KR100529092B1 - Positive electrode for rechargeable lithium battery and rechargeable lithium battery comprising same - Google Patents

Abstract

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same, wherein the positive electrode includes a positive electrode active material capable of an electrochemically reversible oxidation / reduction reaction and an alkali metal dispersed in the positive electrode active material.

Since the positive electrode for a rechargeable lithium battery of the present invention further includes an alkali metal capable of supplying lithium ions consumed due to the irreversibility of the negative electrode active material during initial charge and discharge, the initial irreversible capacity of the battery may be reduced. In addition, the positive electrode for a lithium secondary battery of the present invention can provide a lithium secondary battery having a high energy density.

Description

A positive electrode for a lithium secondary battery and a lithium secondary battery including the same {POSITIVE ELECTRODE FOR RECHARGEABLE LITHIUM BATTERY AND RECHARGEABLE LITHIUM BATTERY COMPRISING SAME}

[Industrial use]

The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, an initial irreversible capacity is reduced, and a lithium secondary battery positive electrode and a lithium secondary battery including the same, which can provide a battery having excellent energy density. It is about.

[Prior art]

Lithium secondary batteries are prepared by reversibly inserting and detaching lithium ions as a positive electrode and a negative electrode, and filling an organic or polymer electrolyte between the positive electrode and the negative electrode, and lithium ions are inserted / desorbed at the positive electrode and the negative electrode. When produced, electrical energy is generated by oxidation and reduction reactions.

As a cathode active material, a chalcogenide compound is used. Examples thereof include a composite metal such as LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _1-x Co _x O ₂ (0 <x <1), and LiMnO ₂ . Oxides are being studied.

Lithium metal is used as a negative electrode active material of a lithium secondary battery. However, when lithium metal is used, there is a risk of explosion due to a short circuit of the battery due to the formation of dendrite. It is going to be replaced. However, these carbonaceous materials exhibit an irreversible characteristic of 5 to 30% during the initial several cycles, and these irreversible capacities consume lithium ions, preventing them from fully charging or discharging at least one active material, which is disadvantageous in terms of energy density of the battery. It works.

In addition, metal negative electrode active materials such as Si and Sn, which have recently been studied as high-capacity negative electrode active materials, have a larger problem of irreversible characteristics.

In order to solve this problem, U.S. Patent No. 5,948,569 discloses that a Group 1 element is attached to a separator or an electrode using a vacuum deposit method such as evaporation, sputtering, or the like. One method is described for positioning between the positive and negative electrodes. However, because this method uses a deposition process, the initial investment is high, the equipment is difficult to maintain, and the process speed is slow because it takes a lot of time to put the separator or electrode into the vacuum chamber and to make or exhaust the vacuum. have. In addition, Group 1 elements, in particular lithium metal adsorbed in the vacuum chamber during the deposition of lithium metal should be removed regularly, the reactivity is very high, there is a safety problem.

US Patent Publication No. 20020119373 describes a method for producing a negative electrode by uniformly mixing lithium metal powder with an active material. In this case, however, the density difference between the lithium metal and the active material is large, making it difficult to maintain a uniform form in the slurry manufacturing, coating, and drying process. Also, the lithium metal powder is dissolved during the initial charging and discharging process, and the remaining pores cause deformation of the electrode plate in the long term. There is a problem that may lower the life and battery reliability.

An object of the present invention is to provide a positive electrode for a lithium secondary battery that can reduce the initial irreversible capacity during battery charging and discharging.

Another object of the present invention is to provide a lithium secondary battery having an excellent energy density including the cathode active material.

In order to achieve the above object, the present invention provides a positive electrode active material capable of electrochemically reversible oxidation / reduction reaction and a positive electrode for a lithium secondary battery containing an alkali metal dispersed in the positive electrode active material.

The present invention also the anode; A negative electrode including a negative electrode active material; And it provides a lithium secondary battery comprising an electrolyte solution.

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

In the lithium secondary battery, an alkali metal powder is added to the positive electrode in order to prevent a decrease in efficiency of the battery caused by the initial irreversible capacity of the active material when charging or discharging the battery, or to utilize as a source of lithium ions. It relates to a positive electrode for a lithium secondary battery for reducing the initial irreversible capacity upon discharge and increasing the energy density of the battery.

The positive electrode for a lithium secondary battery of the present invention includes a positive electrode active material capable of an electrochemically reversible oxidation / reduction reaction and an alkali metal dispersed in the positive electrode active material.

As said alkali metal, Li, Na, or K is preferable and Li is the most preferable. In the anode of the present invention, the alkali metal is dispersed in the compound in powder form, the average particle size of the powder is preferably 1 to 30㎛, more preferably 5 to 20㎛, most preferably 5 to 10㎛ desirable. If the average particle size of the powder is larger than 30 μm, the pore size is large due to the dissolution of the alkali metal during filling, which causes local collapse of the electrode plate, which is not preferable because of the disadvantage of reducing stability and lifespan. In addition, when the average particle size of the powder is less than 1 mu m, the alkali metal is very difficult to handle, which is not preferable.

In the positive electrode of the present invention, since the alkali metal is added to fill the irreversible capacities of the positive electrode and the negative electrode, the addition amount of the alkali metal may be adjusted according to the type of the positive electrode active material and the negative electrode active material, but is 1 to 100 parts by weight of the total positive electrode. 30 weight part is preferable, and 3-20 weight part is more preferable. If the addition amount of the alkali metal is less than 1 part by weight, the effect of adding the alkali metal is hardly obtained, and a capacity improvement and safety improvement effect cannot be obtained. In addition, when the addition amount of the alkali metal exceeds 30 parts by weight, the alkali metal fills the irreversible capacity of the positive electrode and the negative electrode, and a surplus amount is generated. It is not preferable because it can cause.

As the positive electrode active material capable of the electrochemically reversible oxidation / reduction reaction, a lithium intercalation compound generally used in a lithium ion battery may be used, and inorganic sulfur (S ₈ ) generally used in lithium sulfur jersey. Or sulfur type compound can also be used.

Examples of the thiolated intercalation compound may be selected from the group consisting of the following Chemical Formulas 1 to 14.

[Formula 1]

LiAO ₂

[Formula 2]

LiMn ₂ O ₄

[Formula 3]

Li _a Ni _b B _c M _d O ₂ (0.95 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0.001 ≤ d ≤ 0.1)

[Formula 4]

Li _a Ni _b Co _c Mn _d M _e O ₂ (0.95 ≤ a ≤ 1.1, 0 ≤ b ≤ 0.9, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.5, 0.001 ≤ e ≤ 0.1)

[Formula 5]

Li _a AM _b O ₂ (0.95 ≤ a ≤ 1.1, 0.001 ≤ b ≤ 0.1)

[Formula 6]

Li _a Mn ₂ M _b O ₄ (0.95 ≤ a ≤ 1.1, 0.001 ≤ b ≤ 0.1)

[Formula 7]

DX ₂

[Formula 8]

LiDS ₂

[Formula 9]

V ₂ O ₅

[Formula 10]

LiV ₂ O ₅

[Formula 11]

LiEO ₂

[Formula 12]

LiNiVO ₄

[Formula 13]

Li _(3-x) F ₂ (PO ₄ ) ₃ (0 ≤ x ≤ 3)

[Formula 14]

Li _(3-x) Fe ₂ (PO ₄ ) ₃ (0 ≤ x ≤ 2)

(In Chemical Formulas 1 to 14,

A is selected from the group consisting of Co, Ni and Mn,

B is Co or Mn,

D is Ti, Mo or Mn,

E is selected from the group consisting of Cr, V, Fe, Sc and Y,

F is selected from the group consisting of V, Cr, M, Co, Ni and Cu,

M is at least one metal of transition metal or lanthanide metal selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr and V,

X is O or S)

In addition, the sulfur-based compound is selected from the group consisting of Li ₂ S _n (n ≥ 1), an organic sulfur compound and a carbon-sulfur polymer ((C ₂ S _x ) _n : x = 2.5 to 50, n ≥ 2) Can be used.

In the positive electrode of the present invention, the active compound and the alkali metal are mixed in a solvent to prepare a positive electrode active material composition, and the composition is applied to a current collector to produce a positive electrode. As the solvent, any solvent that can disperse the lithium metal powder by mixing with the positive electrode active material may be used. For example, acetonitrile, acetone, acetone, tetrahydrofurane, dimethylformamide (dimethyl formamide) and N-methyl pyrrolidinone may be used.

A binder may be further added to the cathode active material composition to effectively fix the alkali metal and the active material to the current collector. As the binder, polytetrafluoroethylene, polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and polyhexafluoropropylene, styrene-butadiene rubber, or the like may be used.

In addition, a conductive material may be added to the positive electrode active material composition in order to facilitate electrical contact between the current collector and the positive electrode active material or the alkali metal. As the conductive material, any material used as a conductive material in a lithium secondary battery may be used, and representative examples thereof include carbon black, carbon nanotube, carbon fiber, graphite, graphite fiber or polyaniline, polythiophene, polypyrrole, and the like. Conductive polymers, metal powders such as copper, nickel, aluminum, silver, or metal fibers may be used.

When the positive electrode of the present invention is used in a lithium secondary battery, it is possible to reduce the irreversible capacity caused by the same amount of charge charge and discharge charge of the negative electrode active material during the first charge / discharge (initial charge-discharge) of the battery. That is, the carbon-based negative electrode active material generally used exhibits irreversible characteristics in which the charge charge amount and the discharge charge amount are not the same during the first charge and discharge. Theoretically, lithium occluded carbon works completely reversibly. In practice, however, the cycling efficiency during the first charge and discharge is only 80 to 95%. The cycle efficiency is defined here as the discharge charge amount / charge charge amount * 100. This irreversible characteristic is due to the following two causes. The first is because lithium is consumed in reaction with water or oxygen or functional groups present on the carbon surface, and the second is because lithiated graphite reacts with the electrolyte. The second is usually the main cause. Since the lithium ions consumed by the initial irreversible carbon should be supplied from the positive electrode, a positive electrode active material more than necessary is required, which causes a decrease in energy density. In particular, cobalt oxide, which is frequently used as a cathode active material, is expensive and disadvantageously in terms of cost. This problem can be similarly observed in Si, Sn, tin oxide, composite tin alloys, transition metal oxides, lithium metal nitrides, or lithium metal oxides, which can be used as negative electrode active materials in addition to carbon-based materials.

This initial irreversible capacity generation problem can be solved by using abundant lithium ions consumed as initial irreversible carbon in the positive electrode when the positive electrode of the present invention is further added to the positive electrode. In addition, when using an active material that does not contain lithium, such as V ₂ O ₅ can act as a source of lithium ions.

The lithium secondary battery using the positive electrode of the present invention includes a negative electrode and an electrolyte. The negative electrode may use a carbon-based material, Si, Sn, tin oxide, tin alloy composites, transition metal oxide, lithium metal nitride, or lithium metal oxide as a negative electrode active material.

The electrolyte solution contains a non-aqueous organic solvent and a lithium salt.

The lithium salt is a substance that dissolves in an organic solvent and acts as a source of lithium ions in the battery to enable the operation of a basic lithium secondary battery and to promote the movement of lithium ions between the positive electrode and the negative electrode. Representative examples of such lithium salts are LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiAlO ₄ , LiAlCl ₄ , LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ), where x and y are natural numbers, one selected from the group consisting of LiCl and LiI Or two or more as supporting electrolytic salts. The concentration of the lithium salt is preferably used within the range of 0.1 to 2.0M. If the concentration of the lithium salt is less than 0.1M, the conductivity of the electrolyte is lowered, the performance of the electrolyte is lowered, if it exceeds 2.0M there is a problem that the mobility of the lithium ion is reduced by increasing the viscosity of the electrolyte.

The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the cell can move. As the non-aqueous organic solvent, benzene, toluene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluoro Robenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1, 2,4-trichlorobenzene, iodobenzene, 1,2-diiobenzene, 1,3-diiobenzene, 1,4-diiobenzene, 1,2,3-triio Dobenzene, 1,2,4-triiodobenzene, fluorotoluene, 1,2-difluorotoluene, 1,3-difluorotoluene, 1,4-difluorotoluene, 1,2,3 -Trifluorotoluene, 1,2,4-trifluorotoluene, chlorotoluene, 1,2-dichlorotoluene, 1,3-dichlorotoluene, 1,4-dichlorotoluene, 1,2,3-trichlorotoluene , 1,2,4-trichlorotoluene, iodotoluene, 1,2-dioodotoluene, 1,3-dioodotoluene, 1,4-diodotoluene, 1,2,3-triio Dotoluene, 1,2,4-triiodotoluene, R-CN (wherein R is a straight-chain, branched, or cyclic hydrocarbon group of 2-50 carbon atoms, double bond, aromatic ring, or ether Bonds), dimethylformamide, dimethylacetate, xylene, cyclohexane, tetrahydrofuran, 2-methyltetrahydrofuran, cyclohexanone, ethanol, isopropyl alcohol, dimethyl carbonate, ethylmethyl carbonate, diethyl Carbonate, methylpropyl carbonate, propylene carbonate, methyl propionate, ethyl propionate, methyl acetate, ethyl acetate, propyl acetate, dimethoxyethane, 1,3-dioxolane, diglyme, tetraglyme, ethylene carbonate, propylene One or more of carbonate, γ-butyrolactone, sulfolane, valerolactone, decanolide, mevalolactone Available. The mixing ratio in the case of mixing one or more of the organic solvents can be appropriately adjusted according to the desired battery performance, which can be widely understood by those skilled in the art.

An example of the lithium secondary battery of the present invention having the above-described configuration is shown in FIG. 1. 1 shows a cathode 2, an anode 3, a separator 4 disposed between the cathode 2 and an anode 3, the cathode 2, the anode 3, and the separator 4. The cylindrical lithium ion battery 1 which consists of an impregnated electrolyte solution, the battery container 5, and the sealing member 6 which encloses the electric container 5 as a main part is shown. Of course, the lithium secondary battery of the present invention is not limited to this shape, and it is natural that any type of square, pouch, etc., including the positive electrode active material of the present invention and capable of operating as a battery, can be formed.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention and the present invention is not limited to the following examples.

(Example 1)

90% by weight of the graphite negative electrode active material and 10% by weight of the polyvinylidene fluoride binder were mixed in N-methylpyrrolidone to prepare a negative electrode active material slurry. This negative electrode active material slurry was coated on a Cu foil current collector and dried to prepare a negative electrode.

A positive electrode active material slurry was prepared by mixing a LiCoO ₂ positive electrode active material, a Li metal having an average particle size of 30 μm, a Super-P conductive material, and a polyvinylidene fluoride binder in a tetrahydrofuran solvent. The cathode active material slurry was coated on an Al-foil current collector, dried, and pressed to prepare a cathode. At this time, the ratio of the positive electrode active material, the conductive material and the binder was used to be 94: 3: 3 by weight ratio. The amount of the Li metal was used to 10 parts by weight with respect to 100 parts by weight of the positive electrode active material, wherein the amount of the Li metal was calculated by dividing the lithium theoretical capacity by calculating the irreversible capacity of the negative electrode active material used. That is, the LiCoO ₂ positive electrode active material is an active material exhibiting a charge capacity of 160mAh / g when the charge up to 0.1V, 4.3V at the initial charge and a discharge capacity of 157mAg / g when discharging up to 3.0V, the graphite negative active material is initial Since the capacity is 370 mAh / g and the reversible capacity is 350 mAh / g, since the negative electrode theoretically has an initial irreversible capacity of 20 mAh / g, Li metal is added to the positive electrode active material slurry in an amount corresponding to the initial irreversible capacity.

By using the positive electrode and the negative electrode, a polyethylene polymer separator is sandwiched between the positive electrode and the negative electrode so that the N / P ratio representing the amount of the negative electrode active material to the positive electrode capacity is 1: 1.2 based on the reversible capacity. A lithium secondary battery having a capacity of 700 mAh was produced. At this time, a mixed solvent (3: 5: 1: 1 volume ratio) of ethylene carbonate, dimethyl carbonate, ethylmethyl carbonate and fluorobenzene in which 1M LiPF ₆ was dissolved was used.

(Comparative Example 1)

The same procedure as in Example 1 was carried out except that Li metal having an average particle size of 30 µm was not added to the cathode active material slurry.

After measuring the charge and discharge capacity of the lithium secondary battery prepared according to Example 1 and Comparative Example 1 for one to three charge and discharge cycles, the results are shown in Table 1 below. In addition, in order to determine the effect on safety, etc., an overcharge through experiment was performed and the results are shown in Table 1 together.

Example 1 Comparative Example 1 Anode Active Material Amount 4.460 g 4.827 g Anode Material 2.200 g 2.200 g First charge 815 770 First discharge 705 720 Second charge 703 703 Second discharge 702 702 Third charge 701 700 Third discharge 701 700 Overcharge Penetration Test Results 5L0 5L3

* L0: no change, L1: leakage, L2: smoke, L3: exothermic 200 ℃ or less, L4: exothermic 200 ℃ or more, L5: explosion

From the results shown in Table 1, it can be seen that the battery of Example 1 exhibits almost the same level of battery charge / discharge capacity even when a smaller amount of the positive electrode active material is used than Comparative Example 1. This result is considered that the battery of Example 1 can reduce the irreversible capacity of the negative electrode by adding lithium metal to the active material slurry, thereby obtaining a capacity improving effect.

In Table 1, 5LO means that all five cells have no change in the overcharge penetration experiment, and thus, the batteries of Example 1 are all safe batteries that passed the overcharge penetration experiment. On the contrary, in the battery of Comparative Example 1, all five batteries exhibit heat generation of 200 ° C. or lower, and thus, the battery of Comparative Example 1 is inferior in safety. As described above, the battery of Example 1 was much better in safety than Comparative Example 1 in the case of Example 1 because it includes a lithium metal coating layer, the irreversible capacity is reduced, the amount of the positive electrode active material to exhibit the same level of capacity It is considered that there is little compared with the comparative example 1.

As described above, the positive electrode for a rechargeable lithium battery of the present invention further includes an alkali metal capable of supplying lithium ions consumed due to the irreversibility of the negative electrode active material during initial charging and discharging, thereby reducing the initial irreversible capacity of the battery. In addition, the positive electrode for a lithium secondary battery of the present invention can provide a lithium secondary battery having a high energy density.

1 is a view showing a schematic structure of a lithium secondary battery of the present invention.

Claims (15)

A cathode active material capable of electrochemically reversible oxidation / reduction reaction; And Alkali metal dispersed in the said positive electrode active material A positive electrode for a lithium secondary battery comprising a. The positive electrode for a rechargeable lithium battery of claim 1, wherein the alkali metal is selected from the group consisting of Li, Na, and K. 3. The positive electrode for a lithium secondary battery according to claim 2, wherein the alkali metal is Li. The positive electrode for a rechargeable lithium battery of claim 1, wherein the alkali metal is in the form of particles having an average particle size of 1 to 30 μm. The cathode of claim 4, wherein the alkali metal is in the form of particles having an average particle size of 5 to 20 μm. The cathode of claim 5, wherein the alkali metal is in the form of particles having an average particle size of 5 to 10 μm. The positive electrode for a lithium secondary battery of claim 1, wherein the compound is selected from the group consisting of a lithium intercalation compound, an inorganic sulfur (S ₈ ) and a sulfur-based compound. A cathode active material capable of electrochemically reversible oxidation / reduction reaction; And an alkali metal dispersed in the positive electrode active material; A negative electrode including a negative electrode active material; And Lithium secondary battery containing an electrolyte solution. The lithium secondary battery of claim 8, wherein the alkali metal is selected from the group consisting of Li, Na, and K. 10. The lithium secondary battery of claim 9, wherein the alkali metal is Li. The lithium secondary battery of claim 8, wherein the alkali metal is in the form of particles having an average particle size of 1 to 30 μm. The lithium secondary battery of claim 11, wherein the alkali metal is in the form of particles having an average particle size of 5 to 20 μm. The lithium secondary battery of claim 12, wherein the alkali metal is in the form of particles having an average particle size of 5 to 10 μm. The lithium secondary battery according to claim 9, wherein the compound is selected from the group consisting of a lithium intercalation compound, an inorganic sulfur (S ₈ ), and a sulfur-based compound. The method of claim 8, wherein the negative electrode active material is selected from the group consisting of carbon-based materials, Si, Sn, tin oxide, composite tin alloys, transition metal oxide, lithium metal nitride and lithium metal oxide Secondary battery.