposition, chemical vapor deposition, physical vapor deposition, atomic layer deposition, arc discharge and dip coating. Method for manufacturing a negative electrode for a secondary battery. The method of claim 7, wherein the magnesium protective layer is formed by at least one method selected from the group consisting of a physical vapor deposition method and a dip coating method. The method of claim 8, wherein the coating solution of the dip coating method includes a magnesium salt, and the magnesium salt is Mg(TFSI) ₂ , MgCl ₂ , MgF ₂ , MgI ₂ , MgBr ₂ , MgO, MgCO ₃ , Mg(OH ) ₂ , MgSO ₄ , Mg ₃ N ₂ , Mg(ClO ₄ ), C ₆ H ₆ MgO ₇ , Mg(NO3) ₂ , Mg ₃ (PO ₄ ) ₂ , [CH ₃ (CH ₂ ) ₁₆ CO ₂ ] ₂ Mg, (CH ₃ COO) ₂ Mg, (CF ₃ SO ₃ ) ₂ Mg, MgB ₂ , Mg(OCH ₃ ) ₂ One or more selected from the group consisting of a cathode for a lithium secondary battery characterized in that it comprises at least one . The method of claim 9, wherein the magnesium salt is contained in a concentration of 5mM to 1M. anode; The negative electrode of any one of claims 1 to 5; And a separator interposed between the anode and the cathode. And an electrolyte solution. The lithium secondary battery of claim 11, wherein the negative electrode has a magnesium protective layer formed on a surface facing the positive electrode.

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