KR101147601B1 - Cathode Active Material with Modified Surface - Google Patents

Abstract

The present invention is a cathode active material comprising a lithium nickel-manganese-cobalt oxide represented by the following formula (1), wherein the lithium nickel-manganese-cobalt oxide has a nickel content of at least 40% of the total transition metal, Li-containing by-products Provided is a cathode active material in which Al ₂ O ₃ is coated on a surface of the oxide to suppress reactivity of the oxide.

Li _x M _y O ₂ (1)

Where

M is Ni _1-ab Mn _a Co _b (0.05 ≦ a ≦ 0.4, 0.1 ≦ b ≦ 0.4),

2로서 0.95 ≤ x ≤ 1.05 이다.x + y

2 as 0.95 ≦ x ≦ 1.05.

In the cathode active material according to the present invention, Al ₂ O ₃ is coated on the surface of the oxide, thereby reducing the content of water-soluble bases such as Li ₂ CO ₃ and LiOH, which are reaction residues of a Li source, in the process of preparing the oxide. It is possible to suppress the generation of gas generated by decomposition or reaction with the electrolyte during charging.

Description

Cathode Active Material with Modified Surface

The present invention relates to a positive electrode active material for secondary batteries, and more particularly, to a positive electrode active material comprising lithium nickel-manganese-cobalt oxide having a specific composition, wherein the lithium nickel-manganese-cobalt oxide is selected from nickel in the entire transition metal. The present invention relates to a lithium secondary battery comprising a positive electrode active material having an amount of 40% or more and coated with Al ₂ O ₃ on the surface of the oxide so as to suppress reactivity of a Li-containing byproduct.

As the development and demand for mobile devices increases, the demand for secondary batteries as energy sources is rapidly increasing. Among them, lithium secondary batteries with high energy density and voltage, long cycle life, and low self discharge rate It is commercially used and widely used.

Lithium-containing cobalt oxide (LiCoO ₂ ) is mainly used as a positive electrode active material of a lithium secondary battery. In addition, lithium-containing manganese oxides such as LiMnO _{2 having a} layered crystal structure and LiMn ₂ O _{4 having a} spinel crystal structure, and lithium-containing nickel oxide The use of (LiNiO ₂ ) is also contemplated.

Among the positive electrode active materials, LiCoO ₂ is widely used because of its excellent physical properties such as cycle characteristics. However, LiCoO ₂ is low in safety, expensive due to resource limitations of cobalt as a raw material, and limited for mass use as a power source in fields such as electric vehicles. There is.

Lithium manganese oxides such as LiMnO ₂ and LiMn ₂ O ₄ have the advantage of using a resource-rich and environmentally friendly manganese as a raw material, attracting a lot of attention as a cathode active material that can replace LiCoO ₂ . However, these lithium manganese oxides have disadvantages of small capacity and poor cycle characteristics.

On the other hand, lithium nickel-based oxides such as LiNiO ₂ have a lower discharge cost than the cobalt-based oxides, and show a high discharge capacity when charged to 4.3 V. Thus, the reversible capacity of the doped LiNiO ₂ is the capacity of LiCoO ₂ (about 165). mAh / g) in excess of about 200 mAh / g. Thus, despite slightly lower average discharge voltages and volumetric densities, commercialized cells containing LiNiO ₂ positive electrode active materials have improved energy densities. Research is active. However, the LiNiO ₂ based positive electrode active material is limited in practical use due to the following problems.

First, the LiNiO ₂ -based oxide has a sudden phase transition of the crystal structure according to the volume change accompanying the charge-discharge cycle, and thus voids may occur in the cracks or grain boundaries of the particles. Therefore, since the occlusion and release of lithium ions are disturbed to increase the polarization resistance, there is a problem that the charge and discharge performance is lowered. In order to prevent this, conventionally, LiNiO ₂ based oxides were prepared by excessively adding a Li source and reacting in an oxygen atmosphere in the manufacturing process. The cathode active material prepared in this manner has a structure by repulsive force between oxygen atoms in a charged state. There is a problem in that it becomes unstable while expanding and the cycle characteristics are seriously deteriorated by repetition of charge and discharge.

Secondly, LiNiO ₂ has a problem that excess gas is generated during storage or cycle, which is a reaction residue because LiNiO ₂ is excessively added and heat treated during the manufacturing of LiNiO ₂ system to form a crystal structure well. _This is because water-soluble bases such as ₂ CO ₃ and LiOH remain between the primary particles to decompose or to react with the electrolyte during charge to generate CO ₂ gas. In addition, since LiNiO ₂ particles have a structure of secondary particles in which primary particles are agglomerated, the contact area of the electrolyte is large, which causes the phenomenon to become more serious, thereby causing a swelling phenomenon of the battery and causing a high temperature. It has a problem of degrading safety.

Third, LiNiO ₂ has a problem in that the chemical resistance is sharply lowered at the surface when exposed to air and moisture, and gelation of the slurry occurs as the NMP-PVDF slurry starts to polymerize due to high pH. These properties cause serious process problems during cell production.

Fourth, the quality LiNiO ₂ is a simple solid-phase reaction, such as that used in the production of LiCoO ₂ can not be produced, LiNiMO ₂ cathode active material containing the required dopant is cobalt, and that the other dopant of manganese, aluminum and the like are LiOH? H _The production cost is high because a lithium raw material such as ₂ O and a mixed transition metal hydroxide are produced by reacting in an oxygen or syngas atmosphere (that is, a CO ₂ deficient atmosphere). In addition, the production cost is further increased when additional steps such as washing or coating to remove impurities in the production of LiNiO ₂ . Therefore, many prior arts focus on improving the properties of LiNiO ₂ based cathode active materials and the manufacturing process of LiNiO ₂ .

Accordingly, lithium transition metal oxides in which a part of nickel is substituted with other transition metals such as manganese and cobalt have been proposed. However, these metal-substituted nickel-based lithium transition metal oxides have advantages in that they have relatively superior cycle characteristics and capacity characteristics, but even in this case, the cycle characteristics deteriorate rapidly during long-term use, and swelling by gas generation in a battery, Problems such as low chemical stability have not been sufficiently solved.

The reason is that the nickel-based lithium transition metal oxide is in the form of secondary particles in which small primary particles are agglomerated, so that lithium ions move to the surface of the active material and react with moisture or CO ₂ in air to react with Li _2. Swelling of the battery by forming impurities such as CO ₃ and LiOH, or by forming impurities of nickel-based lithium transition metal oxides to reduce battery capacity or decomposing in the battery to generate gas It is understood to generate.

Therefore, there is a high need for a technology capable of solving the high temperature safety problem caused by impurities while using a lithium nickel-based cathode active material suitable for high capacity.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems of the prior art and the technical problems required from the past.

After extensive research and various experiments, the inventors of the present application include lithium nickel-manganese-cobalt oxide having a specific composition in which at least 40% of the total transition metal is nickel and Al ₂ O ₃ is coated on the surface. The positive electrode active material exhibits excellent cycle characteristics and high battery capacity due to its stable crystal structure and high nickel content, while Li ₂ CO, a by-product of Li, caused by the excess Li source used in the process of preparing the oxide. _It has been found that the reactivity of ₃ , LiOH and the like can be greatly suppressed, and the generation of gases generated by these decomposition or reaction with the electrolytic solution at the time of charging can be suppressed, and the present invention has been completed.

Accordingly, the cathode active material according to the present invention is a cathode active material comprising lithium nickel-manganese-cobalt oxide represented by Formula 1, wherein the lithium nickel-manganese-cobalt oxide has a content of 40% of nickel in the total transition metal. or more, there is on the surface of the oxides Al ₂ O ₃ to inhibit the reactivity of the Li-containing by-products consists in that the coating.

Li _x M _y O ₂ (1)

Where

M is Ni _1-ab Mn _a Co _b (0.05 ≦ a ≦ 0.4, 0.1 ≦ b ≦ 0.4),

2로서 0.95 ≤ x ≤ 1.05 이다.x + y

2 as 0.95 ≦ x ≦ 1.05.

Since the positive electrode active material according to the present invention has a Ni content of 40% or more of the transition metal, it is possible to exhibit high battery capacity and to prevent a decrease in cycle characteristics by a stable crystal structure, and thus a conventional lithium transition metal of high nickel content. The problem of oxide can be solved.

In addition, safety can be greatly improved by suppressing the reactivity of Li-containing by-products, such as self-decomposition reactions and electrolyte solution decomposition reactions, which are generally a problem in high nickel transition metal oxides.

Representative examples of the Li-containing by-products include, but are not limited to, Li ₂ CO ₃ , LiOH, and the like, unreacted materials remaining in the preparation of lithium nickel-manganese-cobalt oxide, side reaction products in the manufacturing process, and preparation. After that, the concept includes all of the reaction products caused by the additional reaction.

In the present invention, "inhibiting the reactivity of the Li-containing by-products" means reducing the residual amount of the Li-containing by-products, chemically blocking the reaction sites of the Li-containing by-products, or physically wrapping the Li-containing by-products, It is used as a concept that includes preventing the self-reaction of Li-containing by-products, induced reaction to other substances, and mutual reactions with other substances. Therefore, in the present invention, Al ₂ O ₃ to be applied to the surface of the oxide should be interpreted to include all of inhibiting the reactivity of the Li-containing by-products in any one of the above-mentioned manner or two or more ways.

In the lithium nickel-manganese-cobalt oxide of Chemical Formula 1, the total mole fraction (1-a-b) of Ni is preferably 0.4 to 0.9 as a composition of nickel excess relative to manganese and cobalt. If the nickel content is less than 0.4, it is difficult to expect a high capacity. On the contrary, if the nickel content is more than 0.9, the safety may be greatly reduced, which is not preferable.

The content (b) of cobalt is 0.1 to 0.4, and when the cobalt content is too high at b> 0.4, the cost of the raw material is increased as a whole and the reversible capacity is slightly reduced. On the other hand, if the content of cobalt is too low (b <0.1), it may be difficult to achieve sufficient rate characteristics and high powder density of the battery at the same time.

If the lithium content is too high, i.e., x> 1.05, the safety may be low during the cycle, especially at high voltage (U = 4.35 V) at T = 60 ° C., on the contrary, if the lithium content is too low, i.e. If x <0.95, it shows low rate characteristics and the reversible capacity can be reduced.

The average particle diameter (D50) of the lithium nickel-manganese-cobalt oxide is preferably 30 μm or less, more preferably 3 to 20 μm.

The lithium nickel-manganese-cobalt oxide is preferably in the form of an agglomerated structure, that is, in the form of agglomerates of fine powders, and thus may have a structure having internal pores. Such agglomerated particle structure is characterized by maximizing the surface area reacting with the electrolyte, exerting high rate characteristics, and increasing the reversible capacity of the positive electrode.

In this case, the lithium nickel-manganese-cobalt oxide may be composed of agglomerates of fine particles having an average particle diameter (D90) of 0.01 to 5 μm.

The method of applying Al ₂ O ₃ to the surface of the lithium nickel-manganese-cobalt oxide may be various, and may be preferably applied by a wet method. Wet methods may be desirable in terms of uniform application. An example of this wet method is as follows.

First, aluminum alkoxide, which is a precursor of Al ₂ O ₃ , is dissolved in an organic solvent to prepare a coating solution. The organic solvent can be appropriately selected from solvents capable of dissolving aluminum alkoxides. The prepared coating solution and the lithium transition metal oxide of Chemical Formula 1 are mixed and stirred, and the obtained mixture is heat treated. This heat treatment may be carried out at a temperature at which the aluminum alkoxide may be changed to aluminum oxide by heat, for example, may be performed at about 600 ~ 1000 ℃, preferably about 700 ℃. The heat treatment time is also related to the heat treatment temperature and the like, for example, may be performed for 3 to 7 hours. However, the heat treatment temperature and time should be performed under conditions that do not cause chemical and / or physical property changes of the lithium transition metal oxide.

The form of Al ₂ O ₃ coated on the surface of the lithium nickel-manganese-cobalt oxide may vary, and includes all of the partially coated form and the whole coated form. 1 schematically illustrates one exemplary form in which Al ₂ O ₃ has applied the entire surface of the lithium nickel-manganese-cobalt oxide.

When the Al ₂ O ₃ coating area is too large, the mobility of lithium ions may be reduced, and the rate characteristic may be reduced, and when the coating area is too small, it may be difficult to achieve a desired effect, so that the entire lithium nickel-manganese-cobalt oxide Preference is given to 20% to 80% of the surface.

In addition, the coating thickness of Al ₂ O ₃ may be determined by various factors such as a coating method, an amount or a proportion of reactants, and may preferably be in the range of 0.1 to 10 μm.

In the cathode active material according to the present invention, lithium nickel-manganese-cobalt oxide has an excellent crystal structure, as described above, by a specific composition.

Specifically, as shown in FIG. 2, the lithium nickel-manganese-cobalt oxide has lithium ions occluded and released between the mixed transition metal oxide layers, and the mixed transition metal oxide is contained in the reversible lithium layer for occluding and releasing the lithium ions. Ni ions derived from the layer are inserted to bond the mixed transition metal oxide layers to each other.

Generally, in the case of using a nickel high content lithium transition metal oxide having a layered crystal structure as a positive electrode active material, the crystal structure is formed by the repulsive force between oxygen atoms of the mixed transition metal oxide layers while lithium is released from the reversible lithium layer in a charged state. There is a problem that the capacity becomes unstable as it expands, and thus the crystal structure changes due to charge and discharge repeats, thereby rapidly decreasing capacity and cycle characteristics.

On the other hand, according to the present invention, some Ni in the reversible lithium layer Since the ions are inserted and bonded, even when lithium is released during the charging process, the oxidation number of the Ni ions inserted into the reversible lithium layer is maintained while the crystal structure is not collapsed and the well-developed layered structure can be maintained. Therefore, a battery containing lithium nickel-manganese-cobalt oxide having such a structure as a positive electrode active material can exhibit high capacity and excellent cycle characteristics.

This result is completely different from the conventional notion that the occlusion and release of lithium will be disturbed when some nickel is lowered into the reversible lithium layer and fixed, and no further structural breakdown by oxygen desorption occurs and no further Ni By preventing the occurrence of ²⁺ , the life characteristics and the safety are improved at the same time, the battery capacity and the cycle characteristics can be greatly improved, and there is an advantage that the desired level characteristics can be exhibited.

In one preferred example, the lithium nickel-manganese-cobalt oxide layer may have a structure in which Ni ²⁺ and Ni ³⁺ coexist, and some Ni ²⁺ is inserted into the reversible lithium layer. That is, in this structure, the Ni ions inserted into the reversible lithium layer are all Ni ²⁺ and the oxidation number value of the ions does not change during the charging process.

Specifically, when Ni ²⁺ and Ni ³⁺ coexist in the excess nickel mixed transition metal oxide, if the oxygen atom deprivation state is provided under a predetermined condition (reaction atmosphere, Li content, etc.), the oxidation number value of Ni is As it changes, some Ni ²⁺ can be intercalated into the reversible lithium layer.

The mole fraction of Ni ²⁺ inserted into and bonded to the reversible lithium layer may be preferably 0.03 to 0.07 based on the total amount of Li bonding sites of the reversible lithium layer. If the mole fraction is too small, cycle characteristics may be degraded due to instability of the crystal structure, and if too large, problems such as capacity reduction may occur.

The preferred content of nickel in the lithium nickel-manganese-cobalt oxide may be 40 to 70% based on the total amount of the transition metal (molar ratio).

The lithium nickel-manganese-cobalt oxide having a unique crystal structure as described above may be prepared by, for example, solid-phase reaction of a lithium precursor such as Li ₂ CO ₃ and a mixed transition metal precursor in an oxygen deficient atmosphere such as an air atmosphere.

The positive electrode active material according to the present invention may be used alone or as a mixture with other positive electrode active materials.

In one preferred example, the positive electrode active material (a) may be used in admixture with the positive electrode active material (b) having the formula LiCoO ₂ . In this case, the mixing ratio of the positive electrode active materials (positive electrode active material (a): positive electrode active material (b)) may be 5: 95 to 90: 10 by weight.

Secondary batteries using a combination of the positive electrode active materials as described above, in comparison with secondary batteries using a high nickel content of lithium transition metal oxide, generates a very small amount of gas at a high temperature to suppress the increase in thickness of the battery. By doing so, excellent safety can be exhibited and the amount of reduction in the capacity of the battery can be further lowered.

The present invention also provides a lithium secondary battery comprising the cathode active material. A lithium secondary battery is comprised with a positive electrode, a negative electrode, a separator, lithium salt containing nonaqueous electrolyte, etc., for example.

The positive electrode is produced by, for example, applying a mixture of a positive electrode active material, a conductive material, and a binder onto a positive electrode current collector, followed by drying, and further, a filler may be further added as necessary. The negative electrode is also manufactured by applying and drying a negative electrode material on the negative electrode current collector, and if necessary, the components as described above may be further included.

Examples of the negative electrode active material include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene, and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and Ti which can be alloyed with lithium and compounds containing these elements; Complexes of metals and their compounds and carbon and graphite materials; Lithium-containing nitrides, and the like. Among them, a carbon-based active material, a silicon-based active material, a tin-based active material, or a silicon-carbon based active material is more preferable, and these may be used singly or in combination of two or more.

The separator is an insulating thin film interposed between the anode and the cathode and having high ion permeability and mechanical strength. The pore diameter of the separator is generally 0.01 to 10 mu m and the thickness is generally 5 to 300 mu m. As such a separation membrane, for example, a sheet or a nonwoven fabric made of an olefin-based polymer such as polypropylene which is chemically resistant and hydrophobic, glass fiber, polyethylene or the like is used. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

Examples of the binder include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butyrene rubber, fluorine rubber, various copolymers, high polymer high polyvinyl alcohol, and the like.

The conductive material is a component for further improving the conductivity of the electrode active material, and may be added in an amount of 1 to 30 wt% based on the total weight of the electrode mixture. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, for example, graphite such as natural graphite or artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Carbon derivatives such as carbon nanotubes and fullerenes, conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The viscosity adjusting agent may be added up to 30% by weight based on the total weight of the electrode mixture, so as to control the viscosity of the electrode mixture so that the mixing process of the electrode mixture and the coating process on the collector may be easy. Examples of such viscosity modifiers include carboxymethylcellulose, polyvinylidene fluoride and the like, but are not limited thereto. In some cases, the above-described solvent may play a role as a viscosity adjusting agent.

The filler is an auxiliary component that suppresses the expansion of the electrode, and is not particularly limited as long as it is a fibrous material without causing chemical changes in the battery. Examples of the filler include olefin polymers such as polyethylene and polypropylene; Fiber-like substances such as glass fibers and carbon fibers are used.

The coupling agent is an auxiliary component for increasing the adhesion between the electrode active material and the binder, characterized in that it has two or more functional groups, it can be used up to 30% by weight based on the weight of the binder. Such coupling agents include, for example, one functional group reacting with a hydroxyl group or a carboxyl group on the surface of a silicon, tin, or graphite-based active material to form a chemical bond, and the other functional group is chemically bonded through a reaction with a polymer binder. It may be a material forming a. Specific examples of the coupling agent include triethoxysilylpropyl tetrasulfide, mercaptopropyl triethoxysilane, aminopropyl triethoxysilane, and chloropropyl triethoxysilane ( chloropropyl triethoxysilane, vinyl triethoxysilane, methacryloxypropyl triethoxysilane, glycidoxypropyl triethoxysilane, isocyanatopropyl triethoxysilane, cyan Although silane coupling agents, such as atopropyl triethoxysilane, are mentioned, It is not limited only to these.

The adhesion promoter is an auxiliary component added to improve adhesion of the active material to the current collector, and may be added in an amount of 10 wt% or less, for example, oxalic acid, adipic acid, Formic acid, acrylic acid derivatives, itaconic acid derivatives, and the like.

As the molecular weight modifier, t-dodecyl mercaptan, n-dodecyl mercaptan, n-octyl mercaptan, etc. may be used, and as a crosslinking agent, 1,3-butanediol diacrylate, 1,3-butanediol dimethacrylate, 1,4-butanediol diacrylate, 1,4-butanediol dimethacrylate, aryl acrylate, aryl methacrylate, trimethylolpropane triacrylate, tetraethyleneglycol diacrylate, tetraethyleneglycol dimethacrylate or Divinylbenzene and the like can be used.

In the electrode, the current collector is a portion where electrons move in the electrochemical reaction of the active material, and a negative electrode current collector and a positive electrode current collector exist according to the type of electrode.

The negative electrode current collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used.

The positive electrode current collector is generally made to a thickness of 3 to 500 ㎛. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver or the like can be used.

These current collectors may form fine concavities and convexities on the surface thereof to enhance the bonding strength of the electrode active material, and may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and nonwoven fabrics.

The lithium-containing non-aqueous electrolyte solution consists of a non-aqueous electrolyte solution and a lithium salt.

Examples of the nonaqueous electrolytic solution include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, But are not limited to, lactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, Nitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives , Tetrahydrofuran derivatives, ether, methyl pyrophosphate, ethyl propionate and the like can be used.

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

In some cases, an organic solid electrolyte, an inorganic solid electrolyte, or the like may be used.

Examples of the organic solid electrolytes include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, poly edgementation lysine, polyester sulfides, polyvinyl alcohols, polyvinylidene fluorides, Polymers containing ionic dissociating groups 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 , Nitrides, halides, sulfates and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

In addition, for the purpose of improving charge / discharge characteristics, flame retardancy, etc., the non-aqueous electrolyte solution includes, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, and hexaphosphate triamide. Nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum trichloride, etc. It may be. In some cases, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further added to impart nonflammability, or a carbon dioxide gas may be further added to improve high-temperature storage characteristics.

The lithium secondary battery according to the present invention can be produced by conventional methods known in the art. In addition, the structure of the positive electrode, the negative electrode, and the separator in the lithium secondary battery according to the present invention is not particularly limited, and, for example, each of these sheets in a winding type (winding type) or stacking (stacking type) cylindrical, rectangular Or it may be a form inserted into the case of the pouch type.

The lithium secondary battery according to the present invention may be preferably used as a power source for a hybrid vehicle, an electric vehicle, a power tool, and the like, which require particularly high rate characteristics and high temperature safety.

Hereinafter, the present invention will be further described with reference to Examples and the like, but the scope of the present invention is not limited thereto.

[Production Example 1]

Mixed hydroxide MOOH (M = Ni _4/15 (Mn _1/2 Ni _1/2 ) _8/15 Co _0.2 ) was used as the mixed transition metal precursor, and the mixed hydroxide and Li ₂ CO ₃ were stoichiometric ratio (Li : M = 1.02: 1) and the mixture was sintered at 920 ° C. for 10 hours in air to prepare LiNi _0.53 Co _0.2 Mn _0.27 O ₂ . At this time, the secondary particles did not collapse at all and remained intact.

X-ray analysis confirmed that all samples had a well-developed layered crystal structure. In addition, the unit cell volume did not change significantly with increasing sintering temperature, which means that no significant oxygen deficiency and no significant increase in cation mixing occurred, and essentially no lithium evaporation occurred.

LiNi _0.53 Co _0.2 Mn _0.27 O ₂ It was confirmed that nickel is inserted into the reversible lithium layer as much as 3.9 ~ 4.5%, and an appropriate amount of Ni ²⁺ is inserted into the lithium layer to obtain structural stability.

Example 1

0.9 g of aluminum isopropoxide was added to 25 ml of propanol and heated to 50 ° C., followed by stirring until completely dissolved in a magnetic bar to prepare a coating solution.

A LiNi prepared in Preparative Example _{1 0.53 Co 0.2 Mn 0.27 O 2} 30 g The heat-treated into the coating liquid for containing a crucible to give good mixing, in dried, 700 ℃ in an oven at 130 ~ 150 ℃ for 5 hours, LiNi _0.53 A lithium nickel-manganese-cobalt oxide having Al ₂ O ₃ coated on the surface of Co _0.2 Mn _0.27 O ₂ was prepared.

The lithium nickel-manganese-cobalt oxide was mixed with Super P as a conductive material and polyvinylidene fluoride as a binder in a weight ratio of 92: 4: 4, and NMP (N-methyl pyrrolidone) was added to prepare a slurry. It was. The positive electrode slurry was applied to an aluminum current collector and then dried in a vacuum oven at 120 ° C. to prepare a positive electrode.

In addition, MCMB (mesocarbon microbead) is used as an active material, super P is used as a conductive material, and PVdF is used as a binder, respectively, in a ratio (weight ratio) of 92: 2: 6, and dispersed in NMP, followed by copper foil. It was coated on to prepare a negative electrode.

An electrode assembly was prepared using a porous separator made of polypropylene between the cathode and the anode thus prepared. The electrode assembly was placed in a pouch-type case and the electrode leads were connected. Then, a volume ratio of 1: 1 ethylene carbonate (EC) and dimethyl carbonate (DMC) solution in which 1 M LiPF ₆ was dissolved was injected into the electrolyte and then sealed. , A lithium secondary battery was produced.

[Example 2]

Except for using the positive electrode active material in which Al ₂ O ₃ obtained in Example 1 was uniformly mixed with lithium nickel-manganese-cobalt oxide and LiCoO ₂ in a ratio of 50:50 (weight ratio), an example was used. A lithium secondary battery was produced in the same manner as in 1.

Comparative Example 1

A lithium secondary battery was manufactured in the same manner as in Example 1, except that lithium nickel-manganese-cobalt oxide obtained in Preparation Example 1 was used as the cathode active material.

Comparative Example 2

A lithium secondary battery was prepared in the same manner as in Example 1, except that a positive electrode active material in which the lithium nickel-manganese-cobalt oxide and LiCoO ₂ obtained in Preparation Example 1 were uniformly mixed at a ratio of 50:50 (weight ratio) was used. The battery was produced.

[Experimental Example 1]

For the positive electrode active material used in Example 1 and the positive electrode active material used in Comparative Example 1 was measured the content of Li-containing by-products by pH titration, the results are shown in Table 1 below.

TABLE 1

Example 1 Comparative Example 1 Al content (ppm) 529 6 Li ₂ CO ₃ (%) 0.055 0.094 LiOH (%) 0.066 0.109 Total Excess Li (%) 0.121 0.203

As shown in Table 1, the positive electrode active material of Comparative Example 1 had a higher content of Li ₂ CO ₃ and LiOH than the positive electrode active material of Example 1, the unreacted lithium in which the positive electrode active material of Example 1 is present on the surface It was found that the amount of impurities was small.

Content difference by pH titration as described above, the Al ₂ O ₃ Li actual surface residual amount of containing a by-product in the process for the surface coating of the positive electrode active material due to decreased or the Li-containing by-products by the Al ₂ O ₃ Chemical Or physically deactivated, indicating that at least its reactivity has been greatly reduced.

[Experimental Example 2]

A high temperature storage test (full charge, 90 ° C., 4 hours) was performed on the lithium secondary battery prepared in Example 2 and the lithium secondary battery of Comparative Example 1, and the results are shown in Table 2 below.

TABLE 2

90 ℃, 4hr Example 2 Comparative Example 2 △ T (mm) 1.92 (1.85-1.99) 2.04 (1.91-2.09) Recovery (%) 84.2 (82.5-86.7) 80.9 (78.9 ~ 84.2)

As shown in Table 2, the battery of Example 2 in the high temperature storage test, compared to the battery of Comparative Example 2, due to the relatively small amount of gas issuance, the increase in the thickness of the battery is significantly lower, the battery capacity retention rate is more It can be seen that excellent.

As described above, the positive electrode active material according to the present invention can exhibit a high battery capacity due to the high nickel content, can prevent the degradation of the cycle characteristics by a stable crystal structure, in particular, a lithium transition metal oxide of high nickel content In general, safety can be greatly improved by suppressing the reactivity of Li-containing by-products, which are generally a problem in the reaction, for example, self-decomposition reaction, electrolyte decomposition reaction, and the like.

Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

1 is a schematic diagram of lithium nickel-manganese-cobalt oxide coated with Al ₂ O ₃ according to one embodiment of the present invention;

2 is a schematic diagram of a crystal structure of a positive electrode active material according to one embodiment of the present invention.

Claims (15)

A cathode active material comprising lithium nickel-manganese-cobalt oxide represented by Formula 1, wherein the lithium nickel-manganese-cobalt oxide has a nickel content of 40% or more of all transition metals, and exhibits reactivity of a Li-containing byproduct. A cathode active material characterized in that Al ₂ O ₃ is coated on the surface of the oxide so as to be suppressed: Li _x M _y O ₂ (1) Where M is Ni _1-ab Mn _a Co _b (0.05 ≦ a ≦ 0.4, 0.1 ≦ b ≦ 0.4), x + y

2 as 0.95 ≦ x ≦ 1.05. The cathode active material according to claim 1, wherein the lithium nickel-manganese-cobalt oxide is particles having an agglomerated structure composed of aggregates of fine particles. The cathode active material according to claim 1, wherein the average particle diameter (D50) of the lithium nickel-manganese-cobalt oxide is 3 to 20 µm. The cathode active material according to claim 2, wherein the lithium nickel-manganese-cobalt oxide is formed of an aggregate of fine particles having an average particle diameter (D90) of 0.01 to 8 µm. The cathode active material according to claim 1, wherein the Al ₂ O ₃ is coated on the surface of the lithium nickel-manganese-cobalt oxide by a wet method. The cathode active material according to claim 1, wherein Al ₂ O ₃ is coated with 20% to 80% of the total surface of lithium nickel-manganese-cobalt oxide. The cathode active material according to claim 1, wherein the coating thickness of Al ₂ O ₃ is in the range of 0.1 to 10 μm. The lithium nickel-manganese-cobalt oxide of claim 1, wherein lithium ions are occluded and released between the mixed transition metal oxide layers, and the reversible lithium layer for occluding and releasing the lithium ions is derived from the mixed transition metal oxide layer. The positive electrode active material, characterized in that the Ni ions are inserted to bond the mixed transition metal oxide layers to each other. The cathode active material according to claim 8, wherein the mole fraction of Ni ions inserted into and bonded to the reversible lithium layer is 0.03 to 0.07. The cathode active material according to claim 1, wherein the nickel content is 40 to 70% based on the total amount of the transition metal (molar ratio). A cathode active material comprising a mixture of a cathode active material (a) according to any one of claims 1 to 10 and a cathode active material (b) of formula LiCoO ₂ . The positive electrode active material according to claim 11, wherein the mixing ratio of the positive electrode active material (positive electrode active material (a): positive electrode active material (b)) is 5: 95 to 90: 10 by weight. A lithium secondary battery comprising the cathode active material according to claim 1. The lithium secondary battery of claim 13, wherein the secondary battery is used as a power source for a hybrid vehicle, an electric vehicle, or a power tool. A lithium secondary battery comprising the cathode active material according to claim 11.

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