Positive electrode active material, its manufacturing method, positive electrode and lithium secondary battery including the same

Abstract

The present invention relates to a positive electrode active material comprising a lithium nickel-based oxide in the form of a single particle, a pseudo-single particle, or a mixture thereof, in which the number of nickel ions having an oxidation number of +3 or more on the surface of the lithium nickel-based oxide is greater than the number of nickel ions having an oxidation number of less than +3, and a method for producing the same.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0121308 filed on September 10, 2021, and Korean Patent Application No. 10-2021-0186563 filed on December 23, 2021, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a positive electrode active material, a manufacturing method thereof, and a positive electrode and a lithium secondary battery including the positive electrode active material. More specifically, the present invention relates to a positive electrode active material in the form of at least one of single particles and pseudo-single particles, which has improved resistance characteristics, a manufacturing method thereof, and a positive electrode and a lithium secondary battery including the positive electrode active material. [Background technology]

[0003] A lithium secondary battery generally comprises a positive electrode, a negative electrode, a separator, and an electrolyte, and the positive electrode and the negative electrode contain active materials capable of intercalating and deintercalating lithium ions.

[0004] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2 or LiMnO4, etc.), and lithium iron phosphate (LiFePO4) have been used as positive electrode active materials for lithium secondary batteries. Among these, lithium cobalt oxide has the advantages of high operating voltage and excellent capacity characteristics, but the high cost and unstable supply of cobalt, the raw material, make its commercial application in large-capacity batteries difficult. Lithium nickel oxide has poor structural stability and is difficult to achieve sufficient life characteristics. On the other hand, lithium manganese oxide has excellent stability but poor capacity characteristics. Therefore, to address the issues of lithium transition metal oxides containing only Ni, Co, or Mn, lithium composite transition metal oxides containing two or more transition metals have been developed. Among these, lithium nickel cobalt manganese oxide containing Ni, Co, and Mn is widely used in the field of electric vehicle batteries.

[0005] Conventional lithium nickel cobalt manganese oxides generally have a spherical secondary particle form, consisting of an agglomeration of tens to hundreds of primary particles. However, lithium nickel cobalt manganese oxides in this secondary particle form, consisting of an agglomeration of many primary particles, are prone to particle cracking, in which the primary particles break off during the rolling process during positive electrode production, and also suffer from internal cracks during charge and discharge. When particle cracking or cracking occurs in the positive electrode active material, the contact area with the electrolyte increases, which increases gas generation and degradation of the active material due to side reactions with the electrolyte, resulting in reduced lifespan.

[0006] In addition, with the recent increasing demand for high-power, high-capacity batteries, such as batteries for electric vehicles, the nickel content in the positive electrode active material has been gradually increasing. Although the initial capacity characteristics are improved when the nickel content in the positive electrode active material is increased, repeated charge and discharge can cause structural collapse of the positive electrode active material, which increases the rate of deterioration of the positive electrode active material, resulting in reduced lifespan characteristics and reduced battery stability.

[0007] To address these issues, a technique has been proposed for producing a single-particle cathode active material rather than a secondary particle by increasing the calcination temperature during the production of lithium nickel cobalt manganese oxide. Single-particle cathode active materials have a smaller contact area with the electrolyte than conventional secondary-particle cathode active materials, resulting in fewer side reactions with the electrolyte and superior particle strength, which reduces particle cracking during electrode fabrication. Therefore, the use of single-particle cathode active materials offers the advantages of less gas generation and excellent lifespan characteristics.

[0008] However, conventional single-particle cathode active materials have poor lithium mobility due to the limited number of interfaces between primary particles that serve as paths for lithium ions to travel within the particles. Furthermore, they are manufactured at relatively high sintering temperatures, resulting in the presence of excessive amounts of lithium by-products on the surface. When excessive amounts of lithium by-products are present on the surface of the cathode active material, gas generation increases during high-temperature storage due to side reactions between the lithium by-products and the electrolyte. While the amount of gas generation can be reduced by removing the lithium by-products through water washing, the surface structure of the cathode active material can be damaged during water washing, resulting in increased resistance. Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been made to solve the above-mentioned problems, and aims to provide a method for manufacturing a cathode active material having at least one form of single particle and quasi-single particle, which can suppress an increase in resistance and effectively remove lithium by-products, and a cathode active material manufactured thereby.

[0010] Another object of the present invention is to provide a positive electrode containing the positive electrode active material, and a lithium secondary battery containing the positive electrode, which generates little gas during high-temperature storage and has excellent resistance characteristics at a low SOC. [Means for solving the problem]

[0011] According to one embodiment, the present invention provides a method for producing a cathode active material, the method comprising the steps of: (A) calcining a mixture of a transition metal precursor and a lithium source material to produce a lithium nickel-based oxide having at least one form of a single particle or a quasi-single particle; (B) milling the lithium nickel-based oxide; and (C) calcining the lithium nickel-based oxide at a temperature of 600°C to 900°C, wherein the single particle is composed of one nodule and the quasi-single particle is a composite of 30 or less nodules.

[0012] According to another embodiment, the present invention provides a positive electrode active material comprising a lithium nickel-based oxide having at least one of a monoparticle and a quasi-monoparticle form, wherein the monoparticle is composed of one nodule, and the quasi-monoparticle is a composite of 30 or less nodules, and the number of nickel ions with an oxidation number of +3 or more on the surface of the lithium nickel-based oxide is greater than the number of nickel ions with an oxidation number of less than +3.

[0013] According to yet another embodiment, the present invention provides a cathode including a cathode active material layer including the cathode active material according to the present invention, and a lithium secondary battery including the cathode. [Effects of the Invention]

[0014] According to the method for producing a cathode active material of the present invention, which includes the steps of (A) calcining a mixture of a transition metal precursor and a lithium source material to produce a lithium nickel-based oxide in the form of at least one of single particles and pseudo-single particles, (B) milling the lithium nickel-based oxide, and then (C) calcining the lithium nickel-based oxide at a temperature of 600 to 900°C, residual lithium can be effectively removed without a water washing process. In the case of high-nickel cathode materials with a high nickel content of 80 mol% or more, excessive lithium by-products are generated during the manufacturing process. When such lithium by-products are present, they react with the electrolyte to generate gas. Therefore, it has been common practice to remove the lithium by-products from high-nickel cathode materials using a water washing process before use. However, when the water washing process is performed, the Ni oxidation state on the surface of the cathode active material particles becomes electrically inactive, resulting in the Ni oxidation state becoming inactive. +2 This causes a problem of increased resistance. In the case of a positive electrode active material in the form of secondary particles, the resistance is not high, so a slight increase in resistance during the water washing process is not a major issue. However, in the case of a positive electrode active material in the form of single particles and / or quasi-single particles, the resistance of the particles themselves is high, so if the resistance increases further during the water washing process, the output characteristics will be significantly reduced. In particular, the resistance (hereinafter referred to as discharge end resistance) in the low SOC region (e.g., the region below 10% SOC) increases significantly, making it difficult to apply to electric vehicle batteries that require high output in the low SOC region. However, the method of the present invention allows for effective removal of lithium by-products without a water washing process that changes the Ni oxidation state.

[0015] The cathode active material of the present invention prepared by the above method includes a lithium nickel-based oxide in the form of a single particle and / or a quasi-single particle, and on the surface of the lithium nickel-based oxide, the number of nickel ions with an oxidation number of +3 or more is greater than the number of nickel ions with an oxidation number of less than +3. As such, the cathode active material of the present invention has low resistance characteristics due to the large number of electrically active nickel ions with an oxidation number of +3 or more on the surface of the lithium nickel-based oxide.

[0016] In addition, the cathode active material according to the present invention includes a lithium nickel-based oxide in the form of a single particle and / or a quasi-single particle having high particle strength, which reduces particle cracking during electrode fabrication and charge / discharge, reduces the amount of residual lithium, and suppresses side reactions with the electrolyte. As a result, when applied to a secondary battery, the amount of gas generation is reduced and high-temperature life characteristics are excellent. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a photograph showing the Ni oxidation number distribution on the surface of the positive electrode active material produced in Example 1, measured by TEM-EELS. [Figure 2] 1 is a photograph showing the Ni oxidation number distribution on the surface of the positive electrode active material produced in Comparative Example 2, measured by TEM-EELS. [Figure 3] 1 is a scanning electron microscope image of the positive electrode active material prepared in Example 1. [Figure 4] 1 is a scanning electron microscope image of the positive electrode active material prepared according to Comparative Example 3. [Figure 5] 1 is a scanning electron microscope image of the positive electrode active material prepared according to Comparative Example 4. [Figure 6] 1 is a graph comparing the resistance characteristics of batteries manufactured using the positive electrode active materials manufactured in Examples 1 and 2 and Comparative Examples 1 to 4 depending on the state of charge (SOC). [Figure 7] 1 is a graph comparing changes in cell volume after high-temperature storage of batteries manufactured using the positive electrode active materials manufactured in Examples 1 and 2 and Comparative Examples 1 to 4. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention will be described in more detail below.

[0019] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as having meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0020] In the present invention, a "single particle" refers to a particle consisting of one single nodule. In the present invention, a "quasi-single particle" refers to a particle that is a complex formed by 30 or less nodules.

[0021] In the present invention, the term "nodule" refers to a particle unit body constituting a single particle or a quasi-single particle, and the nodule may be a single crystal lacking a crystalline grain boundary, or a polycrystal lacking any apparent grain boundary when observed at a magnification of 5,000 to 20,000 times using a scanning electron microscope (SEM).

[0022] In the present invention, the term "secondary particles" refers to particles formed by agglomeration of several tens to several hundreds of primary particles. More specifically, secondary particles are agglomerations of 40 or more primary particles.

[0023] The term "particle" as used in the present invention can include any one or all of single particles, quasi-single particles, primary particles, nodules, and secondary particles.

[0024] In the present invention, the "average particle size D 50 " means the particle size at 50% of the volume cumulative particle size distribution of the positive electrode active material powder. When the lithium composite transition metal oxide is a secondary particle, the average particle size means the average particle size of the secondary particles, and when the lithium composite transition metal oxide is a mixture of single particles and quasi-single particles, the average particle size means the average particle size of the particles in this combination. The average particle size D 50can be measured using a laser diffraction method. For example, the positive electrode active material powder is dispersed in a dispersion medium, and then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000). Ultrasonic waves of about 28 kHz are irradiated at an output of 60 W, and a volume cumulative particle size distribution graph is obtained. The particle size can be measured by determining the particle size corresponding to 50% of the volume cumulative amount.

[0025] In the present invention, the "average crystal grain size" was measured by analyzing the XRD data obtained by X-ray diffraction analysis of the positive electrode active material powder using Rietveld refinement. Here, the X-ray diffraction analysis was performed using a Bruker D8 Endeavor (light source: Cu-Kα, λ=1.54Å) equipped with a LynxEye XE-T-position sensitive detector. The sample was placed in the groove of a general powder holder, and a slide glass was used to even out the sample surface and fill the holder so that the sample height matched the edge of the holder. The X-ray diffraction analysis was performed under conditions of FDS 0.5°, 2θ = 15° to 90°, step size = 0.02°, and total scan time = approximately 20 minutes. Rietveld refinement was performed on the measured data, taking into account the charge at each site (+3 for metals at the transition metal site, +2 for Ni at the Li site) and cation mixing. Specifically, instrumental broadening during grain size analysis was accounted for using the Fundamental Parameter Approach (FPA) implemented in the Bruker TOPAS program, and peaks across the entire measurement range were used for fitting. Peak shape was fitted using only the Lorenzian contribution as the First Principle (FP) of the peak types available in TOPAS, and strain was not considered.

[0026] In the present invention, electron energy loss spectroscopy (EELS) measurements were performed as follows: The cathode active material to be measured was milled to a thickness of 80 nm using a focused ion beam (FIB) device, and a spectrum was obtained for 30 minutes under an accelerating voltage of 200 kV.

[0027] Method for producing positive electrode active material First, a method for producing a positive electrode active material according to the present invention will be described.

[0028] The method for manufacturing a cathode active material according to the present invention includes the steps of (A) calcining a mixture of a transition metal precursor and a lithium source material to produce a lithium nickel-based oxide having at least one of a single particle and a quasi-single particle form, (B) milling the lithium nickel-based oxide, and (C) calcining the lithium nickel-based oxide at a temperature of 600° C. to 900° C. Here, the single particle is composed of one nodule, and the quasi-single particle is a composite of 30 or less nodules.

[0029] (A) Lithium nickel oxide production steps First, a transition metal precursor and a lithium source material are mixed and calcined to prepare a lithium nickel-based oxide in the form of at least one of single particles and semi-single particles.

[0030] Here, the transition metal precursor may be a commercially available precursor such as nickel-cobalt-manganese hydroxide, or may be prepared by a precursor preparation method well known in the art.

[0031] Preferably, the transition metal precursor used in the present invention is a transition metal hydroxide containing nickel and cobalt, with the Ni content being 80 mol% or more of the total transition metals, and more preferably, nickel-cobalt-manganese hydroxide with the Ni content being 80 mol% or more. When the nickel content in the transition metal precursor satisfies this range, high capacity characteristics can be achieved.

[0032] Specifically, the transition metal precursor may have a composition represented by the following [Chemical Formula 2].

[0033] [Chemical formula 2] Ni x Co y M 1 z M 2 w (OH)2

[0034] In the above chemical formula 2, M 1 is Mn, Al or a combination thereof, preferably Mn or a combination of Mn and Al.

[0035] Said M 2 is one or more elements selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta and Nb, preferably one or more elements selected from the group consisting of Zr, Y, Mg and Ti, and more preferably Zr, Y or a combination thereof.

[0036] The x represents the molar ratio of nickel to all metals in the precursor, and may be 0.8≦x<1, 0.82≦x<1, or 0.83≦x<1. When the molar ratio of nickel satisfies this range, high energy density and high capacity can be achieved.

[0037] The y represents the cobalt molar ratio among all the metals in the precursor, and can be 0 < y < 0.2, 0 < y < 0.18, or 0.01 ≦ y ≦ 0.17. When the molar ratio of cobalt satisfies the above range, good resistance characteristics and output characteristics can be realized.

[0038] The z represents the molar ratio of the M element among all the metals in the precursor, and can be 0 < z < 0.2, 0 < z < 0.18, or 0.01 ≦ z ≦ 0.17. 1 When the molar ratio of the M element satisfies the above range, the cathode active material exhibits excellent structural stability. 1 The w represents the molar ratio of the M element among all the metals in the precursor, and can be 0 ≦ w ≦ 0.1 or 0 ≦ w ≦ 0.05.

[0039] For example, the transition metal precursor can be produced by introducing a transition metal aqueous solution, an ammonium cation complexing agent, and a basic compound into a reactor and performing a coprecipitation reaction while stirring. 2 The transition metal aqueous solution can be produced by dissolving a transition metal-containing raw material substance in a solvent such as water. For example, a nickel-containing raw material substance and a cobalt-containing raw material substance can be dissolved in water for production. Also, if necessary, the transition metal aqueous solution can further contain an M 1 raw material substance and / or an M 2 metal-containing raw material substance.

[0041] On the other hand, the transition metal-containing raw material substance can be a transition metal acetate, carbonate, nitrate, sulfate, halide, sulfide, oxide, etc. 1 Specifically, the nickel-containing raw material substance can be, for example, NiO, NiCO3·2Ni(OH)2·4H2O, NiC2O2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, nickel halide, or a combination thereof. 2

[0042]

[0043]

[0044]

[0044] The cobalt-containing source material can be, for example, CoSO4, Co(OCOCH3)2·4H2O, Co(NO3)2·6H2O, CoSO4·7H2O, or a combination thereof.

[0045] Said M 1 The containing source material may be a manganese-containing source material and / or an aluminum-containing source material. The manganese-containing source material may be, for example, Mn2O3, MnO2, Mn3O4, MnCO3, Mn(NO3)2, MnSO4·H2O, manganese acetate, manganese halide, or a combination thereof. The aluminum-containing source material may be, for example, Al2O3, Al(OH)3, Al(NO3)3, Al2(SO4)3, (HO)2AlCH3CO2, HOAl(CH3CO2)2, Al(CH3CO2)3 aluminum halide, or a combination thereof. However, in the case of Al, it may be added together with the lithium source material in the calcination step described below without being added to the transition metal aqueous solution.

[0046] M 2 The raw materials contained are M 2 It can be a metal acetate, carbonate, nitrate, sulfate, halide, sulfide or oxide, or the like.

[0047] The amount of each transition metal-containing raw material to be added may be determined in consideration of the molar ratio of the transition metal in the final cathode active material to be produced.

[0048] The ammonium cation complexing agent may include at least one compound selected from the group consisting of NHOH, (NH)SO, NHNO, NHCl, CHCOONH, and (NH)CO, and may be added to the reactor in the form of a solution in which the compound is dissolved in a solvent, which may be water or a mixture of water and an organic solvent (e.g., alcohol) that is uniformly miscible with water.

[0049] The basic compound may be at least one compound selected from the group consisting of NaOH, KOH, and Ca(OH)2, and may be added to the reactor in the form of a solution in which the compound is dissolved in a solvent, which may be water or a mixture of water and an organic solvent (e.g., alcohol) that is uniformly miscible with water.

[0050] As described above, when the aqueous transition metal solution, the ammonium cation complexing agent, and the basic compound are charged into a reactor and stirred, the transition metal in the aqueous transition metal solution is coprecipitated to produce precursor particles in the form of transition metal hydroxide.

[0051] Here, the aqueous transition metal solution, the ammonium cation complex-forming agent, and the basic compound are added in amounts such that the pH of the reaction solution falls within a desired range.

[0052] Once the precursor particles are formed by the above method, the particles are separated from the reaction solution to obtain the transition metal precursor. For example, the reaction solution is filtered to separate the transition metal precursor from the reaction solution, and the separated transition metal precursor is then washed with water and dried to obtain the transition metal precursor. Here, steps such as pulverization and / or classification can be performed as necessary.

[0053] Next, the transition metal precursor and the lithium source material are mixed and then subjected to primary firing to prepare a lithium nickel-based oxide in the form of a single particle or a similar single particle. 1 Contains raw materials and / or M 2 The raw materials can be mixed together and fired, 1 Contains raw materials and / or M 2 Specific examples of the raw material contained are as described above.

[0054] The lithium source material may be a lithium-containing sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide, or oxyhydroxide, such as Li2CO3, LiNO3, LiNO2, LiOH, LiOH·H2O, LiH, LiF, LiCl, LiBr, LiI, CH3COOLi, Li2O, Li2SO4, CH3COOLi, Li3C6H5O7, or a mixture thereof.

[0055] Meanwhile, the lithium source material and the cathode active material precursor may be mixed so that the molar ratio of Li:total metals in the precursor is 1:1 to 1.1:1, preferably 1.02:1 to 1.05:1. When the mixing ratio of the lithium source material and the metals in the cathode active material precursor satisfies this range, the layered crystalline structure of the cathode active material is well developed, allowing the production of a cathode active material with excellent capacity characteristics and structural stability.

[0056] Meanwhile, the calcination is performed at a temperature that allows the formation of monoparticles and / or pseudo-monoparticles. To form monoparticles and / or pseudo-monoparticles, calcination must be performed at a higher temperature than that used to prepare conventional lithium nickel-based oxides in the form of secondary particles. For example, when the precursor composition is the same, calcination must be performed at a temperature that is 30°C to 100°C higher than that used to prepare conventional lithium nickel-based oxides in the form of secondary particles. The calcination temperature for forming monoparticles and / or pseudo-monoparticles can vary depending on the metal composition of the precursor. For example, when forming a high-nickel (Ni) lithium nickel-based oxide having a nickel (Ni) content of 80 mol% or more in the form of monoparticles or pseudo-monoparticles, the primary calcination temperature can be about 800°C to 1000°C, preferably 800°C to 950°C, and more preferably 800°C to 900°C. When the primary calcination temperature satisfies the above range, monoparticle and / or pseudo-monoparticle lithium nickel-based oxides with excellent electrochemical properties can be prepared. If the primary firing temperature is less than 800°C, a positive electrode active material in the form of secondary particles is produced, and if it exceeds 1000°C, excessive firing occurs, the layered crystal structure is not sufficiently formed, and the electrochemical properties are deteriorated.

[0057] The calcination can be carried out in an oxygen atmosphere for 6 to 35 hours, preferably 6 to 20 hours, and more preferably 6 to 12 hours. When the calcination time is within this range, lithium nickel oxide in the form of single particles and / or quasi-single particles can be formed. If the primary calcination time is too short, particle growth is insufficient, resulting in the formation of lithium nickel oxide in the form of secondary particles. If the primary calcination time is too long, a rock salt phase may form, degrading the electrochemical properties of the active material. In this specification, an oxygen atmosphere refers to an atmosphere containing sufficient oxygen for calcination, including the air atmosphere. It is particularly preferable to perform calcination in an atmosphere with an oxygen partial pressure higher than that of the air atmosphere.

[0058] The lithium nickel-based oxide produced by the above-described calcination has the form of a single particle consisting of one nodule and / or a quasi-single particle which is a composite of 30 or less, preferably 2 to 20, more preferably 2 to 10 nodules.

[0059] In addition, the lithium nickel-based oxide may have a composition in which the nickel content of all metals other than lithium is 80 mol% or more, preferably 82 mol% or more, and more preferably 83 mol% or more, and more specifically, may be a lithium nickel cobalt manganese-based oxide in which the nickel content of all metals other than lithium is 80 mol% or more, preferably 82 mol% or more, and more preferably 83 mol% or more. When the nickel content in the lithium nickel-based oxide particles satisfies this range, high energy density can be achieved.

[0060] More specifically, the lithium nickel-based oxide particles may have a composition represented by the following [Chemical Formula 1].

[0061] [Chemical formula 1] Li a Ni b Co c M 1 d M 2 e O2

[0062] In the above Chemical Formula 1, M 1 can be Mn, Al or a combination thereof, preferably Mn or Mn and Al.

[0063] Said M 2 is at least one element selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta and Nb, preferably at least one element selected from the group consisting of Zr, Y, Mg and Ti, more preferably Zr, Y or a combination thereof. 2The element is not necessarily contained, but when contained in an appropriate amount, it can play a role in promoting grain growth during firing or improving the stability of the crystal structure.

[0064] Said a represents the molar ratio of lithium in the lithium nickel-based oxide, and can be 0.8 ≦ a ≦ 1.2, 0.85 ≦ a ≦ 1.15, or 0.9 ≦ a ≦ 1.2. When the molar ratio of lithium satisfies the above range, the crystal structure of the lithium nickel-based oxide can be stably formed.

[0065] Said b represents the molar ratio of nickel among all the metals other than lithium in the lithium nickel-based oxide, and can be 0.8 ≦ b < 1, 0.82 ≦ b < 1, or 0.83 ≦ b < 1. When the molar ratio of nickel satisfies the above range, it shows a high energy density and high capacity can be realized.

[0066] Said c represents the cobalt molar ratio among all the metals other than lithium in the lithium nickel-based oxide, and can be 0 < c < 0.2, 0 < c < 0.18, or 0.01 ≦ c ≦ 0.17. When the molar ratio of cobalt satisfies the above range, good resistance characteristics and output characteristics can be realized.

[0067] Said d represents the molar ratio of element M among all the metals other than lithium in the lithium nickel-based oxide 1 and can be 0 < d < 0.2, 0 < d < 0.18, or 0.01 ≦ d ≦ 0.17. When the molar ratio of element M 1 satisfies the above range, the positive electrode active material shows excellent structural stability.

[0068] Said e represents the molar ratio of element M among all the metals other than lithium in the lithium nickel-based oxide 2 and can be 0 ≦ e ≦ 0.1, or 0 ≦ e ≦ 0.05.

[0069] (B) Milling step After the step (A), a step of milling the lithium nickel-based oxide is performed.

[0070] The milling process is performed to remove large particles and obtain a desired average particle size. During the high-temperature sintering process in step (A), aggregation and / or entanglement between adjacent particles can occur, resulting in the generation of large particles, which can lead to deterioration of rolling properties. Therefore, in the present invention, the milling process is performed to remove the large particles and ultimately form a lithium nickel-based oxide having a desired average particle size. Furthermore, if secondary sintering is performed without the milling process, the effect of removing lithium by-products cannot be obtained.

[0071] The milling may be performed by a common milling method known in the art, such as a jet mill, under conditions of a milling pressure of 1 to 2 bar, a classifier speed of 500 to 3000 rpm, and a feed rate of 2000 to 6000 g / hr.

[0072] Meanwhile, the milling is preferably performed in a low moisture atmosphere, for example, a dry air atmosphere, because if the lithium nickel-based oxide is exposed to moisture, the generation of lithium by-products increases, which can deteriorate the surface properties of the active material.

[0073] (C) Secondary baking step Next, the lithium nickel-based oxide in the form of single particles and / or quasi-single particles produced by the firing in step (A) and the milling in step (B) is subjected to secondary firing.

[0074] Here, the secondary firing temperature may be about 600°C to 900°C, preferably 600°C to 800°C, and more preferably 650°C to 750°C. When the secondary firing temperature is within this range, the increase in initial resistance can be minimized and lithium by-products can be effectively removed. If the secondary firing temperature is less than 600°C, the lithium by-products may not be sufficiently removed. If the secondary firing temperature is more than 900°C, the crystalline structure of the lithium nickel-based oxide may change due to the secondary firing, resulting in a decrease in electrochemical properties. On the other hand, in terms of reducing resistance, the secondary firing temperature is more preferably 800°C or less. This is because if the secondary firing temperature is more than 800°C, the formation of a rock salt phase on the surface may increase, reducing the effect of improving resistance properties.

[0075] The secondary firing may be performed in an oxygen atmosphere for 2 to 10 hours, preferably 4 to 6 hours. If the secondary firing time is too short, the lithium by-products may not be removed effectively, whereas if the secondary firing time is too long, excessive recrystallization may occur, resulting in a deterioration in the electrochemical properties of the positive electrode active material.

[0076] When the secondary firing is performed under the above conditions, lithium by-products remaining on the surface of the lithium nickel-based oxide are decomposed by heat and diffuse into the lithium nickel-based oxide, reducing the amount of lithium remaining on the surface. Lithium ions react with the surface of the lithium nickel-based oxide to form a stable layered structure, thereby stabilizing the surface structure of the lithium nickel-based oxide.

[0077] Meanwhile, the method for manufacturing a cathode active material according to the present invention preferably does not include a water washing step. Conventionally, when manufacturing a high-nickel (Ni) NCM-based lithium nickel-based oxide having a nickel (Ni) content of 80 mol % or more, a water washing step has been generally performed after calcination to reduce the content of lithium by-products. However, according to the research of the present inventors, when a water washing step is performed during the manufacturing of a cathode active material in the form of a single particle and / or a quasi-single particle, the Ni oxidation number on the surface of the lithium nickel-based oxide changes during the water washing process, resulting in the formation of electrically inactive Ni. +2Therefore, when producing the positive electrode active material of the present invention, the positive electrode active material is not washed with water, but is instead subjected to secondary baking at 600 to 800°C to consume the remaining lithium on the surface of the lithium nickel-based oxide, thereby improving the resistance characteristics.

[0078] positive electrode active material Next, the positive electrode active material according to the present invention will be described.

[0079] The cathode active material according to the present invention includes a lithium nickel-based oxide in the form of a single particle and / or a quasi-single particle, and is characterized in that the number of nickel ions having an oxidation number of +3 or more on the surface of the lithium nickel-based oxide is greater than the number of nickel ions having an oxidation number of less than +3. The cathode active material according to the present invention can be prepared by the above-described method of the present invention.

[0080] The lithium nickel-based oxide particles are single particles consisting of one nodule and / or pseudo-single particles which are composites of 30 or less, preferably 2 to 20, more preferably 2 to 10 nodules.

[0081] Such lithium nickel-based oxide particles in the form of single particles or pseudo-single particles have higher particle strength than existing lithium nickel-based oxide particles in the form of secondary particles, which are composed of agglomerates of tens to hundreds of primary particles, and therefore are less likely to crack during rolling.

[0082] In addition, in the case of the lithium nickel-based oxide in the form of a single particle or quasi-single particle according to the present invention, the number of nodules constituting the particle is small, so there is little change due to volume expansion and contraction of the nodules during charge and discharge, and therefore the occurrence of cracks inside the particle is significantly reduced.

[0083] Meanwhile, the lithium nickel-based oxide particles may have a composition in which the nickel content of all metals other than lithium is 80 mol% or more, preferably 82 mol% or more, and more preferably 83 mol% or more, and more specifically, may be a lithium nickel cobalt manganese-based oxide in which the nickel content of all metals other than lithium is 80 mol% or more, preferably 82 mol% or more, and more preferably 83 mol% or more. When the nickel content in the lithium nickel-based oxide particles satisfies this range, high energy density can be achieved.

[0084] More specifically, the lithium nickel-based oxide particles may have a composition represented by the following [Chemical Formula 1].

[0085] [Chemical formula 1] Li a Ni b Co c M 1 d M 2 e O2

[0086] In the above Chemical Formula 1, M 1 can be Mn, Al or a combination thereof, preferably Mn or Mn and Al.

[0087] Said M 2 is at least one selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta and Nb, preferably at least one selected from the group consisting of Zr, Y, Mg and Ti, more preferably Zr, Y or a combination thereof. 2 Although the element is not necessarily contained, when contained in an appropriate amount, it can play a role in promoting grain growth during firing or improving the stability of the crystal structure.

[0088] The aforesaid a represents the molar ratio of lithium in the lithium nickel-based oxide, and can be 0.8 ≦ a ≦ 1.2, 0.85 ≦ a ≦ 1.15, or 0.9 ≦ a ≦ 1.2. When the molar ratio of lithium satisfies the aforesaid range, the crystal structure of the lithium nickel-based oxide can be stably formed.

[0089] The aforesaid b represents the molar ratio of nickel among all the metals other than lithium in the lithium nickel-based oxide, and can be 0.8 ≦ b < 1, 0.82 ≦ b < 1, or 0.83 ≦ b < 1. When the molar ratio of nickel satisfies the aforesaid range, it shows a high energy density and it is possible to achieve a high capacity.

[0090] The aforesaid c represents the cobalt molar ratio among all the metals other than lithium in the lithium nickel-based oxide, and can be 0 < c < 0.2, 0 < c < 0.18, or 0.01 ≦ c ≦ 0.17. When the molar ratio of cobalt satisfies the aforesaid range, good resistance characteristics and output characteristics can be achieved.

[0091] The aforesaid d represents the molar ratio of M 1 element among all the metals other than lithium in the lithium nickel-based oxide, and can be 0 < d < 0.2, 0 < d < 0.18, or 0.01 ≦ d ≦ 0.17. When the molar ratio of the M 1 element satisfies the aforesaid range, the cathode active material shows excellent structural stability.

[0092] The aforesaid e represents the molar ratio of M 2 element among all the metals other than lithium in the lithium nickel-based oxide, and can be 0 ≦ e ≦ 0.1, or 0 ≦ e ≦ 0.05.

[0093] On the other hand, the lithium nickel-based oxide of the present invention manufactured by performing the secondary firing without performing the water washing process is Ni that is electrochemically inert on the surface. +2Specifically, the number of nickel ions with an oxidation number of +3 or more on the surface of the lithium nickel-based oxide of the present invention is greater than the number of nickel ions with an oxidation number of less than +3, resulting in excellent resistance characteristics. The oxidation number on the surface of the lithium nickel-based oxide can be confirmed by electron energy loss spectroscopy.

[0094] In addition, the cathode active material according to the present invention has a low residual lithium content because residual lithium present on the surface of the lithium-nickel-based oxide is removed during the secondary firing process. Specifically, the cathode active material according to the present invention may have a residual lithium content of 0.5 wt % or less, preferably 0.01 to 0.5 wt %, and more preferably 0.01 to 0.4 wt %. This low residual lithium content minimizes the phenomenon of gas generation caused by the reaction of residual lithium with the electrolyte during charge / discharge and / or at high temperatures, thereby achieving excellent life characteristics.

[0095] Meanwhile, the cathode active material according to the present invention may have an average nodule particle size of 0.5 μm to 3 μm, preferably 0.8 μm to 2.5 μm, and more preferably 0.8 μm to 1.5 μm. When the average nodule particle size satisfies this range, a cathode active material in the form of a single particle and / or a quasi-single particle with excellent electrochemical properties can be formed. If the average nodule particle size is excessively small, the number of agglomerates of nodules forming lithium nickel-based oxide particles increases, reducing the effect of suppressing particle cracking during rolling. If the average nodule particle size is excessively large, the lithium diffusion path within the nodule becomes longer, increasing resistance and potentially reducing output characteristics.

[0096] The positive electrode active material has an average particle size D 50 The D of the positive electrode active material can be 2 μm to 6 μm, preferably 2 μm to 5 μm, and more preferably 3 μm to 5 μm. 50 If D is too small, it is difficult to form an active material layer during electrode production, the impregnation of the electrolyte solution is reduced, and the electrochemical properties are deteriorated. 50If is excessively large, the resistance increases, resulting in a problem of reduced output characteristics.

[0097] Additionally, the positive electrode active material may have an average crystallite size of 150 nm to 300 nm, 200 nm to 280 nm, or 200 nm to 250 nm. When the average crystallite size is within this range, the formation of a rock salt phase during the preparation of the lithium-nickel-based oxide is reduced, allowing the preparation of a single-particle and / or quasi-single-particle positive electrode active material with excellent resistance characteristics. Generally, single-particle and / or quasi-single-particle positive electrode active materials are prepared by increasing the sintering temperature to increase the nodule size. However, increasing the nodule size while the crystallite size is small can result in the formation of a rock salt phase on the surface of the nodules, resulting in increased resistance. However, increasing both the average crystallite size and the average particle size of the nodules can minimize the formation of the rock salt phase, thereby suppressing the increase in resistance.

[0098] positive electrode Next, the positive electrode according to the present invention will be described.

[0099] The positive electrode according to the present invention includes a positive electrode active material layer including the positive electrode active material according to the present invention. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material.

[0100] The positive electrode current collector in the positive electrode is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used. The positive electrode current collector typically has a thickness of 3 to 500 μm, and fine irregularities can be formed on the surface of the positive electrode current collector to enhance the adhesive strength of the positive electrode active material. The positive electrode current collector can be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0101] The positive electrode active material layer may contain a conductive material and a binder in addition to the positive electrode active material.

[0102] The conductive material is used to impart conductivity to the electrode. Any material that does not cause chemical changes in the resulting battery and has electronic conductivity can be used without particular limitations. 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, thermal black, carbon fiber, and carbon nanotubes; metal powders or fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials can be used alone or in combination. The conductive material is typically present in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the positive electrode active material layer.

[0103] The binder improves adhesion between positive electrode active material particles and 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, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination. The binder may be included in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the positive electrode active material layer.

[0104] The positive electrode may be manufactured by a conventional method for manufacturing a positive electrode, for example, by mixing a positive electrode active material, a binder, and / or a conductive material in a solvent to prepare a positive electrode slurry, applying the positive electrode slurry to a positive electrode current collector, and then drying and rolling the slurry.

[0105] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and may be used alone or in combination. The amount of solvent used is determined in consideration of the coating thickness of the slurry and the manufacturing yield, and is sufficient as long as the solvent has a viscosity that can dissolve or disperse the positive electrode active material, conductive material, and binder and provide excellent thickness uniformity during subsequent coating for manufacturing a positive electrode.

[0106] Alternatively, the positive electrode can be produced by casting the positive electrode slurry on a separate support, peeling it off from the support, and laminating the resulting film onto a positive electrode current collector.

[0107] Lithium secondary battery Next, the lithium secondary battery according to the present invention will be described.

[0108] The lithium secondary battery of the present invention includes the positive electrode according to the present invention. Specifically, the lithium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, the positive electrode being as described above. The lithium secondary battery may also optionally further include a battery container that houses the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0109] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0110] The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surfaces treated with carbon, nickel, titanium, silver, or the like, and aluminum-cadmium alloys can be used. The negative electrode current collector typically has a thickness of 3 to 500 μm. As with the positive electrode current collector, the surface of the current collector can be formed with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector can be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0111] The negative electrode active material layer includes a negative electrode active material, and optionally a binder and a conductive material.

[0112] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. 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 alloys, Sn alloys, and Al alloys; and SiO. β (0<β<2), metal oxides that can be doped and dedoped with lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the above 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 may be used.

[0113] The negative electrode active material may be a thin film of metallic lithium. The carbon material may be either low-crystalline or high-crystalline. Typical examples of low-crystalline carbon include soft carbon and hard carbon. Typical examples of high-crystalline carbon include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0114] The conductive material is used to impart conductivity to the electrode. Any material that does not cause chemical changes in the battery and has electronic conductivity can be used without particular limitations. 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, thermal black, carbon fiber, and carbon nanotubes; metal powder or metal fiber, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials can be used alone or in combination. The conductive material is typically present in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0115] The binder improves adhesion between negative electrode active material particles and between the negative electrode active material and the negative 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, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination. The binder may be included in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0116] For example, the negative electrode active material layer may be manufactured by coating a negative electrode slurry containing a negative electrode active material, and optionally a binder and a conductive material, on a negative electrode current collector and drying the coating, or by casting the negative electrode slurry on a separate support, peeling the negative electrode slurry from the support, and laminating the resulting film on the negative electrode current collector.

[0117] Meanwhile, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without particular limitations. In particular, a separator that exhibits low resistance to ion migration and excellent humidification ability for the electrolyte solution is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can also be used, and can be selectively used in a single-layer or multi-layer structure.

[0118] Furthermore, examples of the electrolyte used in the present invention include 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 production of lithium secondary batteries, but are not limited to these.

[0119] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0120] The organic solvent may be any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of solvents that can be used include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a C2-C20 linear, branched, or cyclic hydrocarbon group that may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred.

[0121] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, examples of the lithium salt include LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, and LiB(C, O) . The lithium salt concentration is preferably within a range of 0.1 to 5.0 M, and more preferably 0.1 to 3.0 M. When the lithium salt concentration is within this range, the electrolyte exhibits appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0122] In addition to the electrolyte components, the electrolyte may further contain additives for purposes such as improving battery life characteristics, suppressing battery capacity loss, and improving battery discharge capacity. Examples of additives include, but are not limited to, haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethyl alcohol amine, cyclic ethers, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethyl alcohol, and aluminum trichloride, which may be used alone or in combination. The additives may be present in an amount of 0.1 to 10 wt %, preferably 0.1 to 5 wt %, based on the total weight of the electrolyte.

[0123] As described above, the lithium secondary battery including the cathode active material according to the present invention not only has excellent initial capacity and life characteristics, but also has low discharge terminal resistance measured in the low SOC region, especially at SOC 10% or less, and can be effectively used in the field of electric vehicles.

[0124] While the present invention may be embodied in various different forms, it is to be understood that the invention is not limited to the specific embodiments set forth herein, and that the invention may be embodied in various different forms without departing from the spirit or scope of the present invention.

[0125] Example 1 Transition metal precursor Ni 0.83 Co 0.11 Mn 0.06 (OH)2 and LiOH·H2O were mixed so that the transition metal (Ni+Co+Mn):Li molar ratio was 1:1.05, and the mixture was heated to 850°C at a heating rate of 5°C / min, maintained at 850°C for 10 hours, and then cooled to room temperature at a heating rate of 5°C / min to produce lithium nickel-based oxide (step (A)).

[0126] Thereafter, the lithium nickel-based oxide was milled for 30 minutes under the conditions of a grinding pressure of 1.5 bar, a classification speed of 1500 rpm, and an inflow rate of 3000 g / hour (step (B)).

[0127] Next, the lithium nickel-based oxide was heated to 700°C at a heating rate of 5°C / min, maintained at 700°C for 5 hours, and cooled to room temperature at a cooling rate of 5°C / min to prepare a cathode active material (step (C)).

[0128] A scanning electron microscope photograph of the cathode active material prepared by the above method is shown in Figure 3. From Figure 3, it can be seen that the cathode active material prepared in Example 1 has a single particle morphology.

[0129] Example 2 A positive electrode active material was produced in the same manner as in Example 1, except that the temperature was increased to 850° C. in step (C). The produced positive electrode active material had a single particle form.

[0130] Comparative Example 1 A positive electrode active material was prepared in the same manner as in Example 1, except that step (C) was not performed on the lithium nickel-based oxide. The prepared positive electrode active material had a single particle form.

[0131] Comparative Example 2 A positive electrode active material was prepared in the same manner as in Example 1, except that step (C) was omitted and lithium nickel-based oxide and water were mixed in a weight ratio of 1:1, stirred, washed with water, filtered, and dried to prepare a positive electrode active material. The prepared positive electrode active material had a single particle morphology.

[0132] Comparative Example 3 A positive electrode active material was produced in the same manner as in Example 1, except that in step (A), after raising the temperature to 850° C., the maintenance time was changed to 15 hours, and step (C) was not performed.

[0133] Fig. 4 shows a scanning electron microscope photograph of the positive electrode active material produced by the method of Comparative Example 3. From Fig. 4, it can be seen that the positive electrode active material produced by Comparative Example 3 is a quasi-single particle formed by agglomeration of about 15 to 30 nodules.

[0134] Comparative Example 4 A positive electrode active material was produced in the same manner as in Example 1, except that in step (A), the temperature was raised to 850°C and then maintained for 5 hours, and in step (C), the temperature was raised to 700°C and then maintained for 10 hours.

[0135] Fig. 5 shows a scanning electron microscope image of the cathode active material prepared by the method of Comparative Example 4. From Fig. 5, it can be seen that the cathode active material prepared by Comparative Example 4 has a secondary particle form in which 40 or more primary particles are aggregated.

[0136] Experimental Example 1: Analysis of Ni oxidation number The surfaces of the positive electrode active material particles prepared in Example 1 and Comparative Example 2 were analyzed by TEM-EELS to confirm the Ni oxidation number distribution on the surface.

[0137] FIG. 1 shows the results of TEM-EELS analysis showing the Ni oxidation number distribution on the surface of the positive electrode active material particles of Example 1, and FIG. 2 shows the results of TEM-EELS analysis of the Ni oxidation number distribution on the surface of the positive electrode active material particles prepared in Comparative Example 2.

[0138] 1 and 2, it can be seen that Ni ions with a high oxidation number are mainly distributed on the surface of the positive electrode active material of Example 1, which was subjected to the high-temperature secondary baking, whereas Ni ions with a low oxidation number are mainly distributed on the surface of the positive electrode active material of Comparative Example 2, which was subjected to the water washing step.

[0139] Experimental Example 2: Measurement of residual lithium amount Five grams of the positive electrode active material powders prepared in Examples 1 and 2 and Comparative Examples 1 to 4 were dispersed in 100 mL of water, and the dispersions were titrated with 0.1 M HCl to measure the change in pH, thereby obtaining pH titration curves. The pH titration curves were used to calculate the residual amounts of LiOH and Li2CO3 in each positive electrode active material, and the sum of these values ​​was used to evaluate the residual lithium amount. The measurement results are shown in Table 1 below.

[0140] [Table 1]

[0141] As can be seen from Table 1, the cathode active materials of Examples 1 and 2 prepared by the method of the present invention have significantly lower amounts of residual lithium than the cathode active material of Comparative Example 1, and have lower levels of residual lithium than the cathode active material of Comparative Example 2, which did not undergo a water washing process despite not undergoing a water washing process. On the other hand, the cathode active material of Comparative Example 3, which underwent a long period of primary firing without a secondary firing, actually had an increased amount of residual lithium. The cathode active material of Comparative Example 4 had a similar level of residual lithium to Example 1, but exhibited a higher gas generation rate, as shown in Experimental Example 4 below.

[0142] <Secondary battery manufacturing> The positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 to 4, a conductive material (carbon black, Denka), and a PVDF binder were mixed in a weight ratio of 96:2:2 in N-methylpyrrolidone to prepare positive electrode slurry. The positive electrode slurry was applied to one side of an aluminum current collector, dried at 130°C, and rolled to prepare a positive electrode.

[0143] A lithium metal electrode was used as the negative electrode.

[0144] An electrode assembly was fabricated by interposing a separator between the positive and negative electrodes, and then placed inside a battery case. An electrolyte solution was then injected into the case to fabricate a coin half cell. The electrolyte solution was prepared by dissolving 1M LiPF6 in a mixed organic solvent of ethylene carbonate / dimethyl carbonate / diethyl carbonate in a volume ratio of 1:2:1, and adding 2 wt% vinylene carbonate (VC).

[0145] Experimental example 3: Evaluation of resistance characteristics by SOC Coin half-cells prepared using the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 to 4 were charged and discharged once at 2.5 to 4.25 V under 0.1 C / 0.1 C conditions, then charged again to 4.25 V and discharged in 10% increments of the cell discharge capacity to an SOC of 10, while measuring the resistance (unit: Ω) as a function of SOC. Resistance was measured from the voltage change when a current of 2.5 C was applied for 10 seconds at each SOC. The measurement results are shown in FIG. 6.

[0146] 6, it can be seen that the cells employing the cathode active materials of Examples 1 and 2 prepared by the method of the present invention exhibited superior resistance characteristics compared to the cells employing the cathode active materials of Comparative Examples 1 to 3. In particular, the cell employing the cathode active material of Example 1, which was subjected to secondary firing at temperatures between 600°C and 800°C, exhibited superior discharge terminal resistance characteristics compared to the cell employing the cathode active material of Comparative Example 4 in the form of secondary particles. The cell employing the cathode active material of Example 2, which was subjected to secondary firing at temperatures above 800°C, exhibited superior resistance characteristics compared to Comparative Examples 1 to 3, but inferior resistance characteristics compared to Example 1 and Comparative Example 4. This is believed to be due to the increased formation of a rock salt phase on the surface of the lithium-nickel oxide as the secondary firing temperature increased, resulting in a somewhat reduced improvement in surface resistance.

[0147] Experimental Example 4: Evaluation of gas generation rate Each of the lithium secondary batteries fabricated above was charged to 4.25 V in CC-CV mode at 1C, and then the secondary battery was disassembled to separate the positive electrode. Then, 400 mg of the positive electrode and 15 μL of electrolyte were placed in a pouch-type battery case and sealed to fabricate a cell. The cell was then stored at 60°C for 8 weeks, and the change in cell volume (ΔCell volume, unit: ΔmL) before and after high-temperature storage was measured. The change in cell volume was measured by placing the cell in water and measuring the volume change of the water. The measurement results are shown in Figure 7.

[0148] As shown in FIG. 7, the cells using the positive electrode active materials of Examples 1 and 2, which were prepared by high-temperature secondary baking without washing with water, showed significantly less change in volume than the cells using the positive electrode active materials of Comparative Examples 1 to 4.

[0149] In the cells employing the positive electrode active materials of Comparative Examples 1 to 3, the amount of residual lithium in the positive electrode active material was high, and the amount of initial gas generation within two weeks after high-temperature storage was high. In the cell employing the positive electrode active material of Comparative Example 4, which had a small amount of residual lithium, the amount of initial gas generation was relatively small, but the amount of gas generation rapidly increased as the storage time became longer.

Claims

1. (A) calcining a mixture of a transition metal precursor and a lithium source material to prepare a lithium nickel-based oxide having at least one form of a single particle or a quasi-single particle; (B) milling the lithium nickel-based oxide; (C) firing the lithium nickel-based oxide at a temperature of 600°C to 900°C; The single particle consists of one nodule, and the quasi-single particle is a complex of 30 or less nodules; The firing temperature in step (A) is 800°C to 1000°C, The step (A) is carried out for 6 to 35 hours; The step (C) is carried out for 2 to 10 hours; No water washing step is included after the step (B) and before the step (C), In the method for producing a positive electrode active material, the number of nickel ions having an oxidation number of +3 or more on the surface of the lithium nickel-based oxide is greater than the number of nickel ions having an oxidation number of less than +3.

2. The method for producing a positive electrode active material according to claim 1 , wherein the transition metal precursor is nickel-cobalt-manganese hydroxide having a Ni content of 80 mol % or more.

3. The method for producing a positive electrode active material according to claim 1, wherein the lithium nickel-based oxide is represented by the following [Chemical Formula 1]: [Chemical formula 1] Li ａ Ni ｂ Co ｃ M １ ｄ M ２ ｅ O ２ In the above formula 1, M １ is Mn, Al or a combination thereof, and M ２ is one or more elements selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta, and Nb, and 0.8≦a≦1.2, 0.8≦b<1, 0<c<0.2, 0<d<0.2, 0≦e≦0.

1.

4. The method for producing a positive electrode active material according to claim 1, wherein the firing temperature in step (C) is 600°C to 800°C.

5. The method for producing a positive electrode active material according to claim 1 , wherein the transition metal precursor is produced by a co-precipitation reaction.

6. The lithium nickel-based oxide has at least one of a monoparticle and a quasi-monoparticle form, The single particle consists of one nodule, The pseudo-single particle is a complex of 30 or less nodules, a positive electrode active material, wherein the number of nickel ions having an oxidation number of +3 or more on the surface of the lithium nickel-based oxide is greater than the number of nickel ions having an oxidation number of less than +3;

7. The positive electrode active material according to claim 6, wherein the pseudo-single particle is a composite of 2 to 10 nodules.

8. The lithium nickel-based oxide is the positive electrode active material according to claim 6, represented by the following [Chemical Formula 1]: [Chemical formula 1] Li ａ Ni ｂ Co ｃ M １ ｄ M ２ ｅ O ２ In the above formula 1, M １ is Mn, Al or a combination thereof, and M ２ is one or more elements selected from the group consisting of Zr, W, Y, Ba, Ca, Ti, Mg, Ta, and Nb, and 0.8≦a≦1.2, 0.8≦b<1, 0<c<0.2, 0<d<0.2, 0≦e≦0.

1.

9. The positive electrode active material according to claim 6 , wherein the positive electrode active material has a residual lithium amount of 0.5 wt % or less.

10. The positive electrode active material according to claim 6, wherein the positive electrode active material has nodules with an average particle size of 0.5 μm to 3 μm.

11. A positive electrode comprising the positive electrode active material layer according to claim 6 .

12. A lithium secondary battery comprising the positive electrode according to claim 11.

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