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

Abstract

The present invention relates to a lithium nickel-based oxide in the form of a quasi-single particle, which is a single particle consisting of one nodule or a composite of 30 or less nodules; and a positive electrode active material which includes a coating portion containing Co and is formed in an island shape on a part of the surface of the lithium nickel-based oxide particle, and in a Ni L3-edge spectrum obtained by measuring the surface of the lithium nickel-based oxide in contact with the coating portion by electron energy loss spectroscopy, the intensity at 855.5 eV is greater than the intensity at 853 eV, a manufacturing method thereof, and a positive electrode and a lithium secondary battery including the same.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0002995, filed on January 7, 2022, and all contents disclosed in the documents of this Korean patent application 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 same. More particularly, the present invention relates to a positive electrode active material in a single particle or quasi-single particle form that has excellent life characteristics and resistance characteristics, a manufacturing method thereof, and a positive electrode and a lithium secondary battery including the same. [Background technology]

[0003] A lithium secondary battery typically 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 it difficult to commercially apply to large-capacity batteries. 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 suffers from poor capacity characteristics. 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 are generally in the form of spherical secondary particles formed by the aggregation of tens to hundreds of primary particles. However, lithium nickel cobalt manganese oxides in this secondary particle form, formed by the aggregation of many primary particles, are prone to particle cracking, where primary particles fall off during the rolling process in positive electrode manufacturing, and cracks occur within the particles during charge and discharge. Particle cracking or cracking of the positive electrode active material increases the contact area with the electrolyte, increasing gas generation and active material degradation due to side reactions with the electrolyte, resulting in reduced lifespan.

[0006] Recently, there has been an increasing demand for high-power, high-capacity batteries, such as those for electric vehicles, and as a result, the nickel content in the positive electrode active material has gradually increased. As the nickel content in the positive electrode active material increases, the initial capacity characteristics improve, but the highly reactive Ni +4 A large amount of ions are generated, causing the structure of the positive electrode active material to collapse, which increases the rate of degradation of the positive electrode active material, reducing the life characteristics and battery safety.

[0007] To solve these problems, a new technology has been proposed: increasing the calcination temperature during the production of lithium nickel cobalt manganese oxide to produce a single particle-type positive electrode active material rather than a secondary particle. Single particle-type positive electrode active materials have a smaller contact area with the electrolyte than conventional secondary particle-type positive electrode 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-type positive electrode active materials offers the advantages of excellent gas generation and lifespan characteristics.

[0008] However, compared with conventional lithium composite transition metal oxide particles in the form of secondary particles, single-particle lithium composite transition metal oxide particles have relatively large primary particle sizes and fewer interfaces between primary particles that serve as diffusion paths for lithium ions, resulting in reduced lithium mobility. Additionally, because they are manufactured at relatively high sintering temperatures, a rock salt phase forms on the particle surfaces, resulting in high surface resistance. Therefore, single-particle lithium composite transition metal oxide particles have high lithium diffusion resistance, which can lead to uneven lithium ion migration during charge and discharge, leading to deformation of the crystal structure and the resulting particle cracks. The particle cracks increase resistance during charge and discharge, resulting in reduced life characteristics. Summary of the Invention [Problem to be solved by the invention]

[0009] In order to solve the above problems, the present invention aims to provide a single-particle or quasi-single-particle positive electrode active material that exhibits little increase in resistance during charge and discharge and excellent capacity and life characteristics, a method for manufacturing the same, and a positive electrode and a lithium secondary battery using the positive electrode active material. [Means for solving the problem]

[0010] According to one embodiment, the present invention provides a positive electrode active material comprising: a lithium nickel-based oxide in the form of a quasi-single particle, which is a single particle consisting of one nodule or a composite of 30 or less nodules; and a coating portion containing Co, formed in the shape of islands on a portion of the surface of the lithium nickel-based oxide particle, wherein the intensity at 855.5 eV is greater than the intensity at 853 eV in a Ni L3-edge spectrum obtained by measuring the surface of the lithium nickel-based oxide in contact with the coating portion by electron energy loss spectroscopy.

[0011] According to another embodiment, the present invention provides a method for manufacturing a cathode active material, the method including: mixing a transition metal precursor and a lithium source material, followed by calcining the mixture to prepare a lithium nickel-based oxide in a single particle or quasi-single particle form; and mixing the lithium nickel-based oxide with a cobalt-containing coating material, followed by heat treatment at a temperature of 600°C to 750°C to form a coating portion.

[0012] According to still another embodiment, the present invention provides a positive electrode including a positive electrode active material layer including the positive electrode active material of the present invention, and a lithium secondary battery including the positive electrode. [Effects of the Invention]

[0013] The positive electrode active material according to the present invention includes a lithium nickel-based oxide in the form of a single particle or quasi-single particle having high particle strength, which reduces particle cracking during electrode manufacturing and charge / discharge, and has a small contact area with the electrolyte, thereby suppressing the occurrence of side reactions with the electrolyte. As a result, when used in a secondary battery, the amount of gas generation is reduced and the high-temperature life characteristics are excellent.

[0014] In addition, the positive electrode active material according to the present invention includes an island-shaped Co-containing coating portion on a portion of the surface of a lithium nickel-based oxide particle, and more Ni ions with an oxidation number of +3 or higher are present on the surface of the lithium nickel-based oxide particle in contact with the Co-containing coating portion, thereby minimizing an increase in resistance and improving capacity and life characteristics.

[0015] When a Co-containing coating is formed on the surface of lithium nickel-based oxide particles, the surface structure of the positive electrode active material is formed into a stable layer structure by Co, which facilitates the insertion and desorption of lithium ions, improving the surface stability of the positive electrode active material and the initial capacity characteristics. However, if the Co-containing coating is formed as a continuous film, although the surface stability is improved, there is a problem that the formation of an electrically inactive NiO rock-salt phase increases, resulting in an increase in resistance. Therefore, in the present invention, the Co coating conditions are controlled so that the Co-containing coating is formed in an island shape on a part of the surface of the lithium nickel-based oxide, and Ni is formed on the surface of the lithium nickel-based oxide in contact with the Co-containing coating. 2+ By suppressing the generation of ions, it is possible to minimize the increase in resistance while improving capacity and life characteristics.

[0016] Meanwhile, in the method for manufacturing a cathode active material according to the present invention, a lithium nickel-based oxide in the form of a single particle or quasi-single particle is manufactured, and then the lithium nickel-based oxide is mixed with a cobalt raw material and heat-treated at a specific temperature. This allows the Co-containing coating portion to be formed in an island shape, i.e., a shape in which particles are sporadically distributed, and inhibits the formation of a rock salt phase at the interface between the Co-containing coating portion and the lithium nickel-based oxide, thereby ensuring that the oxidation number of nickel ions at the interface is +3 or more. [Brief explanation of the drawings]

[0017] [Figure 1] 1 shows a scanning electron microscope (SEM) image and an energy dispersive X-ray spectroscopy (EDS) Co mapping image of the positive electrode active material prepared in Example 1. [Figure 2] 1 shows an SEM image and an EDS Co mapping image of a positive electrode active material prepared in Example 2. [Figure 3]1 shows an SEM image and an EDS Co mapping image of a positive electrode active material prepared in Example 3. [Figure 4] 1 shows an SEM image and an EDS Co mapping image of a positive electrode active material prepared according to Comparative Example 1. [Figure 5] 1 shows an SEM image and an EDS Co mapping image of a positive electrode active material prepared according to Comparative Example 2. [Figure 6] 1 shows an SEM image and an EDS Co mapping image of a positive electrode active material prepared according to Comparative Example 3. [Figure 7] 1 shows a High-Angle Annular Dark-Field (HAADF) image and an Electron Energy Loss Spectroscopy (EELS) spectrum image of the positive electrode active material prepared in Example 1. [Figure 8] EELS Ni L3-edge spectra of regions 1 to 3 of the EELS spectrum image in Figure 7. [Figure 9] 1 shows an HAADF image and an EELS spectrum image of a positive electrode active material prepared according to Comparative Example 1. [Figure 10] EELS Ni L3-edge spectra of regions 1 and 2 of the EELS spectrum image in Figure 9. [Figure 11] 1 shows an HAADF image and an EELS spectrum image of a positive electrode active material prepared according to Comparative Example 2. [Figure 12] EELS Ni L3-edge spectra of regions 1 to 3 of the EELS spectrum image in FIG. 11. [Figure 13] 1 is a graph showing the high-temperature life characteristics of secondary batteries to which the positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 to 3 are applied. [Figure 14] 1 is a graph showing the resistance characteristics as a function of SOC of secondary batteries to which the positive electrode active materials of Examples 1 to 3 and Comparative Examples 1 to 3 are applied. 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 in a way that is limited to their ordinary and dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, in accordance with the principle that the inventor himself / herself can appropriately define the concept of terms in order to best explain the invention.

[0020] In the present invention, a "single particle" means a particle consisting of one nodule, and a "quasi-single particle" means a composite particle consisting of 30 or fewer nodules.

[0021] The "nodule" is a lower particle unit constituting a single particle or a quasi-single particle, and may be a single crystal having no crystalline grain boundary, or a polycrystal having no apparent grain boundary when observed at a magnification of 5,000 to 20,000 using a scanning electron microscope.

[0022] In the present invention, "secondary particles" refer to particles formed by agglomeration of a plurality of, for example, tens to hundreds of, primary particles. Specifically, secondary particles may be agglomerates of 50 or more primary particles.

[0023] In the present invention, the term "particle" is a concept that includes any one or all of a single particle, a quasi-single particle, a primary particle, a nodule, and a secondary particle.

[0024] In the present invention, the "average particle size D 50" refers to the particle size at 50% of the volume cumulative particle size distribution of the lithium composite transition metal oxide powder or the positive electrode active material powder, and can be measured using a laser diffraction method. For example, the lithium composite transition metal oxide powder or the positive electrode active material powder is dispersed in a dispersion medium, and then introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT3000), and ultrasonic waves of about 28 kHz are irradiated at an output of 60 W. A volume cumulative particle size distribution graph is obtained, and the particle size corresponding to 50% of the volume cumulative amount is determined.

[0025] In the present invention, the term "crystallite" refers to a particle unit having substantially the same crystal orientation, which can be identified through EBSD (Electron Backscatter Diffraction) analysis. Specifically, the term "crystallite" refers to the smallest particle unit that shows the same color in an IPF map obtained by EBSD analysis of a cross section of a cathode active material cut by ion milling.

[0026] Meanwhile, in the present invention, "average crystallite size" can be quantitatively analyzed using X-ray diffraction analysis (XRD) using Cu Kα X-rays. Specifically, the particles to be measured are placed in a holder, and the resulting diffraction grating is analyzed to quantitatively analyze the average crystallite size of the crystal grains. Sampling was performed by placing a powder sample of the particles to be measured in a recessed groove in a general powder holder, smoothing the surface with a glass slide, and aligning the sample height with the edge of the holder. X-ray diffraction analysis was then performed using a Bruker D8 Endeavor (light source: Cu Kα, λ = 1.54 Å) equipped with a LynxEye XE-T position-sensitive detector under the following conditions: FDS 0.5°, 2θ = 15° to 90°, step size 0.02°, total scan time approximately 20 minutes. The measured data were subjected to Rietveld refinement, taking into account the charge at each site (metal ions at transition metal sites are +3, Ni ions at Li sites are +2) and cation mixing. Instrumental broadening during grain size analysis was considered using the Fundamental Parameter Approach (FPA) implemented in the Bruker TOPAS program, and the entire peak in the measurement range was used for fitting. Peak shape was fitted using only the Lorenzian contribution in the First Principle (FP) peak type available in TOPAS, without considering strain.

[0027] In the present invention, the "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mini II manufactured by BEL Japan.

[0028] positive electrode active material The positive electrode active material according to the present invention will be described below.

[0029] The positive electrode active material according to the present invention includes (1) lithium nickel-based oxide particles in the form of single particles or quasi-single particles, and (2) a Co-containing coating portion formed in an island shape on a portion of the surface of the lithium nickel-based oxide particle.

[0030] (1) Lithium nickel oxide particles The lithium nickel-based oxide particles are single particles consisting of one nodule, or quasi-single particles that are composites of 30 or less, preferably 2 to 20, and more preferably 2 to 10 nodules.

[0031] Such lithium nickel-based oxide particles in the form of single particles or quasi-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.

[0032] 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, and therefore, 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.

[0033] Meanwhile, the lithium nickel-based oxide particles may have a composition in which the nickel content of all metals excluding lithium is 70 mol% or more, preferably 80 mol% or more, and more preferably 82 mol% or more, and more specifically, the lithium nickel cobalt manganese-based oxide may have a nickel content of all metals excluding lithium of 70 mol% or more, preferably 80 mol% or more, and more preferably 82 mol% or more. When the nickel content in the lithium nickel-based oxide particles satisfies this range, a high energy density can be achieved.

[0034] More specifically, the lithium nickel-based oxide particles may have a composition represented by the following [Chemical Formula 1].

[0035] [Chemical formula 1] Li a Ni b Co c M 1 d M 2 e O2

[0036] In the above formula 1, M 1 may be Mn, Al or a combination thereof, preferably Mn or Mn and Al.

[0037] Said M 2 is at least one selected from the group consisting of Zr, W, Y, Ba, Ca, Sr, 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 elements are not essential, when contained in an appropriate amount, they can promote grain growth during firing or improve the stability of the crystal structure.

[0038] The a represents the lithium molar ratio in the lithium nickel-based oxide and may be 0.8≦a≦1.2, 0.85≦a≦1.15, or 0.9≦a≦1.2. When the lithium molar ratio satisfies this range, the crystalline structure of the lithium nickel-based oxide can be stably formed.

[0039] The b represents the molar ratio of nickel to all metals excluding lithium in the lithium nickel-based oxide, and may be 0.7≦b<1, 0.8≦b<1, or 0.82≦b<1. When the molar ratio of nickel satisfies this range, high energy density and high capacity can be realized.

[0040] Said c represents the molar ratio of cobalt in all the metals excluding lithium in the lithium nickel-based oxide, and may be 0 < c < 0.3, 0 < c < 0.2, or 0.01 ≤ c ≤ 0.15. When the molar ratio of cobalt satisfies the above range, good resistance characteristics and output characteristics can be realized.

[0041] Said d represents the molar ratio of element M in all the metals excluding lithium in the lithium nickel-based oxide, and may be 0 < d < 0.3, 0 < d < 0.2, or 0.01 ≤ d ≤ 0.15. 1 When the molar ratio of element M satisfies the above range, excellent structural stability of the positive electrode active material appears. 1 Said e represents the molar ratio of element M in all the metals excluding lithium in the lithium nickel-based oxide, and may be 0 ≤ e ≤ 0.2, 0 ≤ e ≤ 0.1, or 0 ≤ e ≤ 0.05.

[0042] 2

[0043] (2) Coating portion The positive electrode active material according to the present invention includes a coating portion containing Co, which is formed in an island shape on a part of the surface of the lithium nickel-based oxide particles in the form of single particles or quasi-single particles described above. Here, the island shape means a shape in which coating portions in the form of small particles are scattered on the surface of the lithium nickel-based oxide particles, and the coating portions in the form of particles may be spherical, plate-shaped, and / or amorphous.

[0044] ​​High-nickel positive electrode active materials with a high nickel content have problems such as side reactions with the electrolyte at the surface of the positive electrode active material, which can cause transition metal elution and structural collapse. To address these problems, conventional methods have involved coating the surface of high-nickel positive electrode active materials with metal oxides such as aluminum or boron to reduce contact with the electrolyte. However, because such coating layers are electrically inactive, forming a coating layer on the surface of the active material increases the resistance of the positive electrode active material, and the increase in resistance becomes more pronounced as the coating layer thickness increases. In particular, monoparticle or quasi-monoparticle positive electrode active materials have higher lithium diffusion resistance than conventional secondary particle positive electrode active materials. Therefore, applying a conventional coating layer to the surface of a monoparticle positive electrode active material can exacerbate the problem of output degradation due to increased resistance.

[0045] The present inventors have conducted extensive research to improve the resistance characteristics of a positive electrode active material having a monoparticle or quasi-monoparticle form, and have found that both the resistance characteristics and the life characteristics can be improved by forming an island-shaped cobalt (Co)-containing coating portion on the surface of a monoparticle or quasi-monoparticle lithium nickel-based oxide.

[0046] Cobalt (Co), an element advantageous for forming a layered structure, can be used to form a coating portion, which stabilizes the crystalline structure on the surface of the positive electrode active material, thereby reducing lithium ion diffusion resistance and suppressing deformation of the crystalline structure and particle cracking during charge and discharge, thereby improving life characteristics. However, if the cobalt-containing coating portion is formed as a continuous film on the surface of the lithium nickel-based oxide particles, the Co diffuses on the surface of the lithium nickel-based oxide, increasing the formation of an electrically inactive rock salt phase, which again increases resistance. Therefore, in the present invention, the heat treatment temperature during coating formation is adjusted to form the coating portion in an island shape, thereby maximizing the improvement in resistance and life characteristics. Specifically, the coating portion may be formed over an area of ​​10% to 60%, preferably 10% to 50%, of the total surface area of ​​the lithium nickel-based oxide particles. When the area of ​​the coating portion satisfies the above range, the improvement in resistance and life characteristics is most excellent.

[0047] Preferably, the coating portion may be formed by distributing particulate lithium cobalt oxide sporadically on the surface of lithium nickel-based oxide particles. The coating portion according to the present invention is manufactured by mixing a cobalt-containing coating material with a lithium nickel-based oxide in the form of a single particle or quasi-single particle followed by heat treatment. During the heat treatment, the lithium remaining on the surface of the lithium nickel-based oxide reacts with the cobalt of the coating material to produce lithium cobalt oxide. When the coating portion is formed in this manner, the washing process for removing the residual lithium can be omitted, thereby simplifying the process. In addition, during the water washing process, the Ni oxidation number on the surface of the positive electrode active material particles becomes electrically inactive. 2+ However, if the water washing step is omitted, the change in the Ni oxidation number can be minimized, which has the effect of suppressing the increase in resistance.

[0048] Meanwhile, the lithium cobalt oxide may have a particle size of 100 nm to 1000 nm, preferably 300 nm to 700 nm, and more preferably 400 nm to 600 nm. When the particle size of the coating portion satisfies this range, the coating portion effectively suppresses the formation of an inactive rock salt phase on the surface of the lithium nickel-based oxide, resulting in excellent life performance at high temperatures.

[0049] Meanwhile, in the cathode active material of the present invention, the intensity at 855.5 eV is greater than the intensity at 853 eV in the Ni L3-edge spectrum measured by electron energy loss spectroscopy on the surface of the lithium nickel-based oxide in contact with the coating portion.

[0050] Electron energy loss spectroscopy (EELS) is a method for obtaining desired information, such as the element, composition, and chemical bonding state of a reaction site, by measuring the energy loss of inelastically scattered electrons that occur when accelerated electrons interact with a sufficiently thin specimen. EELS spectra can be divided into a low-loss excitation region, which is the region with an energy loss of 0 to 50 eV, and a high-loss region, which is the region with an energy loss of 50 eV or more. The high-loss region reveals the ionization edge, which can be used to analyze the oxidation state of metals. In this invention, the Ni L3-edge spectrum was used to analyze the oxidation state of Ni. The EELS spectrum was obtained using HR-TEM equipment. The horizontal axis represents the loss energy region, and the vertical axis represents the peak intensity.

[0051] In the Ni L3-edge spectrum measured by EELS, 3+ The ions are shown around 855.5 eV, and Ni 2+The peak intensity around 855.5 eV is high, which means that Ni ions are present at around 853 eV. 3+ This means that there are many Ni ions, and when the peak intensity around 853 eV is large, 2+ In other words, the fact that the intensity at 855.5 eV is greater than the intensity at 853 eV in the Ni L3-edge spectrum indicates that there are many Ni ions in that region. 3+ This means that there are more ions, which indicates that the average oxidation number of nickel on the surface of the lithium nickel-based oxide in contact with the coating is +3 or higher. When the average oxidation number of nickel on the coating and the surface of the lithium nickel oxide is high, at +3 or higher, the resistance decreases, resulting in improved capacity and output characteristics.

[0052] Specifically, in the positive electrode active material of the present invention, in the Ni L3-edge spectrum measured by electron energy loss spectroscopy on the surface of the lithium nickel-based oxide in contact with the coating portion, the intensity at 853 eV (I 853 ) at 855.5 eV (I 855.5 ) ratio (I 855.5 / I 853 ) may be 1 to 3, preferably 1.2 to 3, and more preferably 1.5 to 3. I 855.5 / I 853 When the above range is satisfied, the resistance characteristics and life characteristics are particularly excellent.

[0053] On the other hand, in the positive electrode active material of the present invention, in the Ni L3-edge spectrum measured by electron energy loss spectroscopy on the surface of the lithium nickel-based oxide not covered with the coating portion, the intensity at 855.5 eV is preferably smaller than the intensity at 853 eV (i.e., the average oxidation number of nickel is less than +3).

[0054] Ni on the surface of the positive electrode active material 2+ The presence of a large number of ions has the advantage of reducing side reactions with the electrolyte, but 2+ The problem is that the resistance increases because the ions are electrically inactive. 3+ and Ni 4+ When nickel ions with a high oxidation number like this are present on the surface of the positive electrode active material, excellent electrical properties such as capacity and output are exhibited, but the high reactivity increases side reactions with the electrolyte, accelerating the structural collapse of the positive electrode active material and resulting in reduced life characteristics.

[0055] However, if the oxidation number of nickel is low on the surface not covered by the coating and high on the surface covered by the coating, the coating will prevent the highly reactive +3 or +4 valent nickel from coming into contact with the electrolyte, and the less reactive Ni will be present in the area that comes into direct contact with the electrolyte. 2+ The arrangement of ions can reduce resistance while minimizing side reactions with the electrolyte.

[0056] Meanwhile, the positive electrode active material according to the present invention may have an average particle size of nodules 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 particle size of the nodules satisfies this range, a positive electrode active material in the form of a single particle or quasi-single particle with excellent electrochemical properties can be formed. If the average particle size of the nodules is too small, the number of particle agglomerates forming the lithium nickel-based oxide particles increases, reducing the effect of suppressing particle breakage during rolling. If the average particle size of the nodules is too large, the lithium diffusion path within the particles becomes longer, increasing resistance and potentially reducing output characteristics.

[0057] The positive electrode active material has an average particle size D 50 The D of the positive electrode active material may be 2 μm to 6 μm, preferably 2 μm to 5 μm, and more preferably 3 μm to 5 μm. 50 If the D is very small, it is difficult to form an active material layer during electrode manufacturing, which reduces the electrolyte impregnation and the electrochemical properties. 50 If the resistance is too large, the resistance increases and the output characteristics deteriorate.

[0058] 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 for the production of a single-particle or quasi-single-particle positive electrode active material with excellent resistance characteristics. Typically, single-particle 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 alone while the crystallite size is small can result in the formation of a rock salt phase on the nodule surface, resulting in increased resistance. However, increasing both the average crystallite size and the average particle size of the nodules can minimize the formation of a rock salt phase and suppress the increase in resistance.

[0059] Method for producing positive electrode active material Next, a method for producing the positive electrode active material according to the present invention will be described.

[0060] The method for manufacturing a cathode active material according to the present invention includes the steps of (1) mixing a transition metal precursor and a lithium source material and calcining the mixture to prepare a lithium nickel-based oxide in a monoparticle or quasi-monoparticle form, and (2) mixing the lithium nickel-based oxide with a cobalt-containing coating material and then heat-treating the mixture at a temperature of 600°C to 750°C to form a coating portion.

[0061] (1) Lithium nickel oxide manufacturing stage First, a transition metal precursor and a lithium source material are mixed and then primarily fired to prepare a lithium nickel-based oxide in the form of a single particle or a similar single particle.

[0062] In this case, 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 known in the art.

[0063] Preferably, the transition metal precursor used in the present invention may be a transition metal hydroxide containing nickel and cobalt, with the Ni content of the total transition metals being 80 mol% or more, 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 realized.

[0064] Specifically, the transition metal precursor may have a composition represented by the following [Chemical Formula 2].

[0065] [Chemical formula 2] Ni x Co y M 1 z M 2 w (OH)2 In the above formula 2, M 1 is Mn, Al or a combination thereof, preferably Mn or a combination of Mn and Al.

[0066] The above-mentioned M 2 is one or more selected from the group consisting of Zr, W, Y, Ba, Ca, Sr, Ti, Mg, Ta, and Nb, preferably one or more selected from the group consisting of Zr, Y, Mg, and Ti, and more preferably Zr, Y, or a combination thereof.

[0067] The above-mentioned x represents the molar ratio of nickel in all the 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 the above range, it exhibits a high energy density and enables the realization of a high capacity.

[0068] The above-mentioned y represents the molar ratio of cobalt in all the metals in the precursor, and may 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.

[0069] The above-mentioned z represents the molar ratio of the M 1 element in all the metals in the precursor, and may be 0 < z < 0.2, 0 < z < 0.18, or 0.01 ≦ z ≦ 0.17. When the molar ratio of the M 1 element satisfies the above range, excellent structural stability of the positive electrode active material appears.

[0070] The above-mentioned w represents the molar ratio of the M 2 element in all the metals in the precursor, and may be 0 ≦ w ≦ 0.1 or 0 ≦ w ≦ 0.05.

[0071] For example, the transition metal precursor may be produced by introducing a transition metal aqueous solution, an ammonium cation complex-forming agent, and a basic compound into a reactor and stirring while allowing a coprecipitation reaction to proceed.

[0072] 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 contains M1 Raw material and / or M 2 It may further comprise a metal-containing source material.

[0073] Meanwhile, the transition metal-containing raw material may be acetate, carbonate, nitrate, sulfate, halide, sulfide, or oxide of the transition metal.

[0074] Specifically, the nickel-containing source material may be, for example, NiO, NiCO3·2Ni(OH)2·4H2O, NiC2O2·2H2O, Ni(NO3)2·6H2O, NiSO4, NiSO4·6H2O, nickel halides, or combinations thereof.

[0075] The cobalt-containing source material may be, for example, CoSO4, Co(OCOCH3)2·4H2O, Co(NO3)2·6H2O, CoSO4·7H2O, or a combination thereof.

[0076] 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, Mn3O4MnCO3, 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 does not matter if it is not added to the transition metal aqueous solution but is added together with the lithium source material in the calcination step described below.

[0077] M 2 The raw materials contained are M 2 The metal may be an acetate, carbonate, nitrate, sulfate, halide, sulfide, or oxide.

[0078] 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.

[0079] Meanwhile, 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.

[0080] 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.

[0081] 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.

[0082] In this case, the aqueous transition metal solution, the ammonium cation complexing agent, and the basic compound are added in amounts such that the pH of the reaction solution falls within a desired range.

[0083] Once the precursor particles are formed as described above, the particles are separated from the reaction solution to obtain the transition metal precursor. For example, the reaction solution may be filtered to separate the transition metal precursor from the reaction solution, and the separated transition metal precursor may then be washed with water and dried to obtain the transition metal precursor. In this case, processes such as pulverization and / or aliquoting may be performed as necessary.

[0084] Next, the transition metal precursor and the lithium source material are mixed and then calcined 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 may be mixed together and fired, 1 Contains raw materials and / or M 2 Specific examples of the raw material contained are as described above.

[0085] 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.

[0086] 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 crystal structure of the cathode active material is developed, and a cathode active material with excellent capacity characteristics and structural stability can be produced.

[0087] Meanwhile, the calcination is performed at a temperature that allows the formation of single particles or quasi-single particles. To form single particles or quasi-single particles, the calcination must be performed at a higher temperature than when conventional lithium nickel-based oxides in the form of secondary particles are prepared. For example, when the precursor composition is the same, the calcination temperature must be about 30°C to 100°C higher than when conventional lithium nickel-based oxides in the form of secondary particles are prepared. The calcination temperature for forming single particles or quasi-single particles may vary depending on the metal composition of the precursor. For example, when a high-nickel (Ni) lithium nickel-based oxide having a nickel (Ni) content of 80 mol% or more is to be formed as single particles or quasi-single particles, the calcination temperature may be about 800°C to 1000°C, preferably 800°C to 950°C, and more preferably 850°C to 950°C. When the calcination temperature satisfies the above range, a lithium nickel-based oxide in the form of single particles or quasi-single particles with excellent electrochemical properties can be prepared. If the calcination temperature is less than 800°C, a secondary particle-shaped positive electrode active material is produced, and if the temperature exceeds 1000°C, excessive calcination occurs, resulting in the formation of a layered crystal structure and degraded electrochemical properties.

[0088] The firing may be carried out in an oxygen atmosphere for 5 to 35 hours, preferably 5 to 20 hours, and more preferably 6 to 15 hours. In this specification, the term "oxygen atmosphere" refers to an atmosphere containing sufficient oxygen for firing, including air. In particular, firing is preferably carried out in an atmosphere with a higher oxygen partial pressure than air.

[0089] The lithium nickel-based oxide produced by the above-described firing has a single particle consisting of one nodule, or a quasi-single particle form that is a composite of 30 or less nodules, preferably 2 to 20 nodules, and more preferably 2 to 10 nodules.

[0090] In addition, the lithium nickel-based oxide particles may have a composition in which the nickel content of all metals excluding lithium is 80 mol% or more, preferably 82 mol% or more, and more preferably 83 mol% or more, and more specifically, the lithium nickel cobalt manganese-based oxide may have a nickel content of all metals excluding lithium of 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, a high energy density can be achieved.

[0091] More specifically, the lithium nickel-based oxide particles may have a composition represented by the following [Chemical Formula 1].

[0092] [Chemical formula 1] Li a Ni b Co c M 1 d M 2 e O2

[0093] In the above formula 1, M 1 may be Mn, Al or a combination thereof, preferably Mn or Mn and Al.

[0094] Said M 2 is at least one selected from the group consisting of Zr, W, Y, Ba, Ca, Sr, 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 elements are not essential, when contained in an appropriate amount, they can promote grain growth during firing or improve the stability of the crystal structure.

[0095] Said a represents the molar ratio of lithium in the lithium nickel-based oxide, and may 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.

[0096] Said b represents the molar ratio of nickel in all metals excluding lithium in the lithium nickel-based oxide, and may 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.

[0097] Said c represents the cobalt molar ratio in all metals excluding lithium in the lithium nickel-based oxide, and may 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.

[0098] Said d represents the molar ratio of M 1 element in all metals excluding lithium in the lithium nickel-based oxide, and may be 0 < d < 0.2, 0 < d < 0.18, or 0.01 ≦ d ≦ 0.17. When the molar ratio of M 1 element satisfies the above range, the structural stability of the positive electrode active material is excellently manifested.

[0099] Said e represents the molar ratio of M 2 element in all metals excluding lithium in the lithium nickel-based oxide, and may be 0 ≦ e ≦ 0.1, or 0 ≦ e ≦ 0.05.

[0100] (2) Coating part formation stage Next, after mixing the lithium nickel-based oxide in the form of single particles or quasi-single particles and the cobalt-containing coating substance, heat treatment is performed to form a coating part.

[0101] The cobalt-containing coating material may be, for example, one or more selected from the group consisting of Co3O4, Co(OH)2, Co2O3, Co3(PO4)2, CoF3, Co(OCOCH3)2·4H2O, Co(NO3)·6H2O, Co(SO4)2·7H2O, and CoC2O4, preferably one or more selected from Co3O4 and Co(OH)2, more preferably Co(OH)2, but is not limited thereto.

[0102] The cobalt-containing coating material may have an average particle size of 100 nm to 1000 nm, preferably 300 nm to 700 nm, and more preferably 400 nm to 600 nm. When the average particle size of the cobalt-containing coating material is within this range, it can be smoothly coated on the surface of the lithium nickel-based oxide, and the formation of a rock salt phase at the interface between the coating portion and the lithium nickel-based oxide is suppressed, resulting in an excellent improvement in resistance characteristics.

[0103] Meanwhile, the lithium nickel-based oxide and the cobalt-containing coating material may be mixed in a solid phase or a liquid phase. However, in the case of liquid phase mixing, the process is complicated and the lithium nickel-based oxide may be damaged by a solvent during the coating process. Therefore, it is more preferable to mix them in a solid phase.

[0104] In addition, the lithium nickel-based oxide and the cobalt-containing coating material may be mixed in a weight ratio of 100:1 to 100:8, preferably 100:1 to 100:6, and more preferably 100:2 to 100:5. When the mixing ratio of the lithium nickel-based oxide and the cobalt-containing coating material satisfies this range, the nickel content in the positive electrode active material can be maintained at a certain level, and the amount of expensive cobalt used can be reduced, thereby saving costs and effectively suppressing an increase in resistance.

[0105] On the other hand, the heat treatment is preferably carried out in a temperature range of 600° C. to 750° C., preferably 600° C. to 700° C., and more preferably 650° C. to 700° C. When the heat treatment temperature during the formation of the coating layer satisfies the above range, the Co-containing coating portion suppresses the formation of a rock salt phase at the interface between the Co-containing coating portion and the lithium nickel-based oxide, and the coating portion is formed in an island shape on part of the surface of the lithium nickel-based oxide.

[0106] 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 performed after calcination to reduce the content of lithium by-products. However, the inventors' research has shown that performing a water washing step during the manufacture of a single-particle or quasi-single-particle cathode active material can degrade the surface properties of the lithium nickel-based oxide and increase its resistance. Therefore, when manufacturing the cathode active material according to the present invention, water washing is not performed, and the remaining lithium on the surface of the lithium nickel-based oxide is consumed through a coating formation process, which is more effective in improving resistance properties.

[0107] positive electrode Next, the positive electrode according to the present invention will be described.

[0108] 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 according to the present invention. The positive electrode active material has been described above, so the remaining components will be described below.

[0109] The positive electrode current collector in the positive electrode is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector may typically have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the positive electrode current collector to enhance adhesion of the positive electrode active material. The positive electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0110] The positive electrode active material layer may contain a conductive material and a binder in addition to the positive electrode active material.

[0111] The conductive material is used to impart conductivity to the electrode. Any material that exhibits electronic conductivity without causing chemical changes in the resulting battery can be used without any 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 may 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.

[0112] The binder functions to improve 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. One or more of 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.

[0113] The positive electrode may be manufactured by a conventional method for manufacturing a positive electrode. For example, the positive electrode may be manufactured 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 coated positive electrode slurry. In this case, the types and contents of the positive electrode active material, binder, and conductive material are as described above.

[0114] 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 the solvent used is sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, and to provide a viscosity that allows excellent thickness uniformity when the slurry is subsequently applied to produce a positive electrode, taking into consideration the coating thickness and manufacturing yield of the slurry.

[0115] Alternatively, the positive electrode may be manufactured by casting the positive electrode slurry on a separate support, peeling the film from the support, and laminating the film on a positive electrode current collector.

[0116] Lithium secondary battery Next, the lithium secondary battery according to the present invention will be described.

[0117] 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.

[0118] 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.

[0119] The negative electrode current collector may be any material having high conductivity without inducing chemical changes in the battery, and may be, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or aluminum-cadmium alloy. The negative electrode current collector may typically have a thickness of 3 to 500 μm. As with the positive electrode current collector, the surface of the current collector may be formed with fine irregularities to enhance the binding strength of the negative electrode active material. The negative electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0120] The negative electrode active material layer includes a negative electrode active material, and optionally a binder and a conductive material.

[0121] 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.

[0122] The negative electrode active material may be a metallic lithium thin film. The carbon material may be either low-crystalline or high-crystalline carbon. Typical low-crystalline carbons include soft carbon and hard carbon, while typical high-crystalline carbons include amorphous, plate-like, scaly, spherical, or fibrous natural or artificial graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbons such as petroleum or coal tar pitch-derived cokes.

[0123] The conductive material is used to impart conductivity to the electrode. Any material that exhibits electronic conductivity without causing chemical changes in the resulting battery can be used without any 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 may 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.

[0124] The binder functions to improve 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.

[0125] 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 resulting film from the support, and laminating the resulting film on the negative electrode current collector.

[0126] 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 limitation. In particular, a separator that exhibits low resistance to electrolyte ion movement and excellent electrolyte humidification capability is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin 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, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material may be used, and may be used in a single-layer or multi-layer structure.

[0127] 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.

[0128] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0129] The organic solvent may be any organic solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the organic solvent may be an ester solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; dibutyl ether; ether-based solvents such as ether or tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; carbonate-based solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylenecarbonate (EC), and propylenecarbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (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; or sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant, which can improve the charge / discharge performance of batteries, and low-viscosity linear carbonate-based compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred.

[0130] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt may be LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, or LiB(C, O) . The lithium salt may be used at a concentration of 0.1 to 5.0 M, preferably 0.1 to 3.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0131] 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, triethanolamine, 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-methoxyethanol, and aluminum trichloride, which may be used alone or in combination. The additives may be present in an amount of 0.1 wt % to 10 wt %, preferably 0.1 wt % to 5 wt %, based on the total weight of the electrolyte.

[0132] As described above, the lithium secondary battery including the cathode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in the fields of portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0133] Therefore, according to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0134] The battery module or battery pack may be used as a power source for one or more medium- to large-sized devices, such as a power tool; an electric vehicle (EV), a hybrid electric vehicle, and an electric plug-in hybrid electric vehicle (PHEV); or a power storage system.

[0135] Although the present invention may be embodied in many different forms, it is to be understood that the invention is not limited to the embodiments set forth herein, and that the invention may be embodied in various different forms, rather than in a limited manner, as described below.

[0136] Manufacturing Example 1 Transition metal precursor Ni 0.86 Co 0.08 Mn 0.06 (OH)2 and LiOH·H2O were added to a 700 L Henschel mixer so that the weight ratio of transition metals (Ni + Co + Mn):Li was 1:1.05, and the mixture was mixed at 400 rpm for 20 minutes. The mixed powder was placed in an alumina crucible measuring 330 mm x 330 mm and fired at 890 °C for 12 hours in an oxygen (O2) atmosphere to produce lithium nickel oxide Li[Ni 0.86 Co 0.08 Mn 0.06 ]O2 was produced.

[0137] Manufacturing Example 2 Transition metal precursor Ni 0.93 Co 0.05 Mn 0.02(OH)2 and LiOH·H2O were added to a 700 L Henschel mixer so that the weight ratio of transition metals (Ni + Co + Mn):Li was 1:1.05, and the mixture was mixed at 400 rpm for 20 minutes. The mixed powder was placed in an alumina crucible measuring 330 mm x 330 mm and fired at 855 °C for 15 hours in an oxygen (O2) atmosphere to obtain lithium nickel oxide Li[Ni 0.93 Co 0.05 Mn 0.02 ]O2 was produced.

[0138] Example 1 The lithium nickel-based oxide prepared in Preparation Example 1 and Co(OH)2 were mixed in a weight ratio of 100:3 and heat-treated at 700°C for 5 hours to prepare a positive electrode active material in which a Co-containing coating portion was formed on the surface of the lithium nickel-based oxide in the form of a single particle or quasi-single particle.

[0139] Example 2 The lithium nickel-based oxide prepared in Preparation Example 1 and Co(OH)2 were mixed in a weight ratio of 100:1.5 and heat-treated at 700°C for 5 hours to prepare a positive electrode active material in which a Co-containing coating portion was formed on the surface of the lithium nickel-based oxide in the form of a single particle or quasi-single particle.

[0140] Example 3 The lithium nickel-based oxide prepared in Preparation Example 2 and Co(OH)2 were mixed in a weight ratio of 100:2 and heat-treated at 660°C for 5 hours to prepare a positive electrode active material in which a Co-containing coating portion was formed on the surface of the lithium nickel-based oxide in the form of a single particle or quasi-single particle.

[0141] Comparative Example 1 The lithium nickel-based oxide prepared in Preparation Example 1 and Co(OH)2 were mixed in a weight ratio of 100:3 and heat-treated at 800°C for 5 hours to prepare a positive electrode active material in which a Co-containing coating portion was formed on the surface of a lithium nickel-based oxide in the form of a single particle or quasi-single particle.

[0142] Comparative Example 2 The lithium nickel-based oxide prepared in Preparation Example 1 was used as a positive electrode active material.

[0143] Comparative Example 3 The lithium nickel-based oxide prepared in Preparation Example 2 and Co(OH)2 were mixed in a weight ratio of 100:2 and heat-treated at 800°C for 5 hours to prepare a cathode active material in which a Co-containing coating portion was formed on the surface of the lithium nickel-based oxide in the form of a single particle or quasi-single particle.

[0144] Experimental example 1: Checking the shape of the coating The particle morphology and coating shape of the positive electrode active materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were observed through SEM and EDS analysis. Figures 1 to 6 show the SEM and EDS analysis results of the positive electrode active materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3, respectively.

[0145] 1 to 6, it can be seen that the positive electrode active materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 contain lithium nickel-based oxide in a quasi-single particle form, which is a complex of 10 or less nodules.

[0146] 1 to 6, it can be seen that the positive electrode active materials prepared in Examples 1 to 3 are coated with sporadically distributed island-like micrometer-sized particles containing Co on a portion of the surface of the quasi-single particle lithium nickel-based oxide. The area of ​​the coated portion is approximately 20% or less of the total surface area of ​​the lithium nickel-based oxide particles.

[0147] Meanwhile, referring to FIGS. 4 and 6, it can be seen that the positive electrode active materials prepared according to Comparative Examples 1 and 3 have a continuous film-like Co-containing coating portion formed on the surface of the quasi-single particle lithium nickel-based oxide.

[0148] Also, referring to FIG. 5, it can be seen that the positive electrode active material of Comparative Example 2 does not have a Co-containing coating portion on the surface of the quasi-single particle lithium nickel-based oxide.

[0149] Experimental Example 2 EELS (Electron Energy Loss Spectroscopy) analysis was performed on the interface between the surface of the lithium nickel-based oxide and the coating portion of each of the positive electrode active materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 to obtain a Ni L3-edge spectrum. In the spectrum, the ratio of the intensity at 855.5 eV to the intensity at 853 eV was calculated as I 855.5 / I 853 The measurement results are shown in Table 1 below.

[0150] [Table 1]

[0151] 7 to 11 show high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) spectrum images of the positive electrode active materials prepared in Example 1 and Comparative Examples 1 and 2.

[0152] FIG. 7 shows a High-Angle Annular Dark-Field (HAADF) image and an Electron Energy Loss Spectroscopy (EELS) spectrum image of the positive electrode active material prepared in Example 1, and FIG. 8 shows the EELS Ni L3-edge spectrum of regions 1 to 3 of the EELS spectrum image of FIG. 7.

[0153] FIG. 9 shows an HAADF image and an EELS spectrum image of the positive electrode active material prepared according to Comparative Example 1, and FIG. 10 shows the EELS Ni L3-edge spectrum of regions 1 and 2 of the EELS spectrum image of FIG. 9.

[0154] FIG. 11 shows an HAADF image and an EELS spectrum image of the positive electrode active material prepared according to Comparative Example 2, and FIG. 12 shows the EELS Ni L3-edge spectrum of regions 1 to 3 of the EELS spectrum image of FIG.

[0155] 7 and 8, in the case of the cathode active material prepared according to Example 1, the intensity at 855.5 eV in the EELS Ni L3-edge spectrum of Region 3, which is the surface of the lithium nickel-based oxide in contact with the Co coating, is greater than the intensity at 853 eV, confirming that the nickel oxidation state in Region 3 is high (+3 or greater) through the EELS spectrum image. In contrast, the intensity at 855.5 eV in the EELS Ni L3-edge spectrum of Region 1, where no coating is present, is less than the intensity at 853 eV, confirming that the nickel oxidation state in Region 3 is low (less than +3) through the EELS spectrum image.

[0156] Meanwhile, referring to FIGS. 9 and 10 , in the case of the cathode active material of Comparative Example 1 in which a Co-containing coating was formed on the entire surface of the lithium nickel-based oxide, the EELS Ni L3-edge spectrum of Region 1, which is the interface between the coating and the lithium nickel-based oxide, shows similar intensities at 855.5 eV and 853 eV, confirming that the nickel oxidation number in Region 1 in the EELS spectrum image is low, being less than +3.

[0157] 11 and 12, in the case of the cathode active material of Comparative Example 2 in which no Co coating portion was formed, it was confirmed that Ni with a low oxidation number, i.e., a valence of +2, was distributed over the entire surface of the lithium nickel-based oxide.

[0158] <Secondary battery manufacturing> Each of the positive electrode active materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3, a carbon black conductive material, and a PVDF binder were mixed in N-methylpyrrolidone at a weight ratio of 95:2:3 to prepare a 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.

[0159] A lithium metal electrode was used as the negative electrode.

[0160] The 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 lithium secondary battery. 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 3:4:3.

[0161] Experimental Example 3: Evaluation of initial charge / discharge The initial charge / discharge capacity of each of the prepared lithium secondary batteries was measured by charging them to 4.25 V in CC-CV mode and then discharging them to 2.5 V in CC mode. In this case, the lithium secondary batteries using the positive electrode active materials of Examples 1 and 2 and Comparative Examples 1 and 2 were charged and discharged at a 0.1 C rate, and the lithium secondary batteries using the positive electrode active materials of Example 3 and Comparative Example 3 were charged and discharged at a 0.2 C rate.

[0162] Experimental example 4: Evaluation of 45℃ cycle characteristics Each lithium secondary battery fabricated as described above was charged at 45°C in CC-CV mode at 0.5 C up to 4.3 V and then discharged at a constant current of 1.0 C down to 2.5 V for 50 cycles, and then the capacity retention rate and the resistance increase rate were measured to evaluate the lifespan characteristics. The measurement results are shown in Table 2 below and FIG. 13.

[0163] [Table 2]

[0164] From Table 2 and FIG. 13, it can be seen that the lithium secondary batteries using the positive electrode active materials of Examples 1 to 3 have superior initial charge / discharge capacity and life characteristics compared to the lithium secondary batteries using the positive electrode active materials of Comparative Examples 1 to 3, and in particular, the resistance increase rate is significantly reduced.

[0165] Experimental example 5: Evaluation of resistance characteristics by SOC Each of the lithium secondary batteries fabricated above was charged and discharged once at 0.2C / 0.2C, and then each SOC state was set at 0.2C. Then, a current of 2.5C was applied for 10 seconds, and the resistance was measured based on the change in voltage with respect to the application of a current of 2.5C. The measurement results are shown in Figure 14.

[0166] From FIG. 14, it can be seen that the lithium secondary battery employing the positive electrode active material of the example in which the Co coating portion is formed in an island shape on a part of the surface of the lithium nickel-based oxide particle and the Ni L3-edge spectrum on the surface of the lithium nickel-based oxide particle in contact with the coating portion satisfies the conditions of the present invention has lower resistance characteristics than the lithium secondary battery employing the positive electrode active material of the comparative example.

Claims

1. a lithium nickel-based oxide in a quasi-single particle form that is a single particle consisting of one nodule or a composite of 30 or less nodules; a coating portion containing Co, the coating portion being formed in an island shape on a part of the surface of the lithium nickel-based oxide particle, In a Ni L3-edge spectrum obtained by measuring the surface of the lithium nickel-based oxide in contact with the coating portion by electron energy loss spectroscopy, the intensity at 855.5 eV is greater than the intensity at 853 eV; The coating portion is a positive electrode active material in which particulate lithium cobalt oxide is distributed.

2. 2. The cathode active material of claim 1, wherein a Ni L3-edge spectrum obtained by measuring a surface of the lithium nickel-based oxide on which the coating portion is not formed by electron energy loss spectroscopy has an intensity at 855.5 eV lower than an intensity at 853 eV.

3. 2. The positive electrode active material according to claim 1, wherein the coating portion is formed over an area of ​​10% to 60% of the total surface area of ​​the lithium nickel-based oxide particles.

4. 2. The positive electrode active material according to claim 1, wherein the lithium cobalt oxide has a particle size of 100 nm to 1000 nm.

5. The positive electrode active material according to claim 1, wherein the lithium nickel-based oxide has a composition 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 Formula 1, M １ is Mn, Al or a combination thereof, M ２ are Zr, W, Ti, Mg, Ca, Sr, and Ba, and 0.8≦a≦1.2, 0.82≦b<1, 0.01≦c≦0.15, 0.01≦d≦0.15, and 0≦e≦0.

1.

6. A method for producing the positive electrode active material according to claim 1, comprising: mixing a transition metal precursor and a lithium source material and calcining the mixture to prepare a lithium nickel-based oxide in a single particle or quasi-single particle form; and mixing the lithium nickel-based oxide with a cobalt-containing coating material, and then heat-treating the mixture at a temperature of 600 to 750° C. to form a coating portion.

7. The cobalt-containing coating material is Co ３ O ４ , Co(OH) ２ , Co ２ O ３ , Co ３ (P.O. ４ ) ２ , CoF ３ , Co(OCOCH ３ ) ２ ・4H ２ O, Co(NO ３ ) 6H ２ O, Co(SO ４ ) ２ ・7H ２ O and CoC ２ O ４ The method for producing a positive electrode active material according to claim 6 , wherein the active material is one or more selected from the group consisting of:

8. 7. The method of claim 6, wherein the cobalt-containing coating material has an average particle size of 100 nm to 1000 nm.

9. 7. The method of claim 6, wherein the lithium nickel-based oxide and the cobalt-containing coating material are mixed in a weight ratio of 100:1 to 100:

8.

10. 10. The method for producing a positive electrode active material according to claim 6, wherein the transition metal precursor is a nickel-cobalt-manganese hydroxide having a Ni content of 80 mol% or more.

11. The method of claim 6, wherein the forming of the coating portion is performed without a water washing process after the forming of the lithium nickel-based oxide.

12. A positive electrode comprising a positive electrode active material layer containing the positive electrode active material according to claim 1 .

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

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