CN109314238B

CN109314238B - Metal-doped positive electrode active material for high voltage

Info

Publication number: CN109314238B
Application number: CN201780036217.XA
Authority: CN
Inventors: 朴星彬; 朴英旭; 朴知伶; 李宝蓝; 赵治皓; 许赫; 郑王谟
Original assignee: LG Chem Ltd
Current assignee: LG Energy Solution Ltd
Priority date: 2016-12-21
Filing date: 2017-12-20
Publication date: 2022-04-01
Anticipated expiration: 2037-12-20
Also published as: CN109314238A; WO2018117634A1

Abstract

The invention relates to a metal element doped positive electrode active material for high voltage and a preparation method thereof, wherein the positive electrode active material can be a material comprising the following components: lithium cobalt oxide having a layered structure; and a metal element (M) doped into the lithium cobalt oxide in an amount of 0.2 to 1 part by weight with respect to 100 parts by weight of the lithium cobalt oxide, wherein a crystal structure can be maintained in a range in which a positive electrode potential at full charge is greater than 4.5V (based on a Li potential).

Description

Metal-doped positive electrode active material for high voltage

Technical Field

Cross Reference to Related Applications

The present application is based on and claims priority from korean patent application nos. 10-2016-.

The invention relates to a metal-doped positive electrode active material for high voltage and a preparation method thereof.

Background

As the technology has been developed and the demand for mobile devices has increased, the demand for secondary batteries as an energy source has rapidly increased, and among these secondary batteries, lithium secondary batteries having high energy density and operating potential, long life span, and low self-discharge rate have been commercialized and widely used.

In addition, with increasing concern about environmental problems, many studies have been made on electric vehicles and hybrid electric vehicles, which can be used to replace vehicles using fossil fuel (e.g., gasoline vehicles, diesel vehicles, etc.) as one of the main causes of air pollution. Although nickel-metal hydride secondary batteries have been mainly used as power sources for electric vehicles and hybrid electric vehicles, the use of lithium secondary batteries having high energy density and discharge voltage has been actively studied, and some of them are now commercially available.

As a positive electrode material for lithium secondary batteries, LiCoO was used₂Ternary material (NMC/NCA), LiMnO₄、LiFePO₄And the like. LiCoO₂Have been frequently used so far because of having excellent physical properties (e.g., high calendering density, etc.) and excellent electrochemical characteristics (e.g., high cycle characteristics). However, due to LiCoO₂Has a charge/discharge current capacity as low as about 150mAh/g, and its structure is unstable at a high voltage of 4.3V or more, so it has a problem of rapid reduction of life characteristics and a problem of ignition caused by reaction with an electrolyte.

Particularly, in order to develop a high-capacity secondary battery, LiCoO is used₂When a high voltage is applied, LiCoO₂The use of Li increases, which may increase the possibility of surface instability and structural instability. To solve these problems, LiCoO has been replaced by replacing part of the cobalt with other elements₂Or by forming a separate coating layer.

However, it is also difficult to improve LiCoO in the positive electrode material having the above substitution or coating layer₂The structural stability of (2). In particular, it is difficult to maintain structural stability at high voltages of greater than 4.5V. In fact, it is difficult to subject LiCoO to₂Is applied to a high-capacity secondary battery.

In addition, for in LiCoO₂A positive electrode material having a coating layer formed on the surface thereof, the coating layer preventing movement of Li ions or reducing LiCoO during charge/discharge cycles₂And thus, there is a problem in that the performance of the secondary battery may be deteriorated.

Therefore, there is an increasing demand for a lithium cobalt oxide-based positive electrode active material capable of ensuring structural stability without deterioration in performance at a high voltage of more than 4.5V.

Disclosure of Invention

[ problem ] to

Accordingly, the present invention provides a positive electrode active material capable of securing structural stability at a high voltage of more than 4.5V without deterioration in performance, and a method for preparing the same.

In addition, the present invention provides a cathode including the cathode active material, and a lithium secondary battery including the cathode to exhibit excellent performance and life characteristics at a high voltage of more than 4.5V.

[ solution ]

Accordingly, the present invention provides a cathode active material comprising a lithium cobalt oxide having a layered structure represented by the following formula 1; and

a metal element (M) doped into the lithium cobalt oxide in an amount of 0.2 to 1 parts by weight, relative to 100 parts by weight of the lithium cobalt oxide, wherein the positive electrode active material maintains a crystal structure at a positive electrode potential (based on Li potential) higher than 4.5V at full charge:

[ formula 1]

Li_1+xCo_1-xO₂

Wherein x satisfies 0 and x is less than or equal to 0.2; and

m is one or more selected from the group consisting of Al, Ti, Mg, Mn, Zr, Ba, Ca, Ta, Mo, Nb and metals having an oxidation number of +2 or + 3.

Further, the present invention provides a lithium secondary battery comprising: a positive electrode containing the positive electrode active material; a negative electrode; and an electrolyte.

Further, the present invention provides a method for preparing the positive active material, the method comprising the step of dry-mixing a cobaltate, a lithium precursor and a doping precursor; and a step of sintering the mixture at a temperature of 900 ℃ or higher.

Hereinafter, a positive electrode active material and a method of preparing the same according to embodiments of the present invention will be described.

According to an embodiment of the present invention, there is provided a positive electrode active material including a lithium cobalt oxide having a layered structure represented by the following formula 1; and

[ formula 1]

Li_1+xCo_1-xO₂

Wherein x satisfies 0 and x is less than or equal to 0.2; and

The inventors of the present invention have made continuous intensive studies, as described in more detail below, and found that when the lithium cobalt oxide of formula 1 having a layered structure is doped with one or more metal elements of a predetermined amount or more, the structural stability of the crystal structure may be improved at a high voltage of more than 4.5V to maintain a stable crystal structure, thereby achieving high voltage characteristics, thereby completing the present invention.

As used herein, "doping" a metal element into lithium cobalt oxide means that the metal element does not form a chemical bond with lithium cobalt oxide and its elements, but at least a part of the metal element M is doped into the lattice structure of lithium cobalt oxide, thereby having a physical/crystallographic connection. In this regard, at least a portion of the metal element M incorporated into the lattice structure of the lithium cobalt oxide may be, for example, incorporated into the void spaces of the lattice structure of the lithium cobalt oxide to have physical/crystallographic connections without forming chemical bonds with the lithium cobalt oxide. In this way, since the metal element M has a physical/crystallographic connection without forming a chemical bond with the lithium cobalt oxide, the metal element M may be mainly distributed in a region near the surface of the lithium cobalt oxide.

Thus, "doping" can be clearly distinguished from a state in which the metal element M forms a chemical bond with the lithium cobalt oxide, for example, from a complex state in which a part of cobalt in the lithium cobalt oxide is substituted with the metal element M and then the metal element M is chemically bonded to the oxide. In the complex state, the metal element M may be uniformly distributed throughout the entire region of the lithium cobalt oxide through chemical bonding or complex formation.

As such, the cathode active material of one embodiment has a structure based on the lithium cobalt oxide of formula 1 doped with one or more metal elements, and thus, a predetermined amount or more of a dopant is doped and placed in a crystal lattice of the lithium cobalt oxide, thereby improving the stability of the crystal structure and the particle surface.

In particular, the cathode active material of one embodiment may include a predetermined amount or more of a dopant, for example, 0.2 parts by weight or more, or 0.2 parts by weight to 1.0 parts by weight, or 0.3 parts by weight to 0.9 parts by weight, so as to stabilize the crystal structure of lithium cobalt oxide even at a high voltage of 4.3V or more than 4.5V. Therefore, it was confirmed that the positive electrode active material can be preferably used as an active material exhibiting excellent capacity and life characteristics at a high voltage.

In contrast, when the doping amount of the dopant is less than 0.2 parts by weight, the positive electrode active material is excellent in both capacity and structural stability up to 4.45V. However, at voltages higher than 4.5V, there is a problem in that the structure collapses or the lifetime rapidly deteriorates. Further, when the doping amount of the dopant is an excessively large amount greater than 1.0 part by weight, or when a metal element forms a complex with lithium cobalt oxide through a chemical bond (for example, a complex in which part of cobalt is replaced with a metal element M) instead of doping, or when a coating layer containing a metal element is formed, the structural stability of the active material may be deteriorated at a high voltage, the replaced element or coating layer may hinder the movement of Li ions during charge/discharge cycles, or the capacity characteristics of the active material based on formula 1 may be deteriorated due to a relative decrease in the cobalt content.

Meanwhile, as shown in fig. 6, it can be confirmed that at least part of the metal element M is doped into the crystal lattice of lithium cobalt oxide by doping, for example, by analyzing the results of the above-described active material by TOF-SIMS (time of flight secondary ion mass spectrometry). It was confirmed that the metal element M was mainly distributed in the region near the surface of the lithium cobalt oxide, indicating that at least a part of the metal element M was doped into the lithium cobalt oxide without forming a complex with the lithium cobalt oxide through a chemical bond.

In addition, in one embodiment, the cathode active material may maintain a crystal structure in a charge range of more than 4.5V and 4.8V or less, specifically, the cathode active material may maintain stability of the crystal structure in a charge range of more than 4.5V and 4.6V or less, and more specifically, the cathode active material may ensure stability of the crystal structure in a charge range of more than 4.5V and 4.55V or less.

The above-mentioned dopant metal element M may be selected from a metal element group consisting of Al, Ti, Mg, Mn, Zr, Ba, Ca, Ta, Mo, Nb, and a metal having an oxidation number of +2 or +3, and is not particularly limited. However, the metal element M may be Al or Mg in view of reducing surface side reactions with an electrolyte or phase stability at a high voltage. In some cases, all Al and Mg may be used as dopants.

Meanwhile, in another embodiment, the positive electrode active material of one embodiment may maintain the stability of the crystal structure when charged at a high voltage, and such stability may be confirmed by XRD analysis. For example, the peak intensity of the (003) plane at 4.55V may be 30% or more of the peak intensity of the (003) plane at 4.50V on the 2 θ scale of the XRD analysis result of the positive electrode active material. More specifically, the peak intensity of the (003) plane at 4.55V may be 40% or more, or 40% to 90% of the peak intensity of the (003) plane at 4.50V on the 2 θ scale of the XRD pattern.

In other words, whether the lithium cobalt oxide maintains the crystal structure can be confirmed by measuring the peak intensity of the (003) plane (a crystal plane representing the lithium ion entrance in the lithium cobalt oxide). When charged to 4.55V, the previous lithium cobalt oxide showed a collapse of the crystal structure, and thus a significantly low peak intensity was measured compared to the peak intensity of 4.5V. Therefore, it can be seen that when the lithium cobalt oxide is not doped with a metal or is doped with a small amount, collapse of the crystal structure may occur at a voltage of more than 4.5V.

However, since the cathode active material of one embodiment includes a specific content and kind of dopant, the peak intensity of the (003) plane at 4.55V may be 30% or more of the peak intensity of the (003) plane at 4.5V, indicating improvement in structural stability at a high voltage.

In yet another embodiment, the cathode active material having the doping content (b) of the metal element (M) of 0.3 parts by weight may exhibit a lower capacity retention rate than the cathode active material having the doping content of the metal element (M) of 0.1 parts by weight at a cathode potential (based on Li potential) of 4.5V at the time of full charge.

Thus, even if the doping content of the metal element is relatively high, the capacity retention rate may be lowered due to the high content of the dopant at a potential of 4.5V at the time of full charge. Therefore, in order to achieve a high capacity retention ratio, the positive active material of one embodiment needs to have a high content of the doping metal and a charge voltage of at least higher than 4.5V.

For example, when the doped metal element is Mg, the cathode active material having a Mg content of 0.3 parts by weight may exhibit a lower capacity retention rate after 30 cycles or more of charge/discharge cycles than the cathode active material having a Mg doping content of 0.1 parts by weight.

That is, when the lithium cobalt cathode active material is doped with Mg and the content of Mg is 0.3 parts by weight, the cathode active material shows a higher capacity retention rate after charge/discharge cycles of less than 30 cycles than the cathode active material having a Mg content of 0.1 parts by weight, but shows a lower capacity retention rate after charge/discharge cycles of more than 30 cycles than the cathode active material having a Mg content of 0.1 parts by weight.

Therefore, at a charging voltage of 4.5V, even if a large amount of dopant is used, the capacity retention rate may decrease and the life characteristics may deteriorate as the number of charge/discharge cycles increases.

In contrast, a cathode active material having a doping content (b) of the metal element (M) of 0.3 parts by weight at a cathode potential (based on Li potential) of 4.55V at full charge can exhibit a higher capacity retention rate than a cathode active material having a doping content of the metal element (M) of 0.1 parts by weight.

In this way, in order to achieve a high capacity retention rate at a high charge voltage of more than 4.5V, the positive electrode active material of one embodiment needs to contain a dopant in a predetermined amount or more. Unlike the case where the charge voltage is 4.5V, the positive electrode active material having a doping content of 0.3 parts by weight may exhibit a higher capacity retention rate after 30 charge/discharge cycles or more than the positive electrode active material having a doping content of 0.1 parts by weight, although the doping metal is Mg.

Further, at a charging voltage of 4.55V, the difference in capacity retention ratio between the positive electrode active material having a dopant metal content of 0.3 parts by weight and the positive electrode active material having a dopant metal content of 0.1 parts by weight gradually increased as the number of charge/discharge cycles increased.

For example, when a positive electrode active material having a dopant metal content of 0.3 parts by weight is used, the capacity retention rate after charge/discharge cycles of 50 cycles at a positive electrode potential (based on the Li potential) of 4.55V at full charge may be 85% or more of the initial capacity, and exhibits high life characteristics as compared with a positive electrode active material having a dopant metal content of 0.1 parts by weight, which generally exhibits a capacity retention rate of less than 75% after the same cycles.

In one embodiment, the positive electrode active material of the above-described one embodiment may further include a coating layer formed on the lithium cobalt oxide particles, and the coating layer may include one or more materials selected from the group consisting of Al, ag, and Al₂O₃、MgO、ZrO、Li₂ZrO₃And TiO₂One or more metal oxides of the group. The formation of the coating layer may further improve the structural stability of the lithium cobalt oxide particles.

In general, when lithium cobalt oxide is used as a cathode active material at a high voltage, a large amount of lithium ions are released from the lithium cobalt oxide particles, and the Li ion concentration on the surface becomes low, so that Co is easily released. As the release of Co increases, the reversible capacity decreases, and the possibility of Co precipitation on the surface of the anode increases, which may increase the anode resistance. Therefore, when the metal oxide coating layer is further formed on the lithium cobalt oxide particles, the metal element contained in the coating layer may react with HF in preference to cobalt to protect the positive electrode active material particles. As a result, the cycle characteristics of the secondary battery can be effectively prevented from being deteriorated at a high voltage.

The content of the metal element contained in the coating layer can be controlled to 300ppmw to 1,200ppmw based on the content of the lithium cobalt oxide of formula 1. When the content of the metal element contained in the coating layer is less than 300ppmw, it is difficult to ensure the structural stability of the positive electrode active material. When the content is more than 1,200ppmw, the capacity and output characteristics of the battery are undesirably deteriorated.

Meanwhile, according to another embodiment of the present invention, there is provided a method of preparing the positive electrode active material of the above-described one embodiment. The preparation method may include a process of dry-mixing the cobaltate, the lithium precursor and the doping precursor; and sintering the mixture at a temperature of 900 ℃ or higher.

Experimental results of the inventors of the present invention show that an active material of an embodiment doped with at least a part of a metal element M can be prepared by dry-mixing precursors with each other and sintering the mixture at a high temperature. Unlike this method, when other wet processes such as coprecipitation are employed, the active material is obtained in a complex form through a chemical bond between the metal element M and formula 1, and thus it is likely that the positive electrode active material of one embodiment cannot be prepared.

In the production method of the above-described another embodiment, the sintering temperature may be, for example, 900 ℃ to 1,200 ℃, specifically, 1,000 ℃ to 1,100 ℃, and the sintering time may be 4 hours to 20 hours, specifically, 5 hours to 15 hours.

When the sintering temperature is lower than 900 deg.c, the structure of the lithium cobalt oxide may not be properly formed, and when the sintering temperature is higher than 1200 deg.c, the lithium cobalt oxide is excessively sintered, thereby causing deterioration of capacity or life characteristics, which is undesirable. In addition, when the sintering time is shorter than 4 hours, a problem of insufficient doping may occur, and when the sintering time is 12 hours or more, physical and chemical properties of lithium cobalt oxide may be changed, thereby causing performance deterioration, which is undesirable.

After the sintering process, the method may further include a process of forming a coating layer on the surface of the lithium cobalt oxide doped with the metal element M. As mentioned above, the coating may comprise a material selected from the group consisting of Al₂O₃、MgO、ZrO、Li₂ZrO₃And TiO₂One or more metal oxides of the group.

In one embodiment, to form the coating, a salt containing the metal element intended to be included in the coating may be mixed with the doped lithium cobalt oxide, and then the mixture may be sintered. That is, the positive electrode active material having the coating layer formed thereon may be prepared by coating a salt containing a metal element onto the surface of particles of the doped lithium cobalt oxide and then sintering.

Meanwhile, the kind of cobalt oxide used in the preparation method of the above-described another embodiment is not particularly limited, but the cobaltate may be, for example, selected from the group consisting of Co₃O₄、CoCO₃、Co(NO₃)₂And Co (OH)₂More than one of the group of compositions, especially Co₃O₄Or Co (OH)₂。

The lithium precursor may be selected from the group consisting of Li₂CO₃、LiOH、LiNO₃、CH₃COOLi and Li₂(COO)₂More than one of the group, especially LiOH or Li₂CO₃。

The doping precursor may be one or more selected from the group consisting of: one or more metals selected from the group consisting of Al, Ti, Mg, Mn, Zr, Ba, Ca, Ta, Mo, Nb, and metals having an oxidation number of +2 or +3, metal oxides thereof, and metal salts thereof; in particular Al and/or Mg.

Meanwhile, according to still another embodiment of the present invention, there is provided a positive electrode for a secondary battery including the positive electrode active material of the above-described one embodiment.

The positive electrode can be manufactured by, for example, applying a positive electrode mixture containing a positive electrode active material composed of positive electrode active material particles, a conductive material, and a binder onto a positive electrode collector, and a filler may be further added to the positive electrode mixture as needed.

The positive electrode collector is generally manufactured to have a thickness of 3 μm to 500 μm. The positive electrode collector is not particularly limited as long as it has high conductivity without causing chemical changes of the corresponding battery. The positive electrode collector may be, for example, any one selected from stainless steel, aluminum, nickel, titanium, and aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. In particular, aluminum may be used. The current collector may increase adhesion of the positive electrode active material by forming fine roughness on the surface. For example, the current collector may be in various forms such as a film, sheet, foil, mesh, porous material, foam, nonwoven material, and the like.

The positive electrode active material may be composed of positive electrode active material particles and the following components: layered structure compound (e.g. lithium nickel oxide (LiNiO)₂) Or a compound substituted with one or more metals; lithium manganese oxides, e.g. of formula Li_1+xMn_2-xO₄(wherein x is 0 to 0.33), LiMnO₃、LiMn₂O₃、LiMnO₂Etc.; lithium copper oxide (Li)₂CuO₂) (ii) a Vanadium oxides, e.g. LiV₃O₈、LiV₃O₄、V₂O₅、Cu₂V₂O₇Etc.; from the formula LiNi_1-xM_xO₂(wherein M is Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x is 0.01 to 0.3); of the formula LiMn_2-xM_xO₂(wherein M is Co, Ni, Fe, Cr, Zn or Ta, and x is 0.01 to 1) or Li₂Mn₃MO₈(wherein M is Fe, Co, Ni, Cu or Zn); LiMn₂O₄Wherein some of the Li in formula (la) is substituted with an alkaline earth metal; a disulfide; fe₂(MoO₄)₃And the like, but are not limited thereto.

The conductive material is generally added in an amount of 0.1 to 30 wt% based on the total weight of the mixture including the positive electrode active material. The conductive material is not particularly limited as long as it has conductivity without causing chemical changes of the corresponding battery. Examples of the conductive material may include: graphite, such as natural or artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, etc.; conductive fibers such as carbon fibers, metal fibers, and the like; metal powders such as carbon fluoride, aluminum, nickel powders, and the like; conductive whiskers such as zinc oxide, potassium titanate, and the like; conductive metal oxides such as titanium oxide and the like; polyphenylene derivatives, and the like.

The binder included in the positive electrode is a component contributing to adhesion between the active material and the conductive agent and adhesion to the current collector, and may be added in an amount of usually 0.1 to 30% by weight based on the total weight of the mixture including the positive electrode active material. Examples of the binder may include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, various copolymers, and the like.

According to still another embodiment, there is provided a lithium secondary battery including the above-described cathode, anode and electrolyte. The kind of the lithium secondary battery is not particularly limited, but the lithium secondary battery may include, for example, a lithium ion battery or a lithium ion polymer battery having advantages of high energy density, discharge voltage, output stability, and the like.

In general, a lithium secondary battery consists of a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte containing a lithium salt.

Hereinafter, other components of the lithium secondary battery will be described.

The anode may be manufactured by coating an anode active material on an anode current collector and then drying. The above-mentioned components may optionally be further included according to need.

The anode current collector is generally made to have a thickness of 3 μm to 500 μm. The anode current collector is not particularly limited as long as it has conductivity without causing chemical changes of the corresponding battery. For 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 can be used. Like the cathode current collector, the anode current collector may have fine roughness on the surface thereof to enhance adhesion of the anode active material, and may be used in various forms, such as a film, a sheet, a foil, a mesh, a porous material, a foam material, a non-woven material, and the like.

The negative active material may include, for example: carbon such as non-graphitizing carbon, etc.; metal complex oxides, e.g. Li_xFe₂O₃(0≤x≤1)、Li_xWO₂(0≤x≤1)、Sn_xMe_1-xMe’_yO_z(Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, an element of group I, II or III of the periodic Table of the elements, halogen; 0<x is less than or equal to 1; y is more than or equal to 1 and less than or equal to 3; z is more than or equal to 1 and less than or equal to 8), and the like; lithium metal; a lithium alloy; a silicon-based alloy; a tin-based alloy; metal oxides, e.g. SnO, SnO₂、PbO、PbO₂、Pb₂O₃、Pb₃O₄、Sb₂O₃、Sb₂O₄、Sb₂O₅、GeO、GeO₂、Bi₂O₃、Bi₂O₄And Bi₂O₅Etc.; conductive polymers such as polyacetylene and the like; Li-Co-Ni based material.

The separator is interposed between the positive electrode and the negative electrode, and an insulating thin film having high ion permeability and mechanical strength is used. The separator typically has a pore size of 0.01 μm to 10 μm and a thickness of 5 μm to 300 μm. As the separator, for example, a sheet or nonwoven fabric formed of an olefin polymer (e.g., polypropylene having chemical resistance and hydrophobicity) or glass fiber or polyethylene is used. When a solid electrolyte such as a polymer is employed as the electrolyte, the solid electrolyte may also serve as both the separator and the electrolyte.

The lithium salt-containing non-aqueous electrolyte is composed of a non-aqueous electrolyte and a lithium salt. The non-aqueous electrolyte may include a non-aqueous organic solvent, an organic solid electrolyte, or an inorganic solid electrolyte, but is not limited thereto.

The non-aqueous organic solvent may be, for example, an aprotic organic solvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ -butyrolactone, 1, 2-dimethoxyethane, tetrahydroxyflane (franc), 2-methyltetrahydrofuran, dimethylsulfoxide, 1, 3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphotriester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1, 3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, diethyl ether, methyl propionate, ethyl propionate and the like.

The organic solid electrolyte may include, for example, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, poly agitation lysine (poly agitation lysine), polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, polymers containing ionic dissociation groups, and the like.

The inorganic solid electrolyte may include, for example, a Li-based nitride, halide or sulfate, e.g., Li₃N、LiI、Li₅NI₂、Li₃N-LiI-LiOH、LiSiO₄、LiSiO₄-LiI-LiOH、Li₂SiS₃、Li₄SiO₄、Li₄SiO₄-LiI-LiOH、Li₃PO₄-Li₂S-SiS₂And the like.

The lithium salt is a substance easily soluble in the non-aqueous electrolyte and may include, for example, LiCl, LiBr, LiI, LiClO₄、LiBF₄、LiB₁₀Cl₁₀、LiPF₆、LiCF₃SO₃、LiCF₃CO₂、LiAsF₆、LiSbF₆、LiAlCl₄、CH₃SO₃Li、CF₃SO₃Li、(CF₃SO₂)₂NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, tetraphenyl lithium borate, imide, and the like.

For the purpose of improving charge/discharge characteristics and flame retardancy, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, N-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinonimine dyes, N-substituted oxazolidinone, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, or the like may be added to the non-aqueous electrolyte. Alternatively, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further added to provide flame retardancy, or carbon dioxide gas may be further added to improve high-temperature retention characteristics, and FEC (fluoroethylene carbonate), PRS (propylene sulfonate lactone), or the like may be further added.

According to still another embodiment of the present invention, there are provided a battery pack including a secondary battery, and an apparatus including the battery pack. The above-described battery pack and device are known in the art, and thus, a detailed description thereof will be omitted in the present invention.

The device may be, for example, a laptop, a netbook, a tablet, a mobile phone, an MP3, a wearable electronic product, an electric tool, an Electric Vehicle (EV), a Hybrid Electric Vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), an electric bicycle, an electric scooter, an electric golf cart, or an electric power storage system, but is not limited thereto.

[ advantageous effects ]

As described above, the cathode active material of the present invention has a structure in which one or more metal elements are doped in the lithium cobalt oxide of a layered structure, and thus, a predetermined amount or more of a dopant is doped and placed in the crystal lattice of the lithium cobalt oxide, thereby preventing the collapse of the crystal structure and securing structural stability at a high voltage of more than 4.5V.

Unlike the ternary system, the capacity of lithium cobalt oxide is increased only by increasing the voltage. However, when the cathode active material of the present invention is used, the problem of stability at high voltage can be solved, thereby achieving high capacity and high cycle characteristics.

Drawings

Fig. 1 is a graph showing the capacity retention of coin-type half cells containing lithium cobalt oxide doped with 1,000ppm or 3,000ppm Mg when charged at an upper limit voltage of 4.5V;

fig. 2 is a graph showing the capacity retention of a coin-type half cell comprising lithium cobalt oxide doped with 1,000ppm or 3,000ppm mg when charged at an upper limit voltage of 4.55V;

fig. 3 is a graph showing the capacity retention of a coin-type half cell comprising lithium cobalt oxide doped with 1,000ppm or 3,000ppm Al when charged at an upper limit voltage of 4.5V;

fig. 4 is a graph showing the capacity retention of a coin-type half cell comprising lithium cobalt oxide doped with 1,000ppm or 3,000ppm Al when charged at an upper limit voltage of 4.55V;

fig. 5 shows the results of TOF-SIMS (time of flight secondary ion mass spectrometry) analysis of the positive electrode active material of example 1; and

fig. 6 is an XRD chart showing the peak intensity of the coin-type half cell containing the positive electrode active material of example 1 or comparative example 1, which was measured when the upper limit voltage was increased from 4.5V to 4.54V at intervals of 0.01V.

Detailed Description

Embodiments of the present invention will be described below with reference to the accompanying drawings. However, these embodiments are provided only for better understanding, but the scope of the present invention is not limited thereto.

Preparation of positive electrode active material

< example 1>

0.294g of MgO, 80.27g of Co₃O₄And 36.94g of Li₂CO₃Are dry-blended with each other so that the content of Mg is 3,000ppm based on the total weight of the cathode active material. Then, the mixture was sintered in a furnace at 1050 ℃ for 10 hours to prepare Mg-doped lithium cobalt oxide.

< example 2>

0.147g of Al₂O₃80.27g of Co₃O₄And 36.94g of Li₂CO₃Are dry-blended with each other so that the content of Al is 3,000ppm based on the total weight of the cathode active material. Then, the mixture was sintered in a furnace at 1050 ℃ for 10 hours to prepare Al-doped lithium cobalt oxide.

< example 3>

0.245g of MgO, 80.27g of Co₃O₄And 36.94g of Li₂CO₃Are dry-blended with each other so that the content of Mg is 5,000ppm based on the total weight of the cathode active material. Then, the mixture was sintered in a furnace at 1050 ℃ for 10 hours to prepare Al-doped lithium cobalt oxide.

< example 4>

0.343g of MgO and 80.27g of Co₃O₄And 36.94g of Li₂CO₃Are dry-blended with each other so that the content of Mg is 7,000ppm based on the total weight of the cathode active material. Then, the mixture was sintered in a furnace at 1050 ℃ for 10 hours to prepare Al-doped lithium cobalt oxide.

< example 5>

0.441g of MgO and 80.27g of Co₃O₄And 36.94g of Li₂CO₃Are dry-blended with each other so that the content of Mg is 9,000ppm based on the total weight of the cathode active material. Then, the mixture was sintered in a furnace at 1050 ℃ for 10 hours to prepare Al-doped lithium cobalt oxide.

< example 6>

To form a coating on the Mg doped lithium cobalt oxide prepared in example 1, 500ppm of Al was added₂O₃Dry-blended with the lithium cobalt oxide particles to coat the lithium cobalt oxide particles, and then sintered in a furnace at 700 c for 5 hours to prepare a positive electrode active material on which a coating layer is formed.

< comparative example 1>

Mg-doped lithium cobalt oxide was prepared in the same manner as in example 1, except that the content of Mg was 1,000ppm based on the total weight of the cathode active material.

< comparative example 2>

Al-doped lithium cobalt oxide was prepared in the same manner as in example 2, except that the content of Al was 1,000ppm based on the total weight of the cathode active material.

< comparative example 3>

Mg-doped lithium cobalt oxide was prepared in the same manner as in example 1, except that the content of Mg was 10,000ppm based on the total weight of the cathode active material.

Manufacture of secondary battery

Each of the metal-doped positive electrode active materials, PVdF binder, and natural graphite conductive material prepared in examples 1 to 6 and comparative examples 1 to 3 was thoroughly mixed in NMP at a weight ratio of 96:2:2 (positive electrode active material: binder: conductive material), and then coated on an Al foil having a thickness of 20 μm and dried at 130 ℃, thereby manufacturing each positive electrode. As the negative electrode, a lithium foil containing 1MLiPF in a solvent of EC: DMC: DEC: 1:2:1 was used₆To manufacture each coin-type half cell.

< Experimental example 1> analysis of Capacity conservation Rate

Each coin-type half cell manufactured above was charged at 25 ℃ to an upper limit voltage of 4.5V or 4.55V at 0.5C, and then discharged to a lower limit voltage of 3V at 1.0C. This process was considered as 1 cycle, and the capacity retention after 50 cycles was measured. The results are shown in table 1 below and fig. 1 to 4. Table 2 shows the initial capacity upon charging at 4.5V and 4.55V, respectively.

[ TABLE 1]

[ TABLE 2 ]

Referring to table 1, fig. 1 to 4, and table 2, when 3,000ppm of Mg or Al is doped as in example 1 or 2, the capacity retention rate after 50 charge/discharge cycles at 4.5V is lower than that of comparative example 1 or 2 doped with 1000ppm of Mg or Al. Fig. 1 shows that, when the positive electrode active material of example 1 was used, the capacity retention rate was higher than that of comparative example 1 up to about 30 cycles, but the capacity retention rate of example 1 rapidly decreased after more than 30 cycles.

However, examples 1 and 2 with higher doping contents exhibited significantly higher capacity retention after 50 charge/discharge cycles at 4.55V.

Example 2 having an Al doping content of 3,000ppm showed a lower capacity retention at 4.5V than at 4.55V. However, the capacity retention rate was decreased at 4.5V and increased at 4.55V, as compared to comparative example 1, in which the Al doping content was 1,000 ppm.

Meanwhile, comparative example 2 having a Mg doping content of 10,000ppm showed a low capacity retention rate as compared to examples.

This is because the positive electrode active material undergoes a reversible phase change during charge/discharge, and as the charge potential increases, the reversibility of the phase change decreases, resulting in a decrease in capacity. However, when a metal within a specific range is doped in the lithium cobalt oxide in order to prepare a high-potential lithium cobalt oxide having a high capacity, irreversible phase transition may be minimized, thereby preventing a decrease in capacity retention rate. However, when the content of the doping metal is too small as in comparative examples 1 and 2, it is difficult to obtain the above effect, and thus the capacity retention rate rapidly decreases when charging/discharging is performed at 4.55V.

Therefore, when the lithium cobalt oxide is doped with Mg or Al in a specific content range, phase transition at a high potential can be minimized, thereby exhibiting high capacity retention rate.

Further, as the content of the doping element increases, the initial efficiency is reduced at a voltage higher than 4.5V, and thus it is expected that the positive electrode and the negative electrode are easily balanced when the battery cell is manufactured. It is also expected that the remaining amount of the positive electrode active material is reduced, which will contribute to reducing the production cost of the battery cell. Therefore, it is expected that materials suitable for the operating voltage conditions of the battery cell may be designed, the desired performance of the battery cell may be achieved, and the production cost may be greatly reduced.

Meanwhile, at 4.55V, the lifetime was slightly improved with an increase in Mg doping amount, which is presumably due to structure stabilization. However, at 4.55V, the initial efficiency decreases with increasing Mg doping. It is expected that when the doping amount of Mg is increased to 10,000ppm or more, the resistance increase of the positive electrode active material may be further promoted instead of the structural stability, so that the capacity may be deteriorated, and the life stability may be reduced.

Meanwhile, the doped lithium cobalt oxide with the coating of example 6 exhibited a high capacity retention rate after 50 charge/discharge cycles at 4.55V, compared to example 1 without the coating.

Therefore, it can be seen that the cathode active material of the example exhibits improved lifespan characteristics at a potential higher than 4.5V.

< Experimental example 2> TOF-SIMS (time of flight Secondary ion Mass Spectrometry) analysis

Fig. 5 shows the results of analyzing the active material obtained in example 2 by TOF-SIMS (time of flight secondary ion mass spectrometry).

Referring to fig. 5, it was confirmed that the doped metallic element Al was distributed on the lithium cobalt oxide, and particularly, the doped metallic element Al was mainly distributed in a region near the surface of the lithium cobalt oxide. These analysis results show that the metallic element Al is doped into lithium cobalt oxide to form a physical/crystallographic connection without forming a complex with lithium cobalt oxide through a chemical bond.

< Experimental example 3> XRD analysis

In order to examine the crystal structure change of the lithium cobalt oxide of example 1 and comparative example 1, coin-type half cells including the same were manufactured, and the peak intensity was measured while increasing the upper limit voltage from 4.5V to 4.55V at intervals of 0.01V. The XRD pattern (2 theta-scale) thus measured is shown in fig. 6.

Referring to fig. 6, the cathode active material of example 1 showed a peak of (003) plane in the range of 23 to 24 degrees at 4.40V to 4.55V, and showed a peak in the range of 24 to 25 degrees at 4.55V. When the charge potential of the positive electrode active material is 4.54V or more, the crystal structure undergoes a phase transition to another phase, and when the charge potential is 4.55V, a new peak indicating the crystal structure after the phase transition is observed in a range of 24 to 25 degrees. However, the peak of the (003) plane was observed to some extent continuously, indicating that the phase transition reversibly occurred. Therefore, even during charge/discharge, the capacity retention rate can be maintained to some extent.

Specifically, fig. 6 shows peaks before/after the phase transition at the respective voltages. The peak intensity of the (003) plane at 4.55V is 30% or more of the peak intensity of the (003) plane at 4.50V. The metal-doped lithium cobalt oxide minimizes phase transition to exhibit high capacity retention, thereby exhibiting significantly improved life characteristics.

However, the cathode active material of comparative example 1 exhibited a significantly low peak intensity of the (003) plane at 4.55V, indicating that the peak indicating the crystal structure after phase transition was higher than the peak indicating the crystal structure before phase transition, indicating that the reversibility of phase transition was significantly reduced. Therefore, it can be seen that when the content of the doping metal is low, the lifetime characteristics may be greatly deteriorated.

It will be apparent to those skilled in the art to which the present invention pertains that various modifications and changes can be made thereto without departing from the scope of the invention.

Claims

1. A positive electrode active material comprising a lithium cobalt oxide having a layered structure represented by the following formula 1, and

a metal element M doped into the lithium cobalt oxide in an amount of 0.2 to 0.9 parts by weight, relative to 100 parts by weight of the lithium cobalt oxide, wherein the positive electrode active material maintains a crystal structure at a positive electrode potential of more than 4.5V and 4.6V or less based on a Li potential at full charge:

[ formula 1]

Li_1+xCo_1-xO₂

Wherein x satisfies 0 and x is less than or equal to 0.2; and

m is Al or Mg, and M is Al or Mg,

wherein the metal element M has a physical/crystallographic connection without forming a chemical bond with the lithium cobalt oxide.

2. The positive electrode active material according to claim 1, wherein a peak intensity of a (003) plane at 4.55V is 30% or more of a peak intensity of a (003) plane at 4.50V on a 2-theta scale of an XRD analysis result of the positive electrode active material.

3. The positive electrode active material according to claim 1, wherein a peak intensity of a (003) plane at 4.55V is 40% or more of a peak intensity of a (003) plane at 4.50V on a 2-theta scale of an XRD analysis result of the positive electrode active material.

4. The positive electrode active material according to claim 1, wherein the positive electrode active material having a doping content of the metal element M of 0.3 parts by weight exhibits a lower capacity retention rate than the positive electrode active material having a doping content of the metal element M of 0.1 parts by weight at a positive electrode potential of 4.5V based on a Li potential at the time of full charge.

5. The positive electrode active material according to claim 4, wherein when the doped metal element M is Mg, the positive electrode active material having an Mg content of 0.3 parts by weight exhibits a lower capacity retention rate after 30 or more charge/discharge cycles than the positive electrode active material having an Mg doping content of 0.1 parts by weight.

6. The positive electrode active material according to claim 1, wherein the positive electrode active material having a doping content of the metal element M of 0.3 parts by weight exhibits a higher capacity retention rate than the positive electrode active material having a doping content of the metal element M of 0.1 parts by weight at a positive electrode potential of 4.55V based on a Li potential at the time of full charge.

7. The positive electrode active material according to claim 1, wherein the capacity retention rate is 85% or more of the initial capacity after 50 charge/discharge cycles of charging at 0.5C and discharging at 1.0C based on a positive electrode potential at which the Li potential is 4.55V at the time of full charge.

8. The positive active material according to claim 1, further comprising a coating layer formed on the lithium cobalt oxide particles, wherein the coating layer comprises one or more selected from the group consisting of Al₂O₃、MgO、ZrO、Li₂ZrO₃And TiO₂One or more metal oxides of the group.

9. A lithium secondary battery, comprising: a positive electrode containing the positive electrode active material according to any one of claims 1 to 8; a negative electrode; and an electrolyte.

10. A method of preparing the positive electrode active material of claim 1, the method comprising:

a step of dry-mixing a cobalt precursor, a lithium precursor and a doping precursor, the cobalt precursor being selected from the group consisting of Co₃O₄、CoCO₃、Co(NO₃)₂And Co (OH)₂One or more of the group consisting of; and

and sintering the obtained mixture at a temperature of 900 ℃ or higher.

11. The method of claim 10, further comprising a step of forming a metal oxide coating on the lithium cobalt oxide particles after the sintering step.