WO2020091448A1 - Lithium secondary battery - Google Patents

Abstract

The present invention relates to a lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte, wherein the electrolyte comprises a first electrolyte layer facing the negative electrode and a second electrolyte layer located on the first electrolyte layer and facing the positive electrode, and the ion conductivity of the first electrolyte layer is higher than that of the second electrolyte layer, and lithium ions move from the positive electrode by charging to form lithium metal on a negative electrode current collector in the negative electrode.

Description

Lithium secondary battery

This application claims the benefit of priority based on Korean Patent Application No. 10-2018-0131581 on October 31, 2018 and Korean Patent Application No. 10-2019-0137128 on October 31, 2019. All content disclosed in the literature is included as part of this specification.

The present invention relates to a lithium secondary battery of a negative electrode free (negative electrode free) structure comprising an electrolyte having a differential ion conductivity.

Recently, various devices that require batteries, such as mobile phones, wireless home appliances, and electric vehicles, have been developed, and according to the development of these devices, demand for secondary batteries is also increasing. In particular, in addition to the trend of miniaturization of electronic products, secondary batteries are also becoming lighter and smaller.

In line with this trend, lithium secondary batteries using lithium metal as an active material have recently been spotlighted. Lithium metal has a low redox potential (-3.045V for a standard hydrogen electrode) and a high weight energy density (3,860mAhg ^-1 ), and is expected to be a negative electrode material for a high-capacity secondary battery.

However, when lithium metal is used as a negative electrode of a battery, a battery is generally prepared by attaching lithium foil on a planar current collector. Lithium is an alkali metal, and since it is highly reactive, it reacts explosively with water and reacts with oxygen in the atmosphere. Therefore, it is difficult to manufacture and use in a general environment. In particular, when lithium metal is exposed to the atmosphere, it has an oxide film such as LiOH, Li ₂ O, Li ₂ CO ₃ as a result of oxidation. When a native oxide layer is present on the surface, the oxide film acts as an insulating film to lower electrical conductivity and inhibit smooth movement of lithium ions, thereby increasing electrical resistance.

For this reason, the problem of forming a surface oxide film due to the reactivity of lithium metal has been partially improved by performing a vacuum deposition process to form a lithium anode, but it is still exposed to the atmosphere during the battery assembly process, and it is impossible to fundamentally suppress the formation of a surface oxide film. to be. Accordingly, there is a need to develop a lithium metal electrode that can solve the reactivity problem of lithium and further simplify the process while increasing energy efficiency using lithium metal.

[Advanced technical literature]

[Patent Document]

Korean Patent Publication No. 10-2016-0052323 "Lithium electrode and lithium battery comprising the same"

In order to solve the above-mentioned problems, the present inventors conducted various studies, and as a result, lithium ions transferred from the positive electrode active material by charging after battery assembly so as to fundamentally block the contact of the lithium metal with the atmosphere when assembling the battery are negative electrode collectors. A negative electrode free battery structure capable of forming a lithium metal layer on the whole was designed, and a composition of a positive electrode active material capable of stably forming the lithium metal layer was developed. In addition, by including two or more electrolyte layers having different ion conductivity, a lithium secondary battery has been developed that can suppress the growth of dendrites due to the difference in ion conductivity.

Accordingly, an object of the present invention is to provide a lithium secondary battery with improved performance and lifespan by solving problems caused by reactivity of lithium metal and problems occurring during assembly.

In order to achieve the above object,

The present invention in a lithium secondary battery comprising a positive electrode, a negative electrode and an electrolyte,

The electrolyte includes a first electrolyte layer facing the negative electrode, a second electrolyte layer positioned on the first electrolyte layer and facing the positive electrode,

The first electrolyte layer has a higher ionic conductivity than the second electrolyte layer,

A lithium secondary battery is provided in which lithium ions are moved from the positive electrode by charging to form lithium metal on the negative electrode current collector in the negative electrode.

The lithium secondary battery according to the present invention is coated in a state of being blocked from the atmosphere through the process of forming a lithium metal layer on the negative electrode current collector, so it is possible to suppress the formation of a surface oxide film due to oxygen and moisture in the atmosphere of the lithium metal, , As a result, the cycle life characteristics are improved.

In addition, by including two or more electrolyte layers having different ion conductivity, there is an effect of significantly suppressing the growth of dendrites due to the difference in ion conductivity.

1 is a schematic view of a lithium secondary battery manufactured according to the present invention.

2 is a schematic view after the initial charging of the lithium secondary battery manufactured according to the present invention is completed.

3 schematically shows the structure and mechanism of a conventional lithium secondary battery.

Figure 4 schematically shows the structure and mechanism of the lithium secondary battery of the present invention.

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

In the drawings, parts unrelated to the description are omitted in order to clearly describe the present invention, and similar reference numerals are used for similar parts throughout the specification. In addition, the sizes and relative sizes of components indicated in the drawings are independent of actual scale, and may be reduced or exaggerated for clarity of explanation.

The terms or words used in the present specification and claims should not be interpreted as being limited to ordinary or lexical meanings, and the inventor can appropriately define the concept of terms in order to best describe his or her invention. Based on the principle that it should be interpreted as meanings and concepts consistent with the technical spirit of the present invention.

When a layer is referred to herein as being "on" another layer or substrate, it may be formed directly on another layer or substrate, or a third layer may be interposed between them. In addition, in the present specification, directional expressions such as upper, upper (second), and upper surfaces may be understood as meanings of lower, lower (second), and lower surfaces according to the standard. That is, the expression of the spatial direction should be understood as a relative direction and should not be construed as limiting the absolute direction.

In addition, the terms "include", "include" or "have" are intended to indicate the presence of features, numbers, components or combinations thereof described in the specification, one or more other features or numbers, It should be understood that it does not exclude the possibility of the presence or addition of components or combinations thereof.

In the drawings, the thickness of the layers and regions may be exaggerated or omitted for clarity. Throughout the specification, the same reference numbers indicate the same components.

In addition, in the following description of the present invention, when it is determined that detailed descriptions of related well-known functions or configurations may unnecessarily obscure the subject matter of the present invention, detailed descriptions thereof will be omitted.

The present invention in a lithium secondary battery comprising a positive electrode, a negative electrode and an electrolyte,

The electrolyte includes a first electrolyte layer facing the negative electrode, a second electrolyte layer positioned on the first electrolyte layer and facing the positive electrode,

The first electrolyte layer has a higher ionic conductivity than the second electrolyte layer,

It relates to a lithium secondary battery in which lithium ions move from the positive electrode by charging to form lithium metal on the negative electrode current collector in the negative electrode.

1 is a cross-sectional view of a lithium secondary battery manufactured according to a first embodiment of the present invention, a positive electrode including a positive electrode current collector 11 and a positive electrode mixture 13; Cathode current collector 21; A first electrolyte layer 31 facing the cathode; And a second electrolyte layer 33 positioned on the first electrolyte layer and facing the anode.

The negative electrode of the lithium secondary battery is usually formed on the negative electrode current collector 21, but in the present invention, only the negative electrode current collector 21, the first electrolyte layer 31 and the second electrolyte layer 33 are used. After assembling into a negative electrode free battery structure, lithium ions released from the positive electrode mixture 13 by charging are used as a negative electrode mixture between the negative electrode current collector 21 and the first electrolyte layer 31. (Not shown) to form a negative electrode having a configuration of a known negative electrode current collector / cathode mixture to form a conventional lithium secondary battery.

More specifically, a lithium metal layer may be formed inside the first electrolyte layer 31 formed on the negative electrode current collector 21.

That is, in the present invention, the lithium secondary battery may be a negative electrode-free battery in which a negative electrode is not formed on the negative electrode current collector during initial assembly, or a negative electrode may be formed on the negative electrode current collector depending on use. It may be a concept including all batteries.

In addition, in the negative electrode of the present invention, the form of lithium metal formed as a negative electrode mixture on the negative electrode current collector is a form in which lithium metal is formed in a layer, and a porous structure in which lithium metal is not formed in a layer (for example, lithium metal It includes all of the structures aggregated in the form of particles).

Hereinafter, the present invention will be described based on the shape of the lithium metal layer 23 in which the lithium metal is formed as a layer, but it is clear that this description does not exclude a structure in which the lithium metal is not formed as a layer.

2 is a schematic diagram after initial charging of a lithium secondary battery manufactured according to the first embodiment of the present invention is completed.

Referring to FIG. 2, when charging is performed by applying a voltage above a certain level to a lithium secondary battery having a negative electrode free battery structure, lithium ions are released from the positive electrode mixture 13 in the positive electrode 10, which is second. It passes through the electrolyte layer 33 and the first electrolyte layer 31 and moves toward the negative electrode current collector 21, and forms a lithium metal layer 23 made of purely lithium on the negative electrode current collector 21 to form a negative electrode. (20).

Formation of the lithium metal layer 23 through such charging may be performed by adjusting the interfacial characteristics as compared to the negative electrode sputtering the lithium metal layer 23 on the conventional negative electrode current collector 21 or laminating the lithium foil and the negative electrode current collector 21. It has the advantage of being very easy.

Particularly, since the lithium metal is formed in a negative electrode free battery structure and there is no exposure to lithium metal in the air during the battery assembly process, problems such as the formation of an oxide film on the surface due to the high reactivity of the lithium itself and the decrease in the life of the lithium secondary battery accordingly. It can be blocked at the source.

In the negative electrode structure of the present invention, the negative electrode current collector 21 constituting the negative electrode is generally made to a thickness of 3 to 50 μm.

The negative electrode current collector 21 in which the lithium metal layer 23 can be formed by charging is not particularly limited as long as it has conductivity without causing a chemical change in the lithium secondary battery. As an example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, or the like may be used.

At this time, the negative electrode current collector 21 may be used in various forms, such as a film, sheet, foil, net, porous body, foam, non-woven fabric, etc. with fine irregularities formed on the surface.

The electrolyte of the lithium secondary battery according to the present invention includes a first electrolyte layer and a second electrolyte layer, the first electrolyte layer faces the negative electrode, and a second electrolyte layer is located on the first electrolyte layer, 2 The electrolyte layer faces the anode.

In addition, the first electrolyte layer has a higher ionic conductivity than the second electrolyte layer.

In the electrolyte of the present invention, the ion conductivity of the first electrolyte layer is 10 ^-5 to 10 ^-2 S / cm, and the ion conductivity of the second electrolyte layer may be 10 ^-6 to 10 ^-3 S / cm. .

In addition, the difference in ionic conductivity between the first electrolyte layer and the second electrolyte layer may be 2 to 10 ⁴ times, and more preferably 10 to 100 times.

When the difference in the ionic conductivity is lower than the lower limit value of the above-described range, the lithium dendrite growth suppression effect becomes insignificant.

In addition, the first electrolyte layer of the present invention has a lower strength than the second electrolyte layer. The strength of the first electrolyte layer may be 10 ⁵ Pa or less, and the strength of the second electrolyte layer may exceed 10 ⁵ Pa. Preferably, the strength of the first electrolyte layer may be 10 to 10 ⁴ Pa, and the strength of the second electrolyte layer may be 10 ⁶ to 10 ¹⁰ Pa.

In general, in the case of a lithium secondary battery that uses a lithium metal or a material containing lithium metal as a negative electrode for a lithium secondary battery, first, degeneration proceeds rapidly due to the growth of dendrites of lithium, reactivity with lithium electrolyte, and other side reactions. Second, in order to solve the above problem, when a defect is generated in the protective layer by forming a protective layer on the surface of the negative electrode, a short circuit of the battery occurs as lithium dendrite growth is accelerated in the defect generating unit.

The present inventors have made great efforts to solve the above problems, and when the first electrolyte layer has a higher ionic conductivity than the second electrolyte layer, even if a defect occurs in the first electrolyte layer, lithium ions are not concentrated in the defect site. Without (FIG. 3), lithium is plated through the first electrolyte layer around the defect generating unit having higher ion conductivity (FIG. 4), thereby discovering that rapid growth of lithium dendrites can be prevented, thereby completing the present invention. Did.

Therefore, the electrolyte of the lithium secondary battery of the present invention is characterized in that the ion conductivity of the first electrolyte layer (or protective layer) facing the negative electrode is higher than that of the second electrolyte layer.

The protective layer is capable of moving lithium ions and must satisfy a condition in which current does not flow, so it can be understood as an electrolyte layer. Therefore, in the present invention, the first electrolyte layer is also defined as having a function of a protective layer.

In the electrolyte of the present invention, at least one of the first electrolyte layer and the second electrolyte layer is characterized by being a semi-solid electrolyte or a solid electrolyte. This is because, in the case of all liquids, it is difficult to obtain the desired effect in the present invention by mixing the first electrolyte layer and the second electrolyte layer.

In the electrolyte of the present invention, the first electrolyte layer may have a thickness of 0.1 to 20 μm, preferably 0.1 to 10 μm. If the thickness is less than 0.1 μm, it may be difficult to perform a function as a protective layer, and when the thickness exceeds 20 μm, the interface resistance may be increased to cause deterioration of battery characteristics.

 In addition, the second electrolyte layer may have a thickness of 0.1 to 50 μm, preferably 0.1 to 30 μm. If the thickness is less than 0.1 μm, it may be difficult to function as an electrolyte, and when the thickness is greater than 50 μm, the interface resistance may be increased, resulting in deterioration of battery characteristics.

In the electrolyte of the present invention, the electrolyte may further include one or more electrolyte layers formed on the second electrolyte layer. In this case, the ionic conductivity of the one or more electrolyte layers may be higher than that of the first electrolyte layer. Rather, when the ion conductivity is higher, the driving performance of the battery may be further improved. This is because the object of the present invention can be achieved from the ionic conductivity relationship between the first electrolyte layer and the second electrolyte layer.

In the electrolyte of the present invention, among the one or more electrolyte layers formed on the second electrolyte layer, the electrolyte layer facing the anode may have a feature that has higher ionic conductivity than the second electrolyte layer.

When the ionic conductivity of the electrolyte layer facing the positive electrode is high as described above, since the conduction of Li ions inserted and desorbed from the positive electrode is fast, the resistance during charging and discharging becomes small, so that the rate performance characteristics of the battery are improved and the driving performance of the battery is further improved. It is desirable because it can be improved.

In the electrolyte of the present invention, among the one or more electrolyte layers formed on the second electrolyte layer, the electrolyte layer facing the anode may have an ionic conductivity of 10 ^-5 to 10 ^-2 S / cm, and 10 ^-4 to 10 It is more preferable that it is ^-2 S / cm.

In one embodiment of the present invention, at least one electrolyte layer formed on the second electrolyte layer is composed of one electrolyte layer, and the electrolyte layer may be in a form facing an anode.

The electrolyte of the present invention may be in a state in which a separator is interposed between the electrolytes. In addition, in this case, the separator may be interposed in a form impregnated with electrolyte.

In one embodiment of the present invention, the separator may be formed on the second electrolyte layer. However, it is not limited to this form.

In the electrolyte of the present invention, when considering the function as a protective layer, the first electrolyte layer may be preferably formed of a semi-solid electrolyte or a solid electrolyte. As the semi-solid electrolyte and the solid electrolyte, if the ionic conductivity conditions defined above are satisfied, those known in the art may be used without limitation.

In the electrolyte of the present invention, the second electrolyte layer may be formed of a liquid electrolyte, a semi-solid electrolyte or a solid electrolyte. When the liquid electrolyte, the semi-solid electrolyte, and the solid electrolyte satisfy the ionic conductivity condition defined above, an electrolyte known in the art may be used without limitation.

The liquid electrolyte, semi-solid electrolyte, and solid electrolyte may be, for example, in the following form, but are not limited thereto.

The non-aqueous electrolyte containing a lithium salt is composed of a lithium salt and an electrolyte, and a non-aqueous organic solvent, an organic solid electrolyte, and an inorganic solid electrolyte may be used as the electrolyte.

The lithium salt of the present invention is a material that is soluble in a non-aqueous organic solvent, such as LiNO ₃ , LiSCN, LiCl, LiBr, LiI, LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiB ₁₀ Cl ₁₀ , LiCH ₃ SO ₃ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiClO ₄ , LiAlCl ₄ , Li (Ph) ₄ , LiC (CF ₃ SO ₂ ) ₃ , LiN (FSO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (SFO ₂ ) ₂ , LiN (CF ₃ CF ₂ SO ₂ ) ₂ , lithium chloroborane, lower aliphatic lithium carboxylate, lithium 4-phenyl borate, lithium imide, and combinations thereof One or more from the group consisting of.

The concentration of the lithium salt is 0.2 to 3 M, depending on several factors, such as the exact composition of the electrolyte mixture, the solubility of the salt, the conductivity of the dissolved salt, the conditions for charging and discharging the cell, the working temperature and other factors known in the lithium battery field. Specifically, it may be 0.6 to 2 M, more specifically 0.7 to 1.7 M. When used below 0.2 M, the conductivity of the electrolyte may be lowered, resulting in deterioration of electrolyte performance, and when used above 3 M, the viscosity of the electrolyte may increase and mobility of lithium ions (Li ⁺ ) may decrease.

The non-aqueous organic solvent must dissolve a lithium salt well, and such non-aqueous organic solvents include, for example, N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, and diethyl carbonate. , Ethylmethyl carbonate, gamma-butyrolactone, 1,2-dimethoxy ethane, 1,2-diethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3- Dioxolane, 4-methyl-1,3-dioxene, diethyl ether, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxy methane , Dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, aprotic organic solvents such as methyl pyropionate and ethyl propionate Hear The organic solvent may be one or a mixture of two or more organic solvents.

Examples of the organic solid electrolyte include, for example, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphoric acid ester polymers, poly agitation lysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and ionic dissociation groups. Including polymers and the like can be used.

As the inorganic solid electrolyte, for example, 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 , Li ₄ SiO Li-nitrides, halides, sulfates, etc., such as ₄ -LiI-LiOH, Li ₃ PO4-Li ₂ S-SiS ₂ can be used.

In the electrolyte of the present invention, for the purpose of improving charge / discharge characteristics, flame retardance, etc., for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme (glyme), hexaphosphate triamide, nitro Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxy ethanol, aluminum trichloride, etc. may be added. . In some cases, in order to impart non-flammability, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further included, or carbon dioxide gas may be further included to improve high temperature storage characteristics, and FEC (Fluoro-ethylene) carbonate), PRS (Propene sultone), FPC (Fluoro-propylene carbonate), and the like.

Meanwhile, the positive electrode mixture 13 may use various positive electrode active materials depending on the type of battery, and the positive electrode active material used in the present invention is not particularly limited as long as it is a material capable of absorbing and releasing lithium ions. Lithium transition metal oxide is typically used as a positive electrode active material capable of realizing a battery having excellent discharge efficiency.

As a lithium transition metal oxide, a layered compound such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ) containing two or more transition metals and substituted with one or more transition metals; Lithium manganese oxide substituted with one or more transition metals; Lithium nickel-based oxide; Spinel lithium nickel manganese composite oxide; Spinel-based lithium manganese oxide in which a part of Li in the formula is substituted with alkaline earth metal ions; And olivine-based lithium metal phosphate; And the like, but is not limited to these.

Preferably, a lithium transition metal oxide is used, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li ₂ NiO ₂ , Li (Ni _a Co _b Mn _c ) O ₂ (0 <a <1 , 0 <b <1, 0 <c <1, a + b + c = 1), LiNi _1-Y Co _Y O ₂ , LiCo _1-Y Mn _Y O ₂ , LiNi _1-Y Mn _Y O ₂ (here In, 0≤Y <1), Li (Ni _a Co _b Mn _c ) O ₄ (0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), LiMn _{2 -z} Ni _z O ₄ , LiMn _2-z Co _z O ₄ (here, 0 <Z <2), Li _x M _y Mn _2-y O _4-z A _z (here, 0.9≤x≤1.2, 0 <y <2, 0≤z <0.2, M = Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, At least one of Ti and Bi, A is -1 or -divalent one or more anions), Li _{1 + a} Ni _b M ' _1-b O _2-c A' _c (here, 0≤a≤0.1, 0≤ b≤0.8, 0≤c <0.2, M 'is one or more selected from the group consisting of 6-coordinate stable elements such as Mn, Co, Mg, and Al, and A' is one or more anions of -1 or -2 .), LiCoPO ₄ and LiFePO ₄ is selected from the group consisting of one or more selected LiCoO ₂ is most preferably used. In addition, sulfide, selenide and halide may be used in addition to the oxide.

The above-described lithium transition metal oxide is used as a positive electrode active material 13 together with a binder and a conductive material as a positive electrode active material. The lithium source for forming the lithium metal layer 23 in the negative electrode free battery structure of the present invention becomes the lithium transition metal oxide. That is, when the lithium ions in the lithium transition metal oxide are charged in a specific range of voltage, lithium ions are desorbed to form the lithium metal layer 23 on the negative electrode current collector 21.

In practice, however, lithium ions in the lithium transition metal oxide do not easily generate desorption on their own, or there is no lithium that can be related to charge and discharge at the operating voltage level, so that the formation of the lithium metal layer 23 is very difficult, and the lithium transition metal oxide When only the bay is used, the irreversible capacity is greatly reduced, causing a problem that the capacity and life characteristics of the lithium secondary battery are lowered.

Accordingly, in the present invention, as an additive capable of providing a lithium source to a lithium transition metal oxide, the initial charge capacity is 200 mAh / g or more when one charge is performed at 0.01 to 0.2C in a voltage range of 4.5V to 2.5V Alternatively, a lithium metal compound which is a high irreversible material having an initial irreversible of 30% or more is used together.

The term 'high irreversible material' referred to in the present invention may be used in the same way as 'large capacity irreversible material' in other terms, which is the irreversible capacity ratio of the first cycle of charge / discharge, that is, "(first cycle charge capacity-first cycle discharge capacity). / First cycle charge capacity "means a large material. That is, the highly irreversible material may irreversibly provide excessive lithium ions during the first cycle of charging and discharging. For example, the lithium transition metal compound capable of occluding and releasing lithium ions may be a positive electrode material having a large irreversible capacity (first cycle charge capacity-first cycle discharge capacity) of the first cycle of charge and discharge.

Generally, the irreversible capacity of the positive electrode active material used is about 2 to 10% of the initial charge capacity, but in the present invention, the lithium metal compound which is a high irreversible material, that is, the initial irreversible capacity is 30% or more of the initial charge capacity, preferably 50% or more. Lithium metal compounds can be used together. In addition, as the lithium metal compound, an initial charge capacity of 200 mAh / g or more, preferably 230 mAh / g or more, may be used. Due to the use of such a lithium metal compound, it acts as a lithium source capable of forming the lithium metal layer 23 while increasing the irreversible capacity of the lithium transition metal oxide as a positive electrode active material.

The lithium metal compound presented in the present invention may be a compound represented by the following Chemical Formulas 1 to 8.

[Formula 1]

Li ₂ Ni _1-a M ¹ _a O ₂

(In the above formula, a is 0≤a <1, and M ¹ is one or more elements selected from the group consisting of Mn, Fe, Co, Cu, Zn, Mg, and Cd.)

[Formula 2]

Li _{2 + b} Ni _1-c M ² _c O _{2 + d}

(In the above formula, -0.5≤b≤0.5, 0≤c≤1, 0≤d <0.3, M ² is P, B, C, Al, Sc, Sr, Ti, V, Zr, Mn, Fe, Co, It is one or more elements selected from the group consisting of Cu, Zn, Cr, Mg, Nb, Mo and Cd.)

[Formula 3]

LiM ³ _e Mn _1-e O ₂

(In the above formula, 0≤e <0.5, M ³ is one or more elements selected from the group consisting of Cr, Al, Ni, Mn, and Co.),

[Formula 4]

Li ₂ M ⁴ O ₂

(In the above formula, M ⁴ is one or more elements selected from the group consisting of Cu and Ni.)

[Formula 5]

Li _{3 + f} Nb _1-g M ⁵ _g S _4-h

(In the above formula, -0.1≤f≤1, 0≤g≤0.5, -0.1≤h≤0.5, M ⁵ is one or more elements selected from the group consisting of Mn, Fe, Co, Cu, Zn, Mg and Cd to be)

[Formula 6]

LiM ⁶ _i Mn _1-i O ₂

(In the above formula, i is 0.05≤i <0.5, and M ⁶ is one or more elements selected from the group consisting of Cr, Al, Ni, Mn, and Co.)

[Formula 7]

LiM ⁷ _2j Mn _2-2j O ₄

(In the above formula, j is 0.05≤j <0.5, and M ⁷ is one or more elements selected from the group consisting of Cr, Al, Ni, Mn, and Co.)

[Formula 8]

Li _k -M ⁸ _m -N _n

(In the above formula, M ⁸ represents an alkaline earth metal, k / (k + m + n) is 0.10 to 0.40, m / (k + m + n) is 0.20 to 0.50, and n / (k + m + n) is 0.20 to 0.50.)

The lithium metal compounds of Chemical Formulas 1 to 8 have different irreversible capacity depending on their structure, and they can be used alone or in combination, and serve to increase the irreversible capacity of the positive electrode active material.

As an example, the highly irreversible materials represented by Chemical Formulas 1 and 3 have different irreversible capacities according to their types, and have numerical values as shown in Table 1 below as an example.

	Initial charging capacity (mAh / g)	Initial discharge capacity (mAh / g)	Initial Coulomb Efficiency	Initial irreversible capacity ratio
[Formula 1] Li 2 NiO 2	370	110	29.7%	70.3%
[Formula 3] LiMnO 2	230	100	43.5%	56.5%
[Chemical Formula 3] LiCr e Mn 1-e O 2	230	80	34.8%	65.2%

In addition, it is preferable that the lithium metal compound of Formula 2 belongs to the space group Immm, of which Ni, M composite oxide forms a planar coordination (Ni, M) O ₄ , and the planar coordination structure faces each other. It is more preferable to share the opposite side (a side formed of OO) and form a primary chain. The crystal lattice constant of the compound of Formula 2 is preferably a = 3.7 ± 0.5 ±, b = 2.8 ± 0.5 Å, c = 9.2 ± 0.5 Å, α = 90 °, β = 90 °, γ = 90 °.

In addition, the lithium metal compound of the formula (8) has an alkaline earth metal content of 30 to 45 atomic%, and a nitrogen content of 30 to 45 atomic%. At this time, when the content of the alkaline earth metal and the content of nitrogen are within the above range, the thermal properties and lithium ion conduction properties of the compound of Formula 8 are excellent. And, in the formula (8), k / (k + m + n) is 0.15 to 0.35, for example 0.2 to 0.33, m / (k + m + n) is 0.30 to 0.45, for example 0.31 to 0.33, n / (k + m + n) is 0.30 to 0.45, for example 0.31 to 0.33.

According to one embodiment, the electrode active material of the above formula is 0.5 to 1, b is 1, and c is 1.

The positive electrode active material may have a core-shell structure having a surface coated with a compound of any one of Formulas 1 to 8.

When a coating film of any one of the above Chemical Formulas 1 to 8 is formed on the surface of the core active material, the electrode active material exhibits stable characteristics while maintaining low resistance characteristics even in an environment in which lithium ions are continuously inserted and desorpted.

In the electrode active material according to the embodiment of the present invention, the thickness of the coating film is 1 to 100 nm. When the thickness of the coating film is within the above range, the ion-conducting properties of the electrode active material are excellent.

The electrode active material has an average particle diameter of 1 to 30 μm, and according to one embodiment, 8 to 12 μm. When the average particle diameter of the positive electrode active material is within the above range, the capacity characteristics of the battery are excellent.

The core active material doped with the alkaline earth metal may be, for example, LiCoO ₂ doped with magnesium. The magnesium content is 0.01 to 3 parts by weight based on 100 parts by weight of the core active material.

The above-described lithium transition metal oxide is used as a positive electrode active material 13 together with a binder and a conductive material as a positive electrode active material. The lithium source for forming the lithium metal layer 23 in the negative electrode free battery structure of the present invention becomes the lithium transition metal oxide. That is, when the lithium ions in the lithium transition metal oxide are charged in a specific range of voltage, lithium ions are desorbed to form the lithium metal layer 23 on the negative electrode current collector 21.

In the present invention, the charging range for forming the lithium metal layer 23 is performed once at a voltage range of 4.5V to 2.5V at 0.01 to 0.2C. If the charging is performed below the above range, it is difficult to form the lithium metal layer 23. On the contrary, when the above-mentioned range is exceeded, charge and discharge are properly performed after over-discharge occurs due to damage of the cell. It does not proceed.

The formed lithium metal layer 23 forms a uniform continuous or discontinuous layer on the negative electrode current collector 21. For example, when the negative electrode current collector 21 has a foil shape, it may have a continuous thin film shape, and when the negative electrode current collector 21 has a three-dimensional porous structure, the lithium metal layer 23 may be discontinuously formed. . That is, the discontinuous layer is in a form of discontinuously distributed, and a region in which the lithium metal layer 23 exists and a region in which a lithium metal layer 23 is present exists in a specific region, but a lithium compound exists in a region where the lithium metal layer 23 does not exist. By distributing the region to be isolated, disconnected or separated like an island type, it means that the region where the lithium metal layer 23 is present is distributed without continuity.

The lithium metal layer 23 formed through such charging and discharging has a thickness of at least 50 nm, 100 μm or less, and preferably 1 μm to 50 μm for functioning as a negative electrode. If the thickness is less than the above range, the efficiency of charging and discharging the battery rapidly decreases. On the contrary, if the thickness exceeds the above range, life characteristics and the like are stable, but the energy density of the battery is lowered.

In particular, the lithium metal layer 23 presented in the present invention is manufactured by using a negative electrode-free battery without lithium metal when assembling the battery, so that the lithium generated in the assembly process is high compared to a lithium secondary battery assembled using a conventional lithium foil. Due to the reactivity, no or little oxide layer is formed on the lithium metal layer 23. Due to this, it is possible to prevent the degradation of the life of the battery due to the oxide layer.

In addition, the lithium metal layer 23 is moved by charging of a high irreversible material, which can form a more stable lithium metal layer 23 compared to forming the lithium metal layer 23 on the positive electrode. When lithium metal is attached on the positive electrode, a chemical reaction between the positive electrode and lithium metal may occur.

The positive electrode mixture 13 comprises the positive electrode active material and the lithium metal compound, wherein the positive electrode mixture 13 may further include a conductive material, a binder, and other additives commonly used in lithium secondary batteries. have.

The conductive material is used to further improve the conductivity of the electrode active material. The conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, 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 thermal 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; Polyphenylene derivatives and the like can be used.

A binder may be further included to bond the positive electrode active material, the lithium metal compound, and the conductive material 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-pentafluoropropylene 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 not necessarily limited to these It is not possible as long as it can be used as a binder in the art.

Examples of other additives are fillers. The filler is selectively used as a component that suppresses the expansion of the electrode, and is not particularly limited as long as it is a fibrous material without causing a chemical change in the battery. For example, olefinic polymers such as polyethylene and polypropylene, or fibrous materials such as glass fibers and carbon fibers are used.

The positive electrode mixture 13 of the present invention is formed on the positive electrode current collector 11.

The positive electrode current collector is generally made to a thickness of 3 μm to 500 μm. The positive electrode current collector 11 is not particularly limited as long as it has high conductivity without causing a chemical change in the lithium secondary battery, and examples thereof include stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel. Surfaces treated with carbon, nickel, titanium, silver, or the like may be used. At this time, the positive electrode current collector 11 may be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric, etc. with fine irregularities formed on the surface to increase the adhesion with the positive electrode active material.

The method of applying the positive electrode mixture 13 on the current collector is a method in which the electrode mixture slurry is distributed over the current collector and then uniformly dispersed using a doctor blade, etc., die casting, comma coating and methods such as (comma coating) and screen printing. In addition, after molding on a separate substrate, the electrode mixture slurry may be bonded to the current collector by pressing or lamination, but is not limited thereto.

The separator used in the lithium secondary battery of the present invention separates or insulates the positive electrode and the negative electrode from each other and enables transport of lithium ions between the positive electrode and the negative electrode, and may be made of a porous non-conductive or insulating material. The separator is an insulator having high ion permeability and mechanical strength, and may be an independent member such as a thin film or a film, or a coating layer added to the anode and / or the cathode. In addition, when a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The pore diameter of the separator is generally 0.01 to 10 μm, and the thickness is generally 5 to 300 μm, and as the separator, a glass electrolyte, a polymer electrolyte, or a ceramic electrolyte may be used. For example, olefin-based polymers such as chemically and hydrophobic polypropylene, sheets or non-woven fabrics made of glass fiber or polyethylene, or the like are used. Typical examples currently on the market include the Celgard series (Celgard ^R 2400, 2300 Hoechest Celanese Corp.), polypropylene separator (manufactured by Ube Industries Ltd. or Pall RAI), and polyethylene series (Tonen or Entek).

The solid electrolyte separator may contain less than about 20% by weight of a non-aqueous organic solvent, and in this case, may further include an appropriate gel-forming compound to reduce the fluidity of the organic solvent. Representative examples of such gel-forming compounds include polyethylene oxide, polyvinylidene fluoride, and polyacrylonitrile.

The production of a lithium secondary battery having the above-described configuration is not particularly limited in the present invention, and can be manufactured through a known method.

In one example, in the case of an all-solid-state battery, the electrolyte of the present invention is disposed between the positive electrode and the negative electrode, and then compression molded to assemble the cell.

The assembled cell is sealed by heat compression after being installed in the exterior material. As the exterior material, a laminate pack such as aluminum or stainless steel, a cylindrical or square metal container may be suitably used.

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 cathode-free lithium secondary battery>

Example 1.

A mixture of LCO (LiCoO ₂ ) and L ₂ N (Li ₂ NiO ₂ ) in a weight ratio of 9: 1 to N-methylpyrrolidone (N-Methyl-2-pyrrolidone) was used as a positive electrode active material, and the positive electrode active material : A conductive material (super-P): binder (PVdF) was mixed in a weight ratio of 95: 2.5: 2.5 and then mixed with a paste face mixer for 30 minutes to prepare a slurry composition.

The slurry composition prepared above was coated on a current collector (Al Foil, 20 μm thick) and dried at 130 ° C. for 12 hours to prepare a positive electrode having a loading of 3 mAh / cm ² .

LiFSI 2.8M was mixed with dimethyl carbonate (DMC) to prepare a first electrolyte, which was injected into a copper current collector, that is, a cathode, and used as a first electrolyte layer.

Poly (ethylene glycol) methyl ether acrylate (PEGMEA), poly (ethylene glycol) diacrylate (PEGDA), SN (Succinonitrile) and LiTFSI 15: A second electrolyte was prepared by mixing in a weight ratio of 5:40:40, which was impregnated into a 48.8% separator with porosity to form a second electrolyte layer, and was interposed between the first electrolyte layer and the third electrolyte layer. .

1M LiPF ₆ and 2% by weight of 1M LiPF ₆ in a mixed solvent of ethylene carbonate (EC), diethylene carbonate (DEC) and dimethyl carbonate (DMC) in a volume ratio of 1: 2: 1 A third electrolyte was prepared by dissolving VC (Vinylene Carbonate), and the third electrolyte layer was formed by injecting it to the positive electrode.

The anode prepared above was placed on the third electrolyte layer to prepare the anode-free lithium secondary battery of Example 1.

Example 2.

A cathode-free lithium secondary battery of Example 2 was manufactured in the same manner as in Example 1, except that L ₂ N (Li ₂ NiO ₂ ) was not used as the positive electrode active material.

Comparative Example 1.

A mixture of LCO (LiCoO ₂ ) and L ₂ N (Li ₂ NiO ₂ ) in a weight ratio of 9: 1 to N-methylpyrrolidone (N-Methyl-2-pyrrolidone) was used as a positive electrode active material, and the positive electrode active material : A conductive material (super-P): binder (PVdF) was mixed in a weight ratio of 95: 2.5: 2.5 and then mixed with a paste face mixer for 30 minutes to prepare a slurry composition.

The slurry composition prepared above was coated on a current collector (Al Foil, 20 μm thick) and dried at 130 ° C. for 12 hours to prepare a positive electrode.

1M LiPF ₆ and 2% by weight in a mixed solvent of ethylene carbonate (EC)), diethylene carbonate (DEC) and dimethyl carbonate (DMC) in a volume ratio of 1: 2: 1 The electrolyte was prepared by dissolving VC (Vinylene Carbonate), and interposed between the positive electrode and the negative electrode by pouring into a 48.8% separator of porosity.

The anode prepared above was placed on a separator to prepare a negative electrode-free lithium secondary battery of Comparative Example 1.

Experimental Example 1. Measurement of ionic conductivity of the electrolyte layer

The ionic conductivity of Example 1 and Comparative Example 1 was measured. The ion conductivity of the first electrolyte layer and the third electrolyte layer of Example 1 was measured using a METTLER TOLEDO conductivity meter, and the ion conductivity of the second electrolyte layer was measured using a SUS / SUS cell. In addition, the ion conductivity of the electrolyte layer of Comparative Example 1 was measured using a METTLER TOLEDO conductivity meter.

The results are shown in Table 2 below.

	Example 1	Comparative Example 1
First electrolyte layer	10X10 -3 S / cm	8X10 -3 S / cm
Second electrolyte layer	2X10 -4 S / cm
3rd electrolyte layer	8X10 -3 S / cm

Experimental Example 2. Characteristic analysis of lithium secondary battery

The negative electrode-free lithium secondary battery prepared in Example 1, Example 2 and Comparative Example 1 was charged once with CC / CV (5% current cut at 1C) of 0.1C and 4.25V to provide a lithium secondary battery having a lithium metal layer formed thereon. It was prepared.

The lithium secondary battery is charged and discharged under the conditions of discharge 3mAh / cm ² based on 0.2C / 0.5C to measure the number of cycles in which the capacity retention rate is 50% or more compared to the initial discharge capacity of the lithium secondary battery in which the lithium metal layer 23 is formed. The results are shown in Table 3 below.

	L 2 N use	When a short occurs	The number of cycles in which the capacity retention rate is 50% or more compared to the initial discharge capacity
Example 1	O	-	17
Example 2	X	-	10
Comparative Example 1	O	2 cycles	Stop short circuit

In the results of Table 3, Example 1 using the high-reversible material L ₂ N did not short, and the number of cycles having a capacity retention ratio of 50% or more compared to the initial discharge capacity was measured to be the highest with 17 cycles. Example 2 did not use the highly irreversible material, L ₂ N, so the number of cycles was lower than that of Example 1. On the other hand, in Comparative Example 1, only one electrolyte layer was included, and short-circuit occurred in 2 cycles, making it impossible to measure capacity retention, and showed very unstable charge / discharge characteristics.

[Description of codes]

11: anode current collector

13: anode mixture

20: cathode

21: cathode current collector

23: lithium metal layer

31: first electrolyte layer

33: second electrolyte layer

Claims (14)

1. In the lithium secondary battery comprising a positive electrode, a negative electrode and an electrolyte,

   The electrolyte includes a first electrolyte layer facing the negative electrode, a second electrolyte layer positioned on the first electrolyte layer and facing the positive electrode,

   The first electrolyte layer has a higher ionic conductivity than the second electrolyte layer,

   A lithium secondary battery in which lithium ions are moved from the positive electrode by charging to form lithium metal on the negative electrode current collector in the negative electrode.
2. According to claim 1,

   The ion conductivity of the first electrolyte layer is 10 ^-5 to 10 ^-2 S / cm,

   Lithium secondary battery, characterized in that the ion conductivity of the second electrolyte layer is 10 ^-6 to 10 ^-3 S / cm.
3. According to claim 1,

   Lithium secondary battery, characterized in that the difference in ionic conductivity between the first electrolyte layer and the second electrolyte layer is 2 to 10 ⁴ times.
4. According to claim 1,

   At least one of the first electrolyte layer and the second electrolyte layer is a lithium secondary battery, characterized in that a semi-solid electrolyte or a solid electrolyte.
5. According to claim 1,

   The first electrolyte layer has a thickness of 0.1 to 20μm, the second electrolyte layer is a lithium secondary battery, characterized in that the thickness is 0.1 to 50μm.
6. According to claim 1,

   The electrolyte is a lithium secondary battery, characterized in that it further comprises at least one electrolyte layer formed on the second electrolyte layer.
7. The method of claim 6,

   A lithium secondary battery characterized in that the electrolyte layer facing the positive electrode of the one or more electrolyte layers formed on the second electrolyte layer has a higher ionic conductivity than the second electrolyte layer.
8. The method of claim 7,

   A lithium secondary battery, characterized in that at least one electrolyte layer formed on the second electrolyte layer is composed of one electrolyte layer.
9. The method of claim 7,

   A lithium secondary battery, characterized in that the electrolyte layer facing the positive electrode among the one or more electrolyte layers formed on the second electrolyte layer has an ionic conductivity of 10 ^-5 to 10 ^-2 S / cm.
10. According to claim 1,

    The electrolyte is a lithium secondary battery, characterized in that a separator is interposed between the electrolyte.
11. The method of claim 10,

    The separator is a lithium secondary battery, characterized in that interposed in a form impregnated with electrolyte.
12. The lithium secondary battery according to claim 1, wherein the lithium metal is formed through one charge in a voltage range of 4.5 to 2.5V.
13. According to claim 1,

    The positive electrode is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _a Co _b Mn _c ) O ₂ (0 <a <1, 0 <b <1, 0 <c <1, a + b + c = 1), LiNi _1-Y Co _Y O ₂ , LiCo _1-Y Mn _Y O ₂ , LiNi _1-Y Mn _Y O ₂ (where 0≤Y <1), Li (Ni _a Co _b Mn _c ) O ₄ (0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), LiMn _2-z Ni _z O ₄ , LiMn _2-z Co _z O ₄ ( Here, 0 <Z <2), Li _x M _y Mn _2-y O _4-z A _z (where 0.9≤x≤1.2, 0 <y <2, 0≤z <0.2, M = Al, Mg, Ni, Co, Fe, Cr, V, Ti, Cu, B, Ca, Zn, Zr, Nb, Mo, Sr, Sb, W, Ti and Bi, one or more, A is -1 or -2 Ideal anion), Li _{1 + a} Ni _b M ' _1-b O _2-c A' _c (where 0≤a≤0.1, 0≤b≤0.8, 0≤c <0.2, M 'is Mn, 1 or more selected from the group consisting of stable elements of 6 coordination such as Co, Mg, and Al, and A 'is -1 or -2 or more anions.), 1 selected from the group consisting of LiCoPO ₄ and LiFePO ₄ A lithium secondary battery comprising the above positive electrode active material.
14. According to claim 1,

    The positive electrode is a lithium secondary battery characterized in that it comprises a lithium metal compound represented by any one of the following formulas 1 to 8:

    [Formula 1]

    Li ₂ Ni _1-a M ¹ _a O ₂

    (In the above formula, a is 0≤a <1, and M ¹ is one or more elements selected from the group consisting of Mn, Fe, Co, Cu, Zn, Mg, and Cd.)

    [Formula 2]

    Li _{2 + b} Ni _1-c M ² _c O _{2 + d}

    (In the above formula, -0.5≤b≤0.5, 0≤c≤1, 0≤d <0.3, M ² is P, B, C, Al, Sc, Sr, Ti, V, Zr, Mn, Fe, Co, It is one or more elements selected from the group consisting of Cu, Zn, Cr, Mg, Nb, Mo and Cd.)

    [Formula 3]

    LiM ³ _e Mn _1-e O ₂

    (In the above formula, 0≤e <0.5, M ³ is one or more elements selected from the group consisting of Cr, Al, Ni, Mn, and Co.),

    [Formula 4]

    Li ₂ M ⁴ O ₂

    (In the above formula, M ⁴ is one or more elements selected from the group consisting of Cu and Ni.)

    [Formula 5]

    Li _{3 + f} Nb _1-g M ⁵ _g S _4-h

    (In the above formula, -0.1≤f≤1, 0≤g≤0.5, -0.1≤h≤0.5, M ⁵ is one or more elements selected from the group consisting of Mn, Fe, Co, Cu, Zn, Mg and Cd to be)

    [Formula 6]

    LiM ⁶ _i Mn _1-i O ₂

    (In the above formula, i is 0.05≤i <0.5, and M ⁶ is one or more elements selected from the group consisting of Cr, Al, Ni, Mn, and Co.)

    [Formula 7]

    LiM ⁷ _2j Mn _2-2j O ₄

    (In the above formula, j is 0.05≤j <0.5, and M ⁷ is one or more elements selected from the group consisting of Cr, Al, Ni, Mn, and Co.)

    [Formula 8]

    Li _k -M ⁸ _m -N _n

    (In the above formula, M ⁸ represents an alkaline earth metal, k / (k + m + n) is 0.10 to 0.40, m / (k + m + n) is 0.20 to 0.50, and n / (k + m + n) is 0.20 to 0.50.)

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