KR20230013396A - Positive electrode active material for lithium secondary battery, preparing method of the same - Google Patents

Abstract

The present invention relates to a positive electrode active material for a lithium secondary battery and a method for manufacturing the positive electrode active material. The positive electrode active material of the present invention includes: a lithium transition metal oxide containing 60 mol% or more of nickel based on the total number of moles of the transition metal excluding lithium; and a coating layer containing boron. The present invention can improve battery performance such as capacity retention rates and lifespan characteristics.

Description

Cathode active material for lithium secondary battery and manufacturing method thereof

The present invention relates to a cathode active material for a lithium secondary battery and a method for manufacturing the cathode active material.

As technology development and demand for mobile devices increase, demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle life, and low self-discharge rate have been commercialized and widely used.

A lithium transition metal composite oxide is used as a cathode active material of a lithium secondary battery, and among them, a lithium cobalt composite metal oxide such as LiCoO ₂ having a high operating voltage and excellent capacity characteristics is mainly used. However, LiCoO ₂ has very poor thermal properties due to destabilization of the crystal structure due to delithiation. In addition, since the LiCoO ₂ is expensive, there is a limit to mass use as a power source in fields such as electric vehicles.

As materials to replace the LiCoO ₂ , lithium manganese composite metal oxides (LiMnO ₂ or LiMn ₂ O ₄ , etc.), lithium iron phosphate compounds (LiFePO ₄ , etc.) or lithium nickel composite metal oxides (LiNiO ₂ , etc.) have been developed. . Among them, research and development on lithium-nickel composite metal oxide, which has a high reversible capacity of about 200 mAh/g and facilitates implementation of a large-capacity battery, is being actively researched. However, LiNiO ₂ has inferior thermal stability compared to LiCoO ₂ , and when an internal short circuit occurs due to pressure from the outside in a charged state, the cathode active material itself is decomposed, resulting in rupture and ignition of the battery. Accordingly, as a method for improving low thermal stability while maintaining excellent reversible capacity of LiNiO ₂ , a lithium transition metal oxide in which Ni is partially substituted with Co, Mn, or Al has been developed.

However, in the case of the lithium transition metal oxide, when the nickel content is increased to increase the capacity characteristics, the thermal stability is further deteriorated, and the nickel in the lithium transition metal oxide tends to be maintained as Ni ²⁺ , so LiOH is formed on the surface thereof. Alternatively, there is a problem in that a large amount of lithium by-products such as Li ₂ CO ₃ are generated. As such, when a lithium transition metal oxide having a high content of lithium by-products on the surface is used as a cathode active material, it reacts with the electrolyte solution injected into the lithium secondary battery, causing gas generation and swelling of the battery, thereby improving battery life and stability. And battery resistance characteristics, etc. may be deteriorated.

Therefore, there is a demand for the development of a positive electrode active material capable of producing a secondary battery with improved lifespan characteristics and resistance characteristics by exhibiting high-capacity characteristics and improving particle strength and structural stability.

Korean Patent Publication No. 10-2014-0093529

An object of the present invention is to provide a positive electrode active material that compensates for particle breakage caused by rolling by improving particle strength.

In order to solve the above problems, the present invention is a lithium transition metal oxide containing 60 mol% or more of nickel based on the total number of moles of the transition metal except lithium; and a coating layer containing boron, wherein the lithium transition metal oxide is a single particle composed of primary particles having an average particle diameter (D ₅₀ ) of 0.3 to 2 μm, and the average particle diameter (D ₅₀ ) of the single particle is 1 to 10 μm.

In addition, the present invention comprises the steps of preparing a transition metal hydroxide precursor containing 60 mol% or more of nickel based on the total number of moles of the transition metal; preparing a single-particle lithium transition metal oxide by mixing the transition metal hydroxide precursor and a lithium source material and firing at a temperature of 850° C. or higher; and mixing the lithium transition metal oxide with boric acid and heat-treating at a temperature of 350 to 450° C.

According to the present invention, a high-strength single-particle positive electrode active material can be manufactured by adjusting the firing temperature and the heat treatment temperature. Since the cathode active material has excellent stability, battery performance such as capacity retention rate and lifespan characteristics may be improved.

Hereinafter, the present invention will be described in more detail to aid understanding of the present invention.

The terms or words used in the description and claims of the present invention should not be construed as being limited to the ordinary or dictionary meaning, and the inventor appropriately defines the concept of the term in order to explain his/her invention in the best way. Based on the principle that it can be done, it should be interpreted as meaning and concept consistent with the technical idea of the present invention.

<Cathode active material>

The cathode active material of the present invention includes a lithium transition metal oxide containing 60 mol% or more of nickel based on the total number of moles of the transition metal excluding lithium; and a coating layer containing boron, wherein the lithium transition metal oxide is a single particle composed of primary particles having an average particle diameter (D ₅₀ ) of 0.3 to 2 μm, and the average particle diameter (D ₅₀ ) of the single particle is 1 to 10 μm.

The lithium transition metal oxide includes nickel (Ni), cobalt (Co), and manganese (Mn), and the content of nickel (Ni) is 60 mol% or more with respect to the total number of moles of the transition metal excluding lithium (Li). Nickel (High-Ni) NCM-based lithium transition metal oxide. The nickel content may be 70 mol% or more, or 80 mol% or more, based on the total number of moles of the transition metal excluding lithium (Li). As described above, since the content of nickel (Ni) satisfies 60 mol% or more with respect to the total number of moles of transition metals excluding lithium (Li), high capacity can be realized and excellent stability can be secured at the same time.

In the present invention, the lithium transition metal oxide may be represented by Formula 1 below.

[Formula 1]

Li _1+a Ni _x Co _y Mn _z M ¹ _w O ₂

In Formula 1,

M ¹ is at least one selected from the group consisting of Al, Mg, V, Ti, and Zr;

0≤a≤0.5, 0.6≤x<1, 0<y≤0.4, 0<z≤0.4, 0<w≤0.04, x+y+z+w=1.

The M ¹ is an element substituted at a transition metal site in the lithium transition metal oxide represented by Formula 1, and may include at least one selected from the group consisting of Al, Mg, V, Ti, and Zr. .

1+a represents the molar ratio of lithium in the lithium transition metal oxide represented by Formula 1, and may be 0≤a≤0.5, preferably 0≤a≤0.20, or 0≤a≤0.15.

The x represents the molar ratio of nickel among metal components other than lithium in the lithium transition metal oxide represented by Chemical Formula 1, and may be 0.6≤x<1.0, preferably 0.8≤x<1.0.

Wherein y represents the molar ratio of cobalt among metal components other than lithium in the lithium transition metal oxide represented by Formula 1, 0<y≤0.4, and considering the remarkable effect of improving capacity characteristics due to the inclusion of Co, it is preferred Preferably, it may be 0<y≤0.2.

The z represents the molar ratio of manganese among metal components other than lithium in the lithium transition metal oxide represented by Chemical Formula 1, and may be 0<z≤0.4, preferably 0<z≤0.2.

The w represents the molar ratio of the doping element M among metal components other than lithium in the lithium transition metal oxide represented by Chemical Formula 1, and may be 0<w≤0.04, preferably 0≤w≤0.02.

In the present invention, the lithium transition metal oxide is a single particle composed of primary particles having an average particle diameter (D ₅₀ ) of 0.3 to 2 μm.

The 'single particle' is a term used to distinguish from positive electrode active material particles in the form of secondary particles formed by aggregation of tens to hundreds of primary particles, which have been generally used in the past, and are single particles composed of one primary particle. and aggregate particles of 10 or less primary particles. The 'secondary particle' refers to an aggregate in which primary particles are aggregated by physical or chemical bonding between the primary particles, that is, a secondary structure, without intentional aggregation or assembly of the primary particles.

The 'primary particle' refers to a minimum particle unit that is distinguished as one lump when a cross section of the cathode active material is observed through a scanning electron microscope (SEM), and may be composed of a single crystal grain or a plurality of crystal grains. may be In the present invention, the average particle diameter of the primary particles may be measured by measuring the size of each of the particles distinguished from cross-sectional SEM data of the positive electrode active material particles and then obtaining an arithmetic mean value thereof.

The 'average particle diameter (D ₅₀ )' may be defined as a particle diameter at 50% of the volume cumulative particle diameter distribution, and may be measured using a laser diffraction method. Specifically, the average particle diameter (D ₅₀ ) is obtained by dispersing the target particle in a dispersion medium, introducing it into a commercially available laser diffraction particle size measuring device (for example, Microtrac MT 3000), irradiating ultrasonic waves of about 28 kHz with an output of 60 W, and then , the average particle diameter (D ₅₀ ) on the basis of 50% of the particle volume cumulative distribution according to the particle diameter in the measuring device can be calculated.

In the present invention, the average particle diameter (D ₅₀ ) of the primary particles constituting the single particles may be 0.3 to 2 μm, preferably 0.4 to 1.5 μm, or 0.5 to 1.2 μm.

The lithium transition metal oxide included in the cathode active material of the present invention is composed of primary particles having an average particle diameter (D ₅₀ ) in the above range, so that particle strength increases to suppress particle breakage during rolling and improve rolling density. In addition, the specific surface area is reduced, and lithium by-products are reduced, thereby reducing the amount of gas generated by side reactions with the electrolyte.

In the present invention, the average particle diameter (D ₅₀ ) of the single particles is 1 to 10 μm. It may preferably be 2 to 7 μm, or 4 to 6 μm. Since the positive active material of the present invention has a single particle form, even if it is formed to have a small particle diameter with an average particle diameter (D ₅₀ ) of 1 to 10 μm, the particle strength may be excellent.

In the present invention, the single particle may be composed of 1 to 30 primary particles, specifically, may be composed of 3 to 20 primary particles.

When the number of primary particles satisfies the above range, there is an effect of preventing particle breakage by dispersing the pressure applied to the particles while increasing the rolling density as single particles attached during electrode rolling fall off.

The cathode active material of the present invention includes a coating layer containing boron.

The boron (B) may be included in an amount of 0.01 to 2% by weight, preferably 0.04 to 1% by weight, based on the total weight of the positive electrode active material. When the boron content satisfies the above range, the effect of improving initial discharge capacity and efficiency characteristics may be more excellent.

The coating layer containing boron blocks contact between the positive electrode active material and the electrolyte included in the lithium secondary battery to suppress side reactions, thereby further improving lifespan characteristics and increasing the filling density of the positive electrode active material.

The boron-containing coating layer may be formed on the entire surface of the lithium transition metal oxide or may be partially formed. Specifically, when the boron-containing coating layer is partially formed on the surface of the lithium transition metal oxide, it may be formed on 20% or more to less than 100% of the total area of the lithium transition metal oxide. When the area of the coating layer is less than 20%, the effect of improving the lifespan characteristics and the packing density according to the formation of the coating layer may be insignificant.

In the present invention, the positive active material may have a minimum pressure of 1200 kgf/cm ² or more at which the rolling density is 120% or more of the tap density.

If the particle strength of the positive electrode active material is weak, the rolling density rapidly increases compared to the density before rolling (tap density) even by weak pressure, and in this case, particle breakage may occur due to rolling. In addition, uncoated lithium transition metal When the surface of the oxide is exposed to the outside, a side reaction with the electrolyte may occur, and the performance of the lithium secondary battery may deteriorate due to physical contact between particles.

As described above, the cathode active material of the present invention has a high minimum pressure of 1200 kgf/cm ² at which the rolling density is 120% or more of the tap density, which has strong resistance to pressure and excellent particle strength, so that the particle is not broken during rolling. This means that it can be suppressed.

In the present invention, the positive electrode active material may have a tap density of 1.8 to 2.8 g/cc, preferably 2.0 to 2.5 g/cc. When the tap density is within the above range, the reaction between the electrolyte and the positive electrode active material particles due to the formation of cavities may be promoted without lowering the energy density of the particles.

The tap density may be measured by putting 5 g of the cathode active material into a cylindrical mold and using a powder resistance meter.

<Method of manufacturing cathode active material>

The manufacturing method of the cathode active material of the present invention includes preparing a transition metal hydroxide precursor containing 60 mol% or more of nickel based on the total number of moles of the transition metal; preparing a single-particle lithium transition metal oxide by mixing the transition metal hydroxide precursor and a lithium source material and firing at a temperature of 850° C. or higher; and mixing the lithium transition metal oxide with boric acid and heat-treating at a temperature of 350 to 450°C.

First, a transition metal hydroxide precursor containing 60 mol% or more of nickel based on the total number of moles of the transition metal is prepared.

The transition metal hydroxide precursor may be purchased and used as a commercially available cathode active material precursor, or prepared according to a method for preparing a cathode active material precursor well known in the art.

For example, the precursor may be prepared by adding an ammonium cation-containing complex forming agent and a basic compound to a transition metal solution containing a nickel-containing raw material, a cobalt-containing raw material, and a manganese-containing raw material, followed by a coprecipitation reaction.

Subsequently, the transition metal hydroxide precursor and the lithium raw material are mixed and calcined at a temperature of 850° C. or higher to prepare single-particle lithium transition metal oxide. That is, the cathode active material may be prepared in the form of a single particle by performing firing at a temperature of 850° C. or higher. Specifically, the firing temperature may be 850 °C or higher, preferably 850 to 870 °C, or 850 to 860 °C.

By firing at a high temperature as described above, the primary particles that were individually present in the secondary particles are combined with each other to grow into large primary particles with high crystallinity, and the positive electrode active material exhibits excellent particle strength due to high structural stability. this can be manufactured. In addition, the cathode active material prepared therefrom suppresses particle breakage due to compression and maintains its shape, thereby reducing the increase in fine particles in the electrode due to particle breakage and improving the lifespan characteristics of the battery.

The lithium raw material is a lithium-containing carbonate (eg, lithium carbonate, etc.), a hydrate (eg, lithium hydroxide I hydrate (LiOH H ₂ O), etc.), a hydroxide (eg, lithium hydroxide, etc.), a nitrate (eg, lithium hydroxide, etc.) For example, it may be lithium nitrate (LiNO ₃ ), etc.), chloride (eg, lithium chloride (LiCl), etc.), but is not limited thereto.

In the present invention, the firing may be performed under an oxygen or air atmosphere, and may be performed for 5 to 13 hours. When calcined in the above atmosphere, the local oxygen partial pressure is increased to improve the crystallinity of the positive electrode active material, and control on the surface may be facilitated.

Subsequently, the lithium transition metal oxide is mixed with boric acid and heat-treated at a temperature of 350 to 450°C. In the above step, a coating layer containing boron located on the surface of the lithium transition metal oxide may be prepared by using boric acid as a coating raw material.

The heat treatment for forming the coating layer may be performed at a temperature of 350 to 450°C, specifically, at 370 to 430°C and 370 to 400°C. The heat treatment may be performed for 1 to 6 hours.

By heat treatment at a high temperature as described above, the amorphous structure is crystallized so that the boron-containing coating layer has higher crystallinity, and thus, the cathode active material of the present invention exhibits excellent particle strength due to high structural stability.

In addition, the heat treatment may be performed in an air atmosphere.

The manufacturing method of the present invention may further include washing the lithium transition metal oxide with water to remove lithium by-products present on the surface of the lithium transition metal oxide.

When a large amount of lithium by-product is present in the cathode active material, the lithium by-product reacts with the electrolyte to generate gas and swelling, which significantly deteriorates high-temperature stability. Accordingly, a water washing process for removing lithium by-products from the lithium transition metal oxide containing high-concentration nickel may be further performed.

The water washing step may be performed, for example, by adding lithium transition metal oxide to ultrapure water and stirring it. At this time, the washing temperature may be 20 ° C or less, preferably 10 to 20 ° C, and the washing time may be about 10 minutes to 1 hour. When the washing temperature and washing time satisfy the above ranges, lithium by-products can be effectively removed.

<Anode>

In addition, the present invention provides a cathode for a lithium secondary battery comprising the cathode active material prepared by the above-described method.

Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector and including the positive electrode active material.

The positive current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon, nickel, titanium on the surface of aluminum or stainless steel. , those surface-treated with silver, etc. may be used. In addition, the cathode current collector may have a thickness of typically 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase adhesion of the cathode active material. For example, it may be used in various forms such as films, sheets, foils, nets, porous materials, foams, and non-woven fabrics.

The positive electrode active material layer may include a conductive material and a binder together with the positive electrode active material.

In this case, the positive electrode active material may be included in an amount of 80 to 99% by weight, more specifically, 85 to 98% by weight based on the total weight of the positive electrode active material layer. When included in the above content range, excellent capacity characteristics can be exhibited.

At this time, the conductive material is used to impart conductivity to the electrode, and in the battery configured, any material that does not cause chemical change and has electronic conductivity can be used without particular limitation. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like, and one of them alone or a mixture of two or more may be used. The conductive material may be included in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer.

The binder serves to improve adhesion between particles of the positive electrode active material and adhesion between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC) ), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and the like, and one type alone or a mixture of two or more types thereof may be used. The binder may be included in an amount of 1 to 30% by weight based on the total weight of the positive electrode active material layer.

The positive electrode may be manufactured according to a conventional positive electrode manufacturing method except for using the positive electrode active material. Specifically, it may be prepared by coating a cathode mixture prepared by dissolving or dispersing the above-described cathode active material and optionally, a binder and a conductive material in a solvent on a cathode current collector, followed by drying and rolling. In this case, the types and contents of the positive electrode active material, the binder, and the conductive material are as described above.

The solvent may be a solvent commonly used in the art, and dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water and the like, and one type alone or a mixture of two or more types of these may be used. The amount of the solvent used is enough to dissolve or disperse the positive electrode active material, conductive material, and binder in consideration of the coating thickness and manufacturing yield of the slurry, and to have a viscosity capable of exhibiting excellent thickness uniformity during subsequent coating for manufacturing the positive electrode. Do.

Alternatively, the positive electrode may be manufactured by casting the positive electrode mixture on a separate support and then laminating a film obtained by peeling from the support on a positive electrode current collector.

<Lithium secondary battery>

In addition, the present invention can manufacture an electrochemical device including the anode. The electrochemical device may be specifically a battery, a capacitor, and the like, and more specifically, may be a lithium secondary battery.

The lithium secondary battery specifically includes a positive electrode, a negative electrode positioned opposite to the positive electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode, and the positive electrode is the same as described above, so a detailed description is omitted, Hereinafter, only the remaining configurations will be described in detail.

In addition, the lithium secondary battery may optionally further include a battery container accommodating the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member sealing the battery container.

In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer positioned on the negative electrode current collector.

The anode current collector is not particularly limited as long as it does not cause chemical change in the battery and has high conductivity. For example, it is formed on the surface of copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel. A surface treated with carbon, nickel, titanium, silver, or the like, an aluminum-cadmium alloy, or the like may be used. In addition, the negative electrode current collector may have a thickness of typically 3 μm to 500 μm, and like the positive electrode current collector, fine irregularities may be formed on the surface of the current collector to enhance bonding strength of the negative electrode active material. For example, it may be used in various forms such as films, sheets, foils, nets, porous materials, foams, and non-woven fabrics.

The anode active material layer optionally includes a binder and a conductive material together with the anode active material.

A compound capable of reversible intercalation and deintercalation of lithium may be used as the anode active material. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of being alloyed with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; metal oxides capable of doping and undoping lithium, such as SiO _β (0<β<2), SnO ₂ , vanadium oxide, and lithium vanadium oxide; or a composite including the metallic compound and the carbonaceous material, such as a Si—C composite or a Sn—C composite, and any one or a mixture of two or more of these may be used. In addition, a metal lithium thin film may be used as the anode active material. Also, as the carbon material, both low crystalline carbon and high crystalline carbon may be used. Soft carbon and hard carbon are typical examples of low crystalline carbon, and high crystalline carbon includes amorphous, platy, scaly, spherical or fibrous natural graphite, artificial graphite, or kish graphite. graphite, pyrolytic carbon, mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar pitch High-temperature calcined carbon such as derived cokes is representative.

The negative active material may be included in an amount of 80% to 99% by weight based on the total weight of the negative active material layer.

The binder is a component that assists in bonding between the conductive material, the active material, and the current collector, and is typically added in an amount of 0.1 part by weight to 10 parts by weight based on 100 parts by weight of the total weight of the negative electrode active material layer. Examples of such binders are polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetra fluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, various copolymers thereof, and the like.

The conductive material is a component for further improving the conductivity of the negative electrode active material, and may be added in an amount of 10% by weight or less, preferably 5% by weight or less, based on the total weight of the negative electrode active material layer. The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery, and examples thereof include graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; fluorinated carbon; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used.

For example, the negative electrode active material layer is prepared by coating a negative electrode composite prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material in a solvent on a negative electrode current collector and drying the negative electrode composite, or by drying the negative electrode composite. It can be produced by casting on a support and then laminating a film obtained by peeling from the support on a negative electrode current collector.

On the other hand, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ion movement. Anything that is normally used as a separator in a lithium secondary battery can be used without particular limitation, especially for the movement of ions in the electrolyte. It is preferable to have low resistance to the electrolyte and excellent ability to absorb the electrolyte. Specifically, a porous polymer film, for example, a porous polymer film made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate copolymer, or these A laminated structure of two or more layers of may be used. In addition, conventional porous non-woven fabrics, for example, non-woven fabrics made of high-melting glass fibers, polyethylene terephthalate fibers, and the like may be used. In addition, a coated separator containing a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and may be selectively used in a single-layer or multi-layer structure.

In addition, the electrolyte used in the present invention includes organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries, and are limited to these. it is not going to be

Specifically, the electrolyte may include an organic solvent and a lithium salt.

The organic solvent may be used without particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvent includes ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; Dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), propylene carbonate, PC) and other carbonate-based solvents; alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (R is a straight-chain, branched or cyclic hydrocarbon group having 2 to 20 carbon atoms and may contain a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; Alternatively, sulfolane or the like may be used. Among them, carbonate-based solvents are preferred, and cyclic carbonates (eg, ethylene carbonate or propylene carbonate, etc.) having high ion conductivity and high dielectric constant capable of increasing the charge and discharge performance of batteries, and low-viscosity linear carbonate-based compounds ( For example, a mixture of ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) is more preferable. In this case, when the cyclic carbonate and the chain carbonate are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte may be excellent.

The lithium salt may be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt is LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN(C ₂ F ₅ SO ₃ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(CF ₃ SO ₂ ) _{2 .} LiCl, LiI, or LiB(C ₂ O ₄ ) ₂ may be used. The concentration of the lithium salt is preferably used within the range of 0.1 to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, so excellent electrolyte performance can be exhibited, and lithium ions can move effectively.

In addition to the above electrolyte components, the electrolyte may include, for example, haloalkylene carbonate-based compounds such as difluoroethylene carbonate, pyridine, and triglycerides for the purpose of improving battery life characteristics, suppressing battery capacity decrease, and improving battery discharge capacity. Ethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imida One or more additives such as zolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol or aluminum trichloride may be further included. In this case, the additive may be included in an amount of 0.1 to 5 parts by weight based on 100 parts by weight of the total weight of the electrolyte.

As described above, since the lithium secondary battery including the cathode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and lifespan characteristics, portable devices such as mobile phones, notebook computers, digital cameras, and hybrid electric vehicles ( It is useful for electric vehicles such as hybrid electric vehicles (HEVs).

Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.

The battery module or battery pack may include a power tool; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); Alternatively, it may be used as a power source for one or more medium or large-sized devices among power storage systems.

The appearance of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape using a can, a prismatic shape, a pouch shape, or a coin shape.

The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but also can be preferably used as a unit cell in a medium-large battery module including a plurality of battery cells.

Example

Hereinafter, the present invention will be described in more detail by examples. However, the following examples are for exemplifying the present invention, and the scope of the present invention is not limited only thereto.

Example 1

Lithium source LiOH was added to and mixed with the cathode active material precursor Ni _0.85 Co _0.05 Mn _0.10 (OH) ₂ , and the mixed powder was put into an alumina crucible for heat treatment. Thereafter, a lithium transition metal oxide was prepared by firing at 850 to 860° C. for 13 hours under an oxygen atmosphere.

Thereafter, the powder subjected to grinding, washing with water, and drying was mixed with the coating material H ₃ BO ₃ and heat-treated at 370 to 400° C. under an oxygen atmosphere to prepare a cathode active material having a single particle form.

Comparative Example 1

A positive electrode active material was prepared in the same manner as in Example 1, except that it was calcined at 800 to 810 °C.

Comparative Example 2

A positive electrode active material was prepared in the same manner as in Example 1, except that heat treatment was performed at 290 to 320 °C.

Comparative Example 3

A positive electrode active material was prepared in the same manner as in Example 1, except for firing at 800 to 810 ° C and heat treatment at 290 to 320 ° C.

Comparative Example 4

A cathode active material was prepared in the same manner as in Example 1, except that the step of forming a coating layer by mixing with the coating material H ₃ BO ₃ was not performed.

Experimental Example 1: Minimum pressure measurement at which the rolling density is 120% or more of the tap density

The rolling density was measured using a powder resistance meter (HPRM-A2). Specifically, after putting 5 g of the cathode active material into a cylindrical mold, 0 kgf/cm ² , 400 kgf/cm ² , 800 kgf/cm ² , 1200 kgf/cm ² , 1600 kgf/cm ² , 2000 kgf/cm After pressurizing the mold containing the cathode active material with ² , the height of the mold was measured to obtain the rolling density.

The tap density value to which no pressure was applied was taken as 100%, and the tap density versus the rolled density value was calculated and described together.

Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Tap density (g/cc) (@ 0 kgf/cm ² ) 2.29 (100%) 2.05 (100%) 2.25 (100%) 1.98 (100%) 2.18 (100%) Rolling density (g/cc) (@ 400 kgf/cm ² ) 2.38 (104%) 2.26 (110%) 2.43 (108%) 2.20 (111%) 2.62 (120%) Rolling density (g/cc) (@ 800 kgf/cm ² ) 2.50 (109%) 2.46 (120%) 2.70 (120%) 2.38 (120%) 2.75 (126%) Rolling density (g/cc) (@ 1200 kgf/cm ² ) 2.61 (114%) 2.60 (127%) 2.81 (125%) 2.49 (126%) 2.83 (130%) Rolling density (g/cc) (@ 1600 kgf/cm ² ) 2.75 (120%) 2.77 (135%) 2.95 (131%) 2.73 (138%) 2.90 (133%) Rolling density (@ 2000 kgf/cm ² ) 3.00 (131%) 3.01 (147%) 3.08 (137%) 2.85 (144%) 2.96 (136%)

As shown in Table 1, Example 1 had a rolling density of 120% of the tap density when a pressure of 1600 kgf/cm ² was applied, and Comparative Examples 1 to 4 had 800 kgf/cm ^{2 and} 800 kgf/cm 2 respectively. When a pressure of cm ^2, 800 kgf/cm ^2, 400 kgf/cm ² was applied, it was confirmed that the rolling density was 120% of the tap density.

These results mean that the positive electrode active material prepared in Examples had higher particle strength than the positive electrode active material of Comparative Example, so that resistance to pressure was high and particle breakage was suppressed.

Experimental Example 2: Evaluation of particle breakage

Cathode active material, carbon black conductive material, and polyvinylidene fluoride (PVdF) binder prepared in Examples and Comparative Examples were mixed in an N-methylpyrrolidone (NMP) solvent at a weight ratio of 96.5: 1.5: 2 to obtain a cathode slurry. was manufactured. The positive electrode slurry was applied to one surface of an aluminum current collector, dried at 100° C., and then rolled to prepare a positive electrode.

After the prepared positive electrode was fired at 500° C. for 10 hours in a firing furnace, only the remaining positive electrode active material was taken and the average particle diameter (D ₅₀ ) was measured using a particle size distribution (PSD). Among them, the ratio (%) of the positive electrode active material having an average particle diameter of 1 μm or less was calculated and shown in Table 2.

Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 1 μm or less ratio (%) 1.96 8.45 7.92 10.71 14.45

Referring to Table 2, in Example 1 compared to Comparative Examples 1 to 4, the ratio (%) of the positive electrode active material having an average particle diameter of 1 μm or less after rolling was much lower. This means that in the cathode active material of Example 1, particle breakage during rolling was significantly suppressed.

Experimental Example 3: High Temperature Life Characteristics

Lithium secondary batteries were prepared using the cathode active materials prepared in Examples and Comparative Examples, and capacity characteristics were evaluated for each.

Specifically, the cathode active material, the carbon black conductive material, and the polyvinylidene fluoride (PVdF) binder prepared in Examples and Comparative Examples were mixed in an N-methylpyrrolidone (NMP) solvent at a weight ratio of 96.5:1.5:2. Thus, a positive electrode slurry was prepared. The positive electrode slurry was applied to one surface of an aluminum current collector, dried at 100° C., and then rolled to prepare a positive electrode.

Meanwhile, lithium metal was used as the negative electrode.

An electrode assembly was prepared by interposing a porous polyethylene separator between the positive electrode and the negative electrode prepared above, and then placed inside a battery case, and then an electrolyte was injected into the case to prepare a lithium secondary battery. At this time, 1.0 M of lithium hexafluorophosphate (LiPF ₆ ) was dissolved in an organic solvent in which ethylene carbonate: ethylmethyl carbonate: diethyl carbonate (EC: EMC: DEC) as an electrolyte was mixed in a volume ratio of 3: 4: 3 A lithium secondary battery was prepared by injecting an electrolyte solution.

Specifically, after manufacturing each battery cell, the battery cell was buffered to a SOC 100 state. Thereafter, after 100 days after leaving the battery cell in an oven at 60 ° C., the initial resistance before leaving and the resistance at 6 weeks were compared to calculate the capacity retention rate (%) of the battery cell during high-temperature storage, thereby evaluating high-temperature life characteristics.

Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 High Temperature Capacity Retention (%) (60℃) 95 82 85 83 72

As shown in Table 3, when the cathode active material prepared in Example 1 was used, a lithium secondary battery with improved lifespan characteristics at high temperature compared to the comparative example could be manufactured. This is because the cathode active material prepared by firing and heat treatment at a high temperature as in Example 1 has high particle strength, so particle breakage is reduced and side reactions with the electrolyte are suppressed.

Experimental Example 4: Evaluation of cycle characteristics

Each of the lithium secondary batteries prepared above was charged at room temperature with a 0.2 C constant current up to 4.25 V at 0.05 C cut off. Then, it was discharged until it became 2.5 V with 0.2 C constant current. The above charging and discharging behavior was set as one cycle, and from the second time, charging with a 0.5 C constant current to 4.25 V with a 0.05 C cut off and discharging to 2.5 V with a 0.5 C constant current was set as one cycle.

After repeating this cycle 30 times, the capacity retention rate according to the cycle of the secondary battery was measured, which is shown in Table 4 below.

Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Capacity retention rate @30cycles (%) 96 83 83 81 71

As shown in Table 4, when the cathode active material prepared in Example 1 is used, a lithium secondary battery showing a higher capacity retention rate than the comparative example can be manufactured.

Claims (10)

a lithium transition metal oxide containing 60 mol% or more of nickel based on the total number of moles of transition metals excluding lithium; And a coating layer containing boron;
The lithium transition metal oxide is a single particle composed of primary particles having an average particle diameter (D ₅₀ ) of 0.3 to 2 μm,
The average particle diameter (D ₅₀ ) of the single particles is 1 to 10 μm of the positive electrode active material.
The method of claim 1,
A cathode active material in which one single particle is composed of 1 to 30 primary particles.
The method of claim 1,
The coating layer containing boron is a positive electrode active material formed on the entire surface of the lithium transition metal oxide.
The method of claim 1,
The lithium transition metal oxide is a positive electrode active material represented by Formula 1 below:
[Formula 1]
Li _1+a Ni _x Co _y Mn _z M ¹ _w O ₂
In Formula 1,
M ¹ is at least one selected from the group consisting of Al, Mg, V, Ti, and Zr;
0≤a≤0.5, 0.6≤x<1, 0<y≤0.4, 0<z≤0.4, 0<w≤0.04, x+y+z+w=1.
The method of claim 1,
A minimum pressure at which a rolling density of the cathode active material is 120% or more of a tap density is 1200 kgf/cm ² or more.
The method of claim 1,
The positive electrode active material has a tap density of 1.8 to 2.8 g / cc.
Preparing a transition metal hydroxide precursor containing 60 mol% or more of nickel based on the total number of moles of the transition metal;
preparing a single-particle lithium transition metal oxide by mixing the transition metal hydroxide precursor and a lithium source material and firing at a temperature of 850° C. or higher; and
Method for producing a cathode active material comprising a; mixing the lithium transition metal oxide with boric acid and heat-treating at a temperature of 350 to 450 ° C.
The method of claim 7,
The firing is a method for producing a positive electrode active material performed in an oxygen or air atmosphere.
A cathode for a lithium secondary battery comprising the cathode active material of any one of claims 1 to 6.
A lithium secondary battery comprising the positive electrode for a lithium secondary battery of claim 9.