WO2025136067A1 - Cathode active material for lithium secondary battery, manufacturing method of same, and lithium secondary battery comprising same - Google Patents

Abstract

A cathode active material for a lithium secondary battery according to an embodiment includes: a metal oxide including nickel and manganese; and a coating layer located on the surface of the metal oxide and containing cobalt, wherein the content of nickel in the metal oxide is 0.75 mol or less based on 1 mol of the total metals excluding lithium, and expression 1 may be satisfied. [Expression 1] 22.0 ≤(A*C)/B ≤ 26.5 (wherein, in expression 1, A and B are an a-axis constant and a c-axis constant measured by XRD analysis with respect to the cathode active material, respectively, and C is a grain size measured by XRD analysis for the cathode active material)

Description

Cathode active material for lithium secondary battery, method for producing same, and lithium secondary battery comprising same

The present invention relates to a positive electrode active material for a lithium secondary battery, a method for producing the same, and a lithium secondary battery including the same.

Recently, driven by the explosive demand for electric vehicles and the need to increase driving range, the development of high-capacity, high-energy-density secondary batteries to meet these demands is actively underway worldwide.

The cathode active materials used in these secondary batteries include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂ or LiMn ₂ O ₄ , etc.), lithium iron phosphate compound (LiFePO ₄ ), lithium nickel cobalt manganese oxide, etc. Among these, lithium cobalt oxide and lithium nickel cobalt manganese oxide have the advantages of high operating voltage and excellent capacity characteristics, so they are widely used and applied as cathode active materials for high voltage.

However, with the recent rapid growth of the lithium secondary battery market, the cost of raw materials is increasing. In particular, the cathode active material accounts for the largest proportion of the cost of lithium secondary batteries, and among them, cobalt (Co) is an expensive metal and has the problem of unstable supply.

Therefore, there is a limit to the mass use of cathode active materials with a high cobalt content as a power source in fields such as electric vehicles, and therefore, there is a need to develop cathode active materials that exclude cobalt or have a reduced cobalt content.

In this embodiment, an object is to provide a cathode active material for a lithium secondary battery having excellent initial capacity, rate characteristics and high-temperature life characteristics, a method for producing the same, and a lithium secondary battery including the same.

According to one embodiment, a cathode active material for a lithium secondary battery includes: a metal oxide including nickel and manganese; and a coating layer located on a surface of the metal oxide and containing cobalt; wherein the content of nickel in the metal oxide is 0.75 mol or less based on 1 mol of the total metal excluding lithium, and may satisfy the following equation 1.

[Formula 1]

22.0 ≤(A*C)/B ≤ 26.5

In the above equation 1, A and B are a-axis and c-axis constants, respectively, measured by XRD analysis for the positive electrode active material, and C means the crystal grain size measured by XRD analysis for the positive electrode active material.

The a-axis constant measured by XRD analysis for the above positive electrode active material may be in the range of 2.880Å to 2.890Å.

The c-axis constant measured by XRD analysis for the above positive electrode active material may be in the range of 14.262Å to 14.281Å.

The crystal grain size of the above positive electrode active material may be in the range of 94.0 nm to 131.0 nm.

In one embodiment, the positive electrode active material may have a ratio of I ₀₀₃ /I ₁₀₄ , which is a peak value I ₀₀₃ corresponding to the (003) plane and a peak value I ₁₀₄ corresponding to the (104) plane, measured by XRD analysis, in a range of 1.1020 to 1.2640.

The content of cobalt may be 0.012 mol or less based on the entire metal oxide on which the coating layer is formed.

The content of cobalt based on the entire metal oxide on which the coating layer is formed may be in the range of 0.001 mol to 0.012 mol.

The above metal oxide may not contain cobalt.

The content of nickel in the above metal oxide may be in the range of 0.65 to 0.75 based on 1 mole of the total metal excluding lithium.

The content of manganese in the above metal oxide may be in the range of 0.25 to 0.35 based on 1 mole of the total metal excluding lithium.

In one embodiment, the metal oxide may be represented by the following chemical formula 1.

[Chemical Formula 1]

Li _a [Ni _x Mn _y M _z ]O ₂

In the chemical formula 1, 0.8≤a≤1.2, 0.67≤x≤0.73, 0.25≤y≤0.38, 0≤z≤0.2, x+y+z=1, and M is Zr, Al, B, Y, Mg, Ti, Nb, W, Sc, Si, V, Fe, Mo, Ce, Hf, Ta, La, Sr, Sn, Sb, Zn, Cu, Ge, Mo, Ru, Ir, Na or a combination thereof.

The average particle diameter (D50) of the above positive electrode active material may be in the range of 10 to 15 μm.

According to another embodiment, a method for manufacturing a cathode active material for a lithium secondary battery comprises the steps of: preparing a metal hydroxide containing nickel and manganese; mixing the metal hydroxide and a lithium raw material to prepare a mixture; firing the mixture to obtain a sintered product; and mixing the sintered product and a coating raw material containing cobalt and then performing a heat treatment to obtain a metal oxide having a coating layer formed thereon; wherein the metal oxide having the coating layer formed thereon may satisfy the following formula 1.

[Formula 1]

22.0 ≤(A*C)/B ≤ 26.5

In the above equation 1, A and B are a-axis and c-axis constants, respectively, measured by XRD analysis for the positive electrode active material, and C means the crystal grain size measured by XRD analysis for the positive electrode active material.

The coating raw material containing the above cobalt has a cobalt content of 0.012 mol or less based on the entire metal oxide on which the coating layer is formed. It can be invested as much as possible.

The above-mentioned cobalt-containing coating raw material may include at least one of Co(OH) ₂ , _CoCl2 , _CoO , _CoF3 , _CoSO4 · _xH2O , CoSO4· _7H2O , ( _CH3COO ) _2Co · _4H2O , Co( _NO3 ) _{2 · 6H2O, (CH3CO2 ) 2Co , CoCO3} · _xH2O , _Co3 ( _PO4 ) ₂ , and combinations thereof.

The average particle diameter (D50) of the coating raw material containing the above cobalt may be in the range of 10 ㎛ to 12 ㎛.

In one embodiment, the heat treatment process may be performed only once.

A cathode for a lithium secondary battery according to another embodiment may include a cathode active material according to one embodiment.

A lithium secondary battery according to another embodiment may include a cathode comprising a cathode active material according to one embodiment.

According to the present embodiment, since a coating layer including a trace amount of cobalt is located on the surface of a metal oxide that does not include cobalt, a cathode active material having excellent initial discharge capacity, rate characteristics, and high-temperature life characteristics can be implemented.

In addition, since the positive electrode active material of the present embodiment contains a small amount of cobalt, it is economically feasible to secure excellent electrochemical characteristics while minimizing production costs.

Figure 1 shows the results of measuring the initial capacity of positive electrode active materials manufactured according to Examples 1 to 3 and Comparative Examples 1 to 7.

Figure 2 shows the results of measuring the high temperature life characteristics of positive electrode active materials manufactured according to Examples 1 to 3 and Comparative Examples 1 to 7.

Figure 3 shows the XRD measurement results for positive electrode active materials manufactured according to Examples 1 to 3 and Comparative Examples 1 to 7.

The terms first, second, and third, etc. are used to describe, but are not limited to, various parts, components, regions, layers, and/or sections. These terms are only used to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Thus, a first part, component, region, layer, or section described below may be referred to as a second part, component, region, layer, or section without departing from the scope of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. As used herein, the singular forms include the plural forms as well, unless the context clearly dictates otherwise. The word "comprising," as used herein, specifies particular features, regions, integers, steps, operations, elements, and/or components, but does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, and/or components.

When a part is referred to as being "on" or "on" another part, it may be directly on or above the other part, or there may be other parts intervening. In contrast, when a part is referred to as being "directly on" another part, there are no other parts intervening.

Unless otherwise defined, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Terms defined in commonly used dictionaries are additionally interpreted as having a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted in an ideal or very formal sense unless defined.

Also, unless otherwise specified, % means weight%, and 1 ppm is 0.0001 weight%.

In this specification, the term "combination(s) thereof" described in the expression in the Makushi format means one or more mixtures or combinations selected from the group consisting of the components described in the expression in the Makushi format, and means including at least one selected from the group consisting of said components.

Hereinafter, embodiments of the present invention will be described in detail so that those with ordinary skill in the art can easily implement the present invention. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

Cathode active material for lithium secondary batteries

According to one embodiment, a cathode active material for a lithium secondary battery includes: a metal oxide including nickel and manganese; and a coating layer located on a surface of the metal oxide and containing cobalt; wherein the content of nickel in the metal oxide is 0.75 mol or less based on 1 mol of the total metal excluding lithium, and may satisfy the following equation 1.

[Formula 1]

22.0 ≤(A*C)/B ≤ 26.5

In the above equation 1, A and B are a-axis and c-axis constants, respectively, measured by XRD analysis for the positive electrode active material, and C means the crystal grain size measured by XRD analysis for the positive electrode active material.

Equation 1 is derived using the a-axis lattice constant, the c-axis lattice constant, and the grain size, and the value of Equation 1 can be in the range of 22.0 to 26.5, and more specifically, can be in the range of 23.5 to 26.0. When the value of Equation 1 satisfies the above range, the interfacial resistance between particles can be reduced, and since the structural stability is excellent, a cathode active material having excellent electrochemical characteristics such as life span and output characteristics can be implemented.

More specifically, the a-axis constant measured by XRD analysis for the positive electrode active material may be in the range of 2.880Å to 2.890Å, and more specifically, in the range of 2.881Å to 2.886Å. When the a-axis lattice constant satisfies the above range, lithium ions can stably diffuse in the horizontal direction, so that a positive electrode active material with excellent ion conductivity can be implemented.

The c-axis constant measured by XRD analysis for the above-mentioned positive electrode active material may be in the range of 14.262Å to 14.281Å, and more specifically, in the range of 14.265Å to 14.280Å. When the c-axis lattice constant satisfies the above-mentioned range, the interlayer distance is appropriately maintained, so that lithium ions are advantageously moved into/out of the crystal structure, thereby improving the charging speed and output characteristics of the lithium secondary battery.

In one embodiment, the crystal grain size of the positive electrode active material may be in a range of 94.0 nm to 131.0 nm, and more specifically, in a range of 115 nm to 130 nm. When the crystal grain size satisfies the above range, the structural stability of the positive electrode active material can be secured, thereby reducing the interfacial resistance and improving the life characteristics.

The above cathode active material may have a ratio of I ₀₀₃ /I ₁₀₄ , which is a peak value I ₀₀₃ corresponding to the (003) plane and a peak value I ₁₀₄ corresponding to the (104) plane, measured by XRD analysis, in a range of 1.1020 to 1.2640, and more specifically, in a range of 1.1000 to 1.2000.

The I ₀₀₃ /I ₁₀₄ value is a characteristic indicating the mixing of lithium ions and transition metal ions (cation mixing), and if it satisfies the above range, it indicates that the layered structure is well formed. Accordingly, the lithium ion diffusion is smooth, the charge/discharge speed is improved, and the initial capacity and efficiency of the lithium secondary battery can be improved, and excellent life characteristics can be secured.

In one embodiment, the content of cobalt based on the entire metal oxide on which the coating layer is formed may be 0.012 mole or less.

The present embodiment includes a metal oxide having a cobalt-free composition that does not contain cobalt. Since the lithium metal oxide does not contain cobalt, the manufacturing cost can be reduced.

The above metal oxide may have a layered crystal structure.

In a case where the metal oxide does not include cobalt in the crystal structure as in this embodiment, there is a problem in that electrochemical characteristics such as rate characteristics and resistance characteristics deteriorate because the structural instability of the particles increases. Therefore, in this embodiment, by forming a coating layer including a trace amount of cobalt on the surface of the metal oxide, it is possible to implement a cathode active material having excellent electrochemical performance while reducing the manufacturing cost.

The above metal oxide may be a secondary particle formed by agglomeration of multiple primary particles.

In this specification, “secondary particle” means an aggregate, i.e., a secondary structure, in which tens to hundreds of primary particles are aggregated together by physical or chemical bonding between the primary particles without any intentional agglomeration or assembly process for the primary particles.

In addition, the term “primary particle” refers to the smallest particle unit that can be distinguished as a single lump when observing the cross-section of a positive electrode active material through a scanning electron microscope (SEM), and may be composed of a single crystal grain or multiple crystal grains.

Additionally, the term “crystal grain” refers to a distinct region in which atoms within a primary particle form a lattice structure with a certain orientation.

The content of cobalt based on the entire metal oxide on which the coating layer is formed may be 0.012 mol or less, more specifically, 0.001 mol to 0.012 mol. When the content of cobalt included in the coating layer satisfies the above range, excellent electrochemical characteristics of the positive electrode active material can be secured without significantly increasing the manufacturing cost.

The content of nickel in the above metal oxide may be in the range of 0.65 to 0.75 based on 1 mole of the total metal excluding lithium. If the content of nickel is too low, the capacity characteristics and high-voltage stability may deteriorate. If the content of nickel is too high, the life characteristics may deteriorate.

The manganese content in the above metal oxide may be in the range of 0.25 to 0.35 based on 1 mole of the total metal excluding lithium. If the manganese content is too low, the structural safety characteristics may deteriorate. If the manganese content is too high, the low-potential region safety characteristics may deteriorate.

According to one embodiment, a metal oxide may be represented by the following chemical formula 1.

[Chemical Formula 1]

Li _a [Ni _x Mn _y M _z ]O ₂

In the chemical formula 1, 0.8≤a≤1.2, 0.67≤x≤0.73, 0.25≤y≤0.38, 0≤z≤0.2, x+y+z=1, and M is Zr, Al, B, Y, Mg, Ti, Nb, W, Sc, Si, V, Fe, Mo, Ce, Hf, Ta, La, Sr, Sn, Sb, Zn, Cu, Ge, Mo, Ru, Ir, Na or a combination thereof.

The average particle diameter (D50) of the above positive electrode active material may be in the range of 10 to 15 μm. When the average particle diameter satisfies the above range, a positive electrode active material having excellent capacity and output characteristics can be realized.

Method for manufacturing positive electrode active material for lithium secondary battery

According to one embodiment, a method for manufacturing a cathode active material for a lithium secondary battery comprises the steps of: preparing a metal hydroxide containing nickel and manganese; mixing the metal hydroxide and a lithium raw material to prepare a mixture; firing the mixture to obtain a sintered product; and mixing the sintered product and a coating raw material containing cobalt and then performing a heat treatment to obtain a metal oxide having a coating layer formed thereon; wherein the metal oxide having the coating layer formed thereon may satisfy the following formula 1.

[Formula 1]

22.0 ≤(A*C)/B ≤ 26.5

In the above equation 1, A and B are the a-axis and c-axis constants measured by XRD analysis for the positive electrode active material, respectively, and C means the crystal grain size measured by XRD analysis for the positive electrode active material.

Regarding equation 1, it is the same as described above and is omitted here.

Let's look at each step in more detail below.

First, a step of preparing a metal hydroxide containing nickel and manganese is performed.

The metal hydroxide containing the nickel and manganese mentioned above can be produced by, for example, adding a solution containing a complexing agent and a solution containing a pH regulator to a solution containing a transition metal containing a nickel raw material and a manganese raw material and performing a co-precipitation reaction.

The above nickel raw material is not particularly limited as long as it is used in the art for manufacturing a positive electrode active material precursor. For example, the nickel raw material may be a nickel-containing sulfate, acetate, nitrate, halide, sulfide, hydroxide, oxide or oxyhydroxide, and specifically, may be, but is not limited to, NiSO ₄ , NiSO ₄ .6H ₂ O, Ni(OH) ₂ , NiO, _NiOOH , NiCO ₃ .2Ni(OH) ₂ .4H 2 O, NiC ₂ O 2 .2H 2 O, Ni ₍ NO ₃ ) ₂ .6H ₂ O, fatty acid nickel salts, nickel halides or combinations thereof.

The above manganese raw material is not particularly limited as long as it is used in the art for manufacturing a positive electrode active material precursor. For example, the manganese raw material may be a manganese-containing sulfate, acetate, nitrate, halide, sulfide, hydroxide, oxide, oxyhydroxide or a combination thereof, and specifically, may be a manganese salt such as MnSO ₄ , MnCO ₃ , Mn(NO ₃ ) ₂ , manganese acetate, manganese dicarboxylic acid salt, manganese citrate and manganese fatty acid salt, manganese oxide such as Mn ₂ O ₃ , MnO ₂ , and Mn ₃ O ₄ , oxyhydroxide, manganese chloride or a combination thereof, but is not limited thereto.

The above transition metal-containing solution may be prepared by adding nickel raw material and manganese raw material to a solvent, specifically, water, or a mixture of water and an organic solvent (e.g., alcohol, etc.) that can be uniformly mixed with water.

The above complexing agent-containing solution performs a complex-forming function and may include, but is not limited to, complexing agents such as NH ₃ , NH ₄ OH, (NH ₄ ) ₂ SO ₄ , NH ₄ NO ₃ , NH ₄ Cl, CH ₃ COONH ₄ , NH ₄ CO ₃ or a combination thereof. Meanwhile, the complexing agent-containing solution may be used in the form of an aqueous solution, and at this time, water or a mixture of water and an organic solvent (e.g., alcohol, etc.) that can be uniformly mixed with water may be used as a solvent.

The above pH adjusting agent may be a caustic soda solution, which may include an alkali compound of a hydroxide of an alkali metal or alkaline earth metal, such as NaOH, KOH, or Ca(OH) ₂ , a hydrate thereof, or a combination thereof. The caustic soda solution may also be used in the form of an aqueous solution, and at this time, water, or a mixture of water and an organic solvent, such as alcohol that can be uniformly mixed with water, may be used as the solvent.

The above coprecipitation reaction can be performed under an inert atmosphere such as nitrogen or argon. Specifically, it can be manufactured by performing the coprecipitation reaction while injecting N ₂ to prevent oxidation of the metal ion.

Next, a step of preparing a mixture by mixing the above metal hydroxide and lithium raw material is performed.

The above lithium raw material is not particularly limited as long as it is commonly used in the art, but may be, for example, LiCO ₃ , LiOH or LiOH H ₂ O.

In addition, the mixture can be prepared so that the molar ratio of lithium (Li) to the total metal (Me) excluding lithium (Li/Me) is in the range of 1.05 to 1.09. When the molar ratio of lithium (Li) to the total metal (Me) excluding lithium (Li/Me) satisfies the above range, the high-temperature life characteristics are very excellent.

At this stage, a doping raw material may be added as needed, and for example, a raw material including at least one of Zr, Al, B, Y, Mg, Ti, Nb, W, Sc, Si, V, Fe, Mo, Ce, Hf, Ta, La, Sr, Sn, Sb, Zn, Cu, Ge, Mo, Ru, Ir, and Na may be used.

Next, a step of calcining the mixture to obtain a calcined product is performed.

Specifically, the sintering can be performed at a temperature ranging from 700 to 850°C for 5 to 20 hours. If the sintering temperature is too high, the structural stability of the positive electrode active material may deteriorate, resulting in a decrease in reversible capacity, and if the sintering temperature is too low, there is a problem of a decrease in particle uniformity.

In addition, if the firing time is excessively long, there are problems in terms of productivity and economic feasibility, and if the firing time is excessively short, there are problems in which the synthetic reaction does not occur completely or the crystal structure does not develop sufficiently.

Next, a step of mixing the above-mentioned sintered product and a coating raw material including cobalt and then performing a heat treatment to obtain a metal oxide on which a coating layer is formed is performed.

In the step of obtaining the metal oxide on which the coating layer is formed, the heat treatment can be performed only once. For example, the heat treatment can be performed at 550° C. to 700° C. for 3 to 10 hours. When the heat treatment process satisfies the above conditions, the amount of lithium remaining on the surface can be reduced while simultaneously stabilizing the surface structure.

The coating raw material containing the above cobalt may be added so that the cobalt content is 0.012 mol or less, more specifically, 0.001 mol to 0.012 mol, based on the entire metal oxide on which the coating layer is formed. The specific details regarding the cobalt content are the same as those described above, and are therefore omitted here.

The above-described cobalt-containing coating raw material may include at _least one of, for example, Co(OH) ₂ , _CoCl2 , _CoO , _CoF3 , _CoSO4 · _xH2O , _CoSO4 ·7H2O, ( _CH3COO ) _2Co · _4H2O , Co _{( NO3} ) ₂ ·6H2O, _{( CH3CO2) 2Co} , CoCO3· _xH2O , _{Co3 ( PO4} ) ₂ , and combinations thereof.

The average particle diameter (D50) of the coating raw material containing the above cobalt may be in the range of 10 ㎛ to 20 ㎛. When the average particle diameter of the coating raw material satisfies the above range, the coating layer can be formed uniformly.

anode

In another embodiment, a cathode is provided, comprising a current collector, and a cathode active material layer positioned on one surface of the current collector and comprising the cathode active material of the above-described embodiment.

The characteristics of the positive electrode active material constituting the positive electrode active material layer are the same as described above. Therefore, a detailed description of the positive electrode active material will be omitted.

The above-mentioned collector may be, for example, made of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc.

Meanwhile, the positive electrode active material layer may include a binder and a conductive material.

At this time, the binder serves to improve the adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one kind alone or a mixture of two or more kinds thereof may be used, but is not limited thereto. The binder may be included in an amount of 1 to 30 wt% with respect to the total weight of the positive electrode active material layer.

And, the conductive material is used to provide conductivity to the electrode, and in the battery to be formed, as long as it does not cause a chemical change and has electronic conductivity, it can be used without special restrictions. 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 powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like, and one type alone or a mixture of two or more types thereof may be used, but is not limited thereto. The conductive material may typically be included in an amount of 1 to 30 wt% with respect to the total weight of the positive electrode active material layer.

The above positive electrode can be manufactured according to a conventional positive electrode manufacturing method, except that the above positive electrode active material is used.

Specifically, the positive electrode can be manufactured by applying a composition for forming a positive electrode active material layer, including the positive electrode active material described above and optionally a binder, a conductive agent or a solvent, on a positive electrode current collector, and then drying and rolling. At this time, the types and contents of the positive electrode active material, the binder and the conductive agent are as described above.

The solvent may be a solvent generally used in the relevant technical field, and may include dimethylsulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water. One of these may be used alone or a mixture of two or more thereof may be used. The amount of the solvent used is sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, taking into account the coating thickness and manufacturing yield of the slurry, and to have a viscosity that can exhibit excellent thickness uniformity during subsequent coating for manufacturing the positive electrode.

Alternatively, the positive electrode may be manufactured by casting the composition for forming the positive electrode active material layer on a separate support, and then laminating the resulting film on a positive electrode current collector by peeling it off from the support.

Lithium secondary battery

In another embodiment, a lithium secondary battery including the positive electrode is provided.

The lithium secondary battery may specifically include a positive electrode, an anode positioned opposite the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is as described above. In addition, the lithium secondary battery may optionally further include a battery container that accommodates an electrode assembly including the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

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

The above negative electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., an aluminum-cadmium alloy, etc. can be used. In addition, the negative electrode current collector can typically have a thickness of 3 to 500 ㎛, and, like the positive electrode current collector, fine unevenness can be formed on the surface of the current collector to strengthen the bonding strength of the negative electrode active material. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a non-woven fabric.

The negative electrode active material layer may optionally include a binder and a conductive material together with the negative electrode active material. The negative electrode active material layer may be manufactured by, for example, applying a composition for forming a negative electrode active material layer comprising the negative electrode active material, and optionally a binder and a conductive material, onto a negative electrode current collector and drying the composition, or by casting the negative electrode forming composition onto a separate support and then peeling the film from the support and laminating the resulting film onto a negative electrode current collector.

As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium can be used. Specific examples thereof include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, and Al alloy; metallic oxides capable of doping and dedoping lithium, such as SiOβ (0 < β < 2), SnO ₂ , vanadium oxide, and lithium vanadium oxide; or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these can be used. In addition, a metallic lithium thin film can be used as the negative electrode active material. In addition, the carbon material can be both low-crystalline carbon and high-crystalline carbon. Representative examples of low-crystallization carbon include soft carbon and hard carbon, and representative examples of high-crystallization carbon include amorphous, plate-like, flaky, spherical or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase pitch microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch derived cokes.

The above binder and the conductive material may be the same as those described above for the anode.

Next, depending on the type of lithium secondary battery, a separator may exist between the positive and negative electrodes. As such a separator, a multilayer film of two or more layers of polyethylene, polypropylene, polyvinylidene fluoride, or these may be used, and a mixed multilayer film such as a polyethylene/polypropylene two-layer separator, a polyethylene/polypropylene/polyethylene three-layer separator, or a polypropylene/polyethylene/polypropylene three-layer separator may also be used.

In addition, in the lithium secondary battery, examples of the electrolyte include, but are not limited to, 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.

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

As the organic solvent, any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move may be used without particular limitation. Specifically, the organic solvent may include 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 solvents such as benzene and fluorobenzene; Examples of solvents that can be used include carbonate solvents, such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents, such as ethyl alcohol and isopropyl alcohol; nitriles, such as R-CN (wherein R represents a C2 to C20 linear, branched, or cyclic hydrocarbon group, which may include a double-bonded aromatic ring or an ether bond); amides, such as dimethylformamide; dioxolanes, such as 1,3-dioxolane; and sulfolanes. Among these, a carbonate solvent is preferable, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant capable of improving the charge/discharge performance of the battery and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate, etc.) is more preferable. In this case, the cyclic carbonate and the chain carbonate can be mixed and used in a volume ratio of about 1:1 to about 1:9 to exhibit excellent electrolyte performance.

The above lithium salt can be used without any particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt may be LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAl0 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 . It is preferable to use the concentration of the lithium salt within the range of 0.1 to 2.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, so that it can exhibit excellent electrolyte performance, and lithium ions can move effectively.

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

Hereinafter, embodiments of the present invention will be described in detail. However, these are presented as examples, and the present invention is not limited thereby, and the present invention is defined only by the scope of the claims described below.

Example 1

(Mixed) A precursor having a composition of Ni _0.70 Mn _0.30 (OH) ₂ was prepared through a conventional co-precipitation process. Next, the precursor and LiOH H ₂ O were placed in a mixer so that the molar ratio of lithium to the total metal in the precursor (Li/Me) was 1.07, and then mechanically mixed to prepare a mixture.

(After calcination), the mixture was heated at 5°C/min in an oxygen (O ₂ ) atmosphere, calcined at a temperature of 850°C for 6 hours, and then naturally cooled to room temperature. The composition of the obtained lithium metal oxide was Li _1.07 Ni _0.70 Mn _0.30 O ₂ .

(Coating) Co(OH) ₂ was dry-mixed as a coating material to the lithium metal oxide obtained above. At this time, the content of Co was mixed so that it was 0.01 mol based on 100 mol of the final positive electrode active material. Thereafter, a positive electrode active material having a coating layer formed was manufactured by heat-treating at 680°C in an _O2 atmosphere. Specifically, the heat treatment was performed by heating for 3 hours and maintaining for 6 hours.

Example 2

In the coating process, a cathode active material was manufactured in the same manner as in Example 1, except that the coating raw material was added so that the Co content was 0.005 mol based on 100 mol of the total final cathode active material.

Example 3

In the coating process, a cathode active material was manufactured in the same manner as in Example 1, except that the coating raw material was added so that the Co content was 0.001 mol based on 100 mol of the total final cathode active material.

Comparative Example 1

A positive electrode active material was manufactured in the same manner as in Example 1, except that the coating process was not performed.

Comparative Example 2

In the coating process, Co(OH) ₂ and _NH4VO3 were dry mixed as coating raw materials to the obtained lithium metal oxide. At this time, the positive electrode active material was manufactured in the same manner as Example 1, except that the Co content was mixed to be 500 ppm and the V content was mixed to be 0.1 mol based on 100 mol _of the total final positive electrode active material.

Comparative Example 3

A cathode active material was manufactured in the same manner as in Example 1, except that a second heat treatment was performed for 3 hours under the same conditions after the first heat treatment at 680°C in the coating process.

Comparative Example 4

A cathode active material was manufactured in the same manner as in Example 1, except that a precursor having the composition of Ni _0.69 Co _0.02 Mn _0.29 (OH) ₂ was used in the mixing process.

Therefore, the composition of the lithium metal oxide obtained in the sintering process was Li _1.07 Ni _0.69 Co _0.02 Mn _0.29 O ₂ .

Comparative Example 5

In the coating process, a cathode active material was manufactured in the same manner as in Example 1, except that the coating raw material was added so that the Co content was 0.05 mol based on 100 mol of the total final cathode active material.

Comparative Example 6

A cathode active material was manufactured in the same manner as in Example 1, except that a precursor having a composition of Ni _0.80 Mn _0.20 (OH) ₂ was used in the mixing process.

Therefore, the composition of the lithium metal oxide obtained in the sintering process was Li _1.07 Ni _0.80 Mn _0.20 O ₂ .

Comparative Example 7

In the coating process, a cathode active material was manufactured in the same manner as in Comparative Example 4, except that the coating raw material was added so that the Co content was 0.005 mol based on 100 mol of the total final cathode active material.

Therefore, the composition of the lithium metal oxide obtained in the sintering process was Li _1.07 Ni _0.69 Co _0.02 Mn _0.29 O ₂ .

Experimental Example 1: Coin cell fabrication and electrochemical characterization

Using the positive electrode active materials manufactured in the examples and comparative examples, a CR2032 coin cell was manufactured by the following method, and the electrochemical characteristics were evaluated, which are shown in Table 1 and Figures 1 to 3 below.

(1) Coin cell manufacturing

The slurry for manufacturing the electrode plate was mixed with the above-mentioned positive electrode active material: conductive material (carbon black, denka black): binder (PVDF, KF1100) = 96.5 : 1.5 : 2 wt%, and the viscosity was adjusted so that the solid content was about 30% by adding NMP (N-Methyl-2-pyrrolidone). The manufactured slurry was coated on a 20 ㎛ thick Al foil using a doctor blade, and then dry rolled. The electrode loading was 15.4 mg/cm ² , and the rolling density (25°C, 20 kN) was 3.6 g/cm ³ .

A 2032 coin-type half-cell was manufactured by a conventional method using the above positive electrode, lithium metal negative electrode (200 μm thick, Honzo metal), electrolyte, and polypropylene polyethylene separator. The electrolyte was manufactured by dissolving 1 M LiPF ₆ in a mixed solvent of ethylene carbonate, dimethyl carbonate, and diethyl carbonate (mixing ratio EC:DMC:DEC=1:2:1 by volume) to prepare a mixed solution, to which 3 wt% of vinylene carbonate (VC) was added and used.

(2) Initial capacity and initial efficiency evaluation

(1) The Coen cell manufactured in was aged at 25°C for 12 hours, and then a charge/discharge test was performed at 25°C. For the initial capacity evaluation, 200 mAh/g was used as the reference capacity, and the battery was charged to 4.45 V at a constant current of 0.1 C, then switched to a constant voltage and charged until the end current reached 0.05 C. After a rest time of 10 minutes after charging, the battery was discharged until 2.5 V was reached at a constant current of 0.1 C, with 200 mAh/g as the reference capacity.

(3) Output characteristics evaluation

A total of 4 cycles were performed at 25℃, and the C-rate was varied during the constant current discharge process within a voltage range of 2.5 to 4.45 V with a reference capacity of 200 mAh/g.

The output characteristics were evaluated by comparing the initial discharge capacity at 0.5C charge/0.1C discharge in the first cycle and the initial discharge capacity at 0.5C charge/2C discharge in the fourth cycle.

(4) Resistance characteristic evaluation

After fabricating the coin cell, it was charged to 4.45 V at a constant current of 0.5 C at 25°C, then switched to constant voltage and charged until the end current reached 0.05 C. After a rest time of 1 minute after charging, it was discharged until it reached 2.5 V at a constant current of 1.0 C. At this time, the impedance was measured to evaluate the resistance characteristics.

(5) Evaluation of high temperature life characteristics

After fabricating the coin cell, it was charged to 4.45 V at a constant current of 0.5 C at 45°C, then switched to constant voltage and charged until the end current reached 0.05 C. After a rest time of 10 minutes after charging, it was discharged until the voltage reached 2.5 V at a constant current of 1.0 C. Under these charge/discharge cycle conditions, 30 charge/discharge cycles were performed, and the capacity retention rate of the 30th cycle compared to the first cycle was calculated.

Meanwhile, FIG. 1 shows the results of measuring the initial capacity of the positive electrode active materials manufactured according to Examples 1 to 3 and Comparative Examples 1 to 7, and FIG. 2 shows the results of measuring the high temperature life characteristics of the positive electrode active materials manufactured according to Examples 1 to 3 and Comparative Examples 1 to 2.

division Comparative Example 1 Example 3 Example 2 Example 1 Comparative Example 2 Comparative Example 3 Comparative example
4 Comparative example
5 Comparative example
6 Comparative example
7 volume beginning
Charging capacity
(mAh/g) 217.1 225.3 225.1 226.7 216.7 221.6 220.0 221.0 229.8 231.7 beginning
Discharge capacity
(mAh/g) 192.7 199.5 200.3 202 192.7 199.1 197.6 196.6 209.2 200.9 Initial efficiency
(%) 88.8 88.6 89 89.2 88.9 89.9 89.8 88.8 91.0 86.7 Output characteristics 0.1C discharge capacity (mAh/g) 193.3 201.5 201.9 203.8 189.3 199.4 198.7 192.7 210.1 201.2 2C discharge capacity (mAh/g) 162.7 169 169.3 171.3 161 167.3 166.5 168.4 178.2 167.8 Rate characteristics
(2C/0.1C, %) 84.2 83.9 83.8 84.1 82.6 83.9 83.8 87.4 84.8 83.4 Initial resistance
(0.2C, 1min, Ω) 41.2 38.7 35.7 34.9 32.5 32.2 32.1 37.0 32.1 30.1 Capacity retention rate, 30 cycles 96.2 98.6 98.8 98.8 97. 97.7 94.6 97.4 77.9 96.5

Referring to Table 1 and FIGS. 1 to 2, it can be seen that the positive electrode active materials of Examples 1 to 3, in which the nickel content and the value of Formula 1 satisfy the range of the present embodiment, have excellent initial charge/discharge capacity and initial efficiency, and thus the energy density and efficiency are improved. In addition, since the output characteristics are excellent, the performance is maintained even under high output conditions, so that rapid charge/discharge is possible, and since the initial resistance value is low, the resistance loss can be minimized, which has an advantageous effect. In addition, since the capacity retention rate is excellent, it can be seen that the life characteristics are also improved. That is, it can be confirmed that the positive electrode active materials of Examples 1 to 3 comprehensively implement capacity, rate characteristics, resistance characteristics, and life characteristics very well.

In contrast, it can be seen that the cathode active material of Comparative Example 1, which did not form a coating layer, has both reduced capacity and output characteristics, and also has a high initial resistance. It can be seen that the cathode active material of Comparative Example 2, in which the cobalt content is outside the range of the examples, and Comparative Example 3, in which heat treatment was performed twice, has deteriorated capacity and output characteristics.

In addition, in the case of Comparative Example 4 containing cobalt in the metal oxide, it can be confirmed that although the initial resistance is good, the life characteristics are significantly reduced because cobalt is contained in the base material.

In addition, it can be seen that the cathode active materials manufactured according to Comparative Example 5 in which the cobalt content is outside the range of the present example, Comparative Example 6 in which the nickel content is outside the range of the present example, and Comparative Example 7 in which a coating layer is formed on a composition including cobalt also have significantly reduced life characteristics.

Experimental Example 2: XRD Measurement Results

For the positive electrode active materials manufactured according to Examples 1 to 3 and Comparative Examples 1 to 7, X-ray diffraction measurements were performed using CuKα rays.

Specifically, the intensities (peak areas) of the (003) plane, (104) plane, and (108) plane were measured at a scan speed (°/s) of 0.328 using XRD equipment (X’pert3 powder diffraction from Panalytical). From these results, I(003)/I(104) was obtained.

In addition, the lattice constants and crystal sizes of each material were measured using XRD Rietveld analysis. The results are shown in Table 2 and Fig. 3 below.

a (Å)
(A) c (Å)
(B) Peak intensity
(I _[003] /I _[104] ) Crystal size
(nm)
(C) Equation 1
(A*C)/B Example 1 2.884 14.271 1.1836 127.3 25.7 Example 2 2.882 14.270 1.1364 120.5 24.3 Example 3 2.884 14.275 1.1036 118.1 23.9 Comparative Example 1 2.878 14.269 1.0886 92.1 18.6 Comparative Example 2 2.871 14.286 1.0983 100.2 20.1 Comparative Example 3 2.874 14.261 1.1015 93.4 18.8 Comparative Example 4 2.866 14.292 1.1153 104.8 21.0 Comparative Example 5 2.888 14.282 1.2647 131.7 26.6 Comparative Example 6 2.982 14.875 1.1922 152.1 30.5 Comparative Example 7 2.875 14.287 1.0365 106.8 21.5

Referring to FIG. 3, patterns in which the (003) peak, which is a typical layered structure, shows a higher intensity than the (104) peak were confirmed in the positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 to 7.

In addition, the XRD patterns of Examples 1 to 3 show relatively higher intensity, which confirms that the crystallinity of the positive electrode active materials of the Examples is excellent and the structure is well formed.

In contrast, in the case of Comparative Example 6, it can be seen that the peak of the (003) plane is relatively broadened, indicating that part of the layered structure has collapsed.

Meanwhile, referring to Table 2, it can be seen that Examples 1 to 3 satisfy the range of the present example in terms of the value of Equation 1. In addition, it can be seen that the layered structure is well formed when considering the I _[003] /I _[104] peak intensity ratio.

In contrast, in the cases of Comparative Examples 1 to 7, the value of Equation 1 is outside the range of the present example. In addition, in the case of Comparative Examples 3 and 4, the I _[003] /I _[104] peak intensity ratio tends to be somewhat lower, which indicates that the layered structure is incomplete or that some of the transition metals are mixed with the lithium layer, resulting in a structural defect.

In addition, it can be confirmed that Examples 1 to 3 have grain sizes that satisfy the range of the present Example, but Comparative Examples 1 to 7 have grain sizes lower than the range of the present Example. Therefore, it is predicted that the interfacial resistance between particles in the Comparative Examples increases, resulting in a decrease in the lifespan or resistance characteristics.

In the case of the examples, since the coating layer is included, it is presumed that the coating raw material penetrates not only the surface of the lithium metal oxide but also the structure, thereby affecting the crystal size.

Although the preferred embodiments of the present invention have been described above, the present invention is not limited thereto, and various modifications may be made within the scope of the patent claims, the detailed description of the invention, and the attached drawings, which also fall within the scope of the present invention.

Accordingly, the substantial scope of the present invention is defined by the appended claims and their equivalents.

Claims (19)

Metal oxides containing nickel and manganese; and A coating layer located on the surface of the above metal oxide and containing cobalt; The content of nickel in the above metal oxide is 0.75 mol or less based on 1 mol of the total metal excluding lithium, A cathode active material for a lithium secondary battery, satisfying the following equation 1. [Formula 1] 22.0 ≤(A*C)/B ≤ 26.5 (In the above equation 1, A and B are the a-axis and c-axis constants measured by XRD analysis for the positive electrode active material, respectively. C refers to the crystal grain size measured by XRD analysis for the above positive electrode active material) In the first paragraph, A cathode active material for a lithium secondary battery, wherein the a-axis constant measured by XRD analysis for the cathode active material is in the range of 2.880Å to 2.890Å. In the first paragraph, A cathode active material for a lithium secondary battery, wherein the c-axis constant measured by XRD analysis for the cathode active material is in the range of 14.262Å to 14.281Å. In the first paragraph, A cathode active material for a lithium secondary battery, wherein the crystal grain size of the cathode active material is in the range of 94.0 nm to 131.0 nm. In the first paragraph, The above cathode active material is a cathode active material for a lithium secondary battery, wherein the ratio of the peak value I ₀₀₃ corresponding to the (003) plane and the peak value I ₁₀₄ corresponding to the (104) plane, I ₀₀₃ /I ₁₀₄ , is in the range of 1.1020 to 1.2640 as measured by XRD analysis. In the first paragraph, A positive electrode active material for a lithium secondary battery, wherein the content of cobalt is 0.012 mol or less based on the entire metal oxide on which the coating layer is formed. In the first paragraph, A positive electrode active material for a lithium secondary battery, wherein the content of cobalt is in the range of 0.001 mol to 0.012 mol based on the entire metal oxide on which the coating layer is formed. In the first paragraph, The above metal oxide is a positive electrode active material for a lithium secondary battery that does not contain cobalt. In the first paragraph, A cathode active material for a lithium secondary battery, wherein the content of nickel in the metal oxide is in the range of 0.65 to 0.75 based on 1 mole of the total metal excluding lithium. In the first paragraph, A cathode active material for a lithium secondary battery, wherein the manganese content in the metal oxide is in the range of 0.25 to 0.35 based on 1 mole of the total metal excluding lithium. In the first paragraph, The above metal oxide is a cathode active material for a lithium secondary battery, represented by the following chemical formula 1: [Chemical Formula 1] Li _a [Ni _x Mn _y M _z ]O ₂ In the chemical formula 1, 0.8≤a≤1.2, 0.67≤x≤0.73, 0.25≤y≤0.38, 0≤z≤0.2, x+y+z=1, and M is Zr, Al, B, Y, Mg, Ti, Nb, W, Sc, Si, V, Fe, Mo, Ce, Hf, Ta, La, Sr, Sn, Sb, Zn, Cu, Ge, Mo, Ru, Ir, Na or a combination thereof. In the first paragraph, A cathode active material for a lithium secondary battery, wherein the average particle diameter (D50) of the cathode active material is in the range of 10 to 15 μm. A step of preparing a metal hydroxide containing nickel and manganese; A step of preparing a mixture by mixing the above metal hydroxide and lithium raw material; A step of calcining the above mixture to obtain a calcined product; and A step of mixing the above-mentioned sintered product and a coating raw material including cobalt and then performing a heat treatment to obtain a metal oxide on which a coating layer is formed; A method for manufacturing a cathode active material for a lithium secondary battery, wherein the metal oxide on which the coating layer is formed satisfies the following formula 1. [Formula 1] 22.0 ≤(A*C)/B ≤ 26.5 (In the above equation 1, A and B are the a-axis and c-axis constants measured by XRD analysis for the positive electrode active material, respectively. C refers to the crystal grain size measured by XRD analysis for the above positive electrode active material) In Article 13, The coating raw material containing the above cobalt has a cobalt content of 0.012 mol or less based on the entire metal oxide on which the coating layer is formed. A method for manufacturing a cathode active material for a lithium secondary battery, which is as much as possible injected. In Article 13, A method for producing a cathode active material for a lithium secondary battery, wherein the coating raw material containing the cobalt comprises at _least one _of Co(OH) ₂ , _CoCl2 , CoO, _CoF3 , _CoSO4 · _xH2O , CoSO4· _7H2O , _{( CH3COO} ) _2Co · _4H2O , Co _{( NO3} ) _{2 · 6H2O} , ( _{CH3CO2 )} 2Co, CoCO3·xH2O, _Co3 ( _PO4 ) ₂ , and combinations thereof. In Article 13, A method for manufacturing a positive electrode active material for a lithium secondary battery, wherein the average particle diameter (D50) of the coating raw material containing the above cobalt is in the range of 10 ㎛ to 12 ㎛. In Article 13, A method for manufacturing a cathode active material for a lithium secondary battery, wherein the above heat treatment process is performed only once. A cathode for a lithium secondary battery comprising a cathode active material according to claim 1. A lithium secondary battery comprising a positive electrode for a lithium secondary battery according to claim 18.

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