Positive electrode active material and lithium secondary battery comprising same

Abstract

The present invention relates to a positive electrode active material and a lithium secondary battery comprising same. More specifically, the present invention relates to a positive electrode active material and a lithium secondary battery comprising same, the positive electrode active material comprising a mid-Ni-type lithium transition metal oxide having a nickel (Ni) content of 50 mol% to 70 mol%, wherein the positive electrode active material is capable of securing excellent particle strength and structural stability even in a polycrystalline structure by suppressing the growth of crystallites and stabilizing grain boundaries in the lithium transition metal oxide.

Description

positive electrode active material and lithium secondary battery containing the same

[0001] The present invention relates to a positive electrode active material and a lithium secondary battery containing the same. More specifically, the present invention relates to a positive electrode active material comprising a Mid-Ni type lithium transition metal oxide having a nickel (Ni) content of 50 mol% or more and 70 mol% or less, wherein the growth of crystallites of the lithium transition metal oxide is suppressed and the grain boundaries are stabilized, thereby enabling excellent particle strength and structural stability even in a polycrystalline structure, and to a positive electrode active material and a lithium secondary battery containing the same.

[0002]

[0003] Due to their high energy density and excellent output characteristics, lithium-ion batteries have established themselves as a key power source in various applications, including portable electronic devices, electric vehicles, and energy storage systems. In particular, the cathode active material is one of the most critical components determining the electrochemical performance of lithium-ion batteries; its structural stability, charge / discharge characteristics, lifespan, and safety all depend on the composition and microstructure of the cathode active material.

[0004] As representative cathode active materials, lithium transition metal oxides containing nickel (Ni), cobalt (Co), and manganese (Mn), known as the NCM series, are being widely researched and commercialized. NCM-based cathode active materials can be classified into High-Ni (nickel content > 70%), Mid-Ni (nickel content 50~70%), and Low-Ni (nickel content ≤ 40%) depending on the composition ratio, and each composition has its own advantages and disadvantages.

[0005] High-Ni compositions offer the advantage of securing high capacity and energy density, but they suffer from problems such as degraded thermal stability and lifespan characteristics due to structural instability, oxygen release, and Li / Ni cation mixing. On the other hand, while low-Ni compositions offer excellent structural stability, their low theoretical capacity limits their application in high-energy applications. Accordingly, mid-Ni compositions, with a nickel (Ni) content in the range of 50–70 mol%, are attracting attention as a balanced composition capable of simultaneously securing relatively high capacity and stability.

[0006] Mid-Ni type cathode active materials generally exhibit good charge / discharge efficiency and cycle characteristics in lithium-ion batteries, making them suitable for applications requiring both high capacity and stability. However, even with these Mid-Ni type cathode active materials, improving particle structure and microstructural characteristics remains a significant challenge.

[0007] Among the microstructures of cathode active materials, the size of primary particles is an important factor. Generally, as the size of primary particles decreases, the diffusion path of lithium during the charge / discharge process becomes shorter, which can improve rate characteristics. However, if the size of primary particles is less than 1 μm, the possibility of adverse reactions with the electrolyte may increase due to the excessively large specific surface area. Furthermore, mechanical stress may concentrate within the particles due to lattice shrinkage and expansion that occur repeatedly during the charge / discharge process, leading to a decrease in particle fracture strength. This mechanical brittleness is more pronounced in Mid-Ni type cathode active materials with a polycrystalline structure, and consequently leads to the fragmentation or cracking of secondary particles, causing a decrease in the battery's lifespan characteristics.

[0008] As such, there is a need for a technology that can effectively secure particle strength and suppress crack formation in Mid-Ni type cathode active materials having a polycrystalline structure with a primary particle size of less than 1 μm.

[0009]

[0010] So-called Mid-Ni type lithium transition metal oxides with a nickel (Ni) content of 50 mol% or more and 70 mol% or less have the advantage of having superior structural stability compared to High-Ni compositions with a nickel (Ni) content of more than 70 mol%, and can secure higher energy density than Low-Ni type lithium transition metal oxides.

[0011] However, there is a problem in that the mechanical strength of Mid-Ni type lithium transition metal oxides is not sufficiently secured in polycrystalline structures (e.g., when the average particle size of primary particles is less than 1 μm). Generally, as the size of primary particles decreases, electrochemical reactivity is favorable, but the particle fracture strength is reduced due to volume changes and lattice deformation during the charging / discharging process, which can consequently cause fracture or cracking of secondary particles and lead to a decrease in lifespan characteristics.

[0012] Meanwhile, improving the strength of the particles through single crystallization can be considered. As the size of the primary particles constituting the lithium transition metal oxide increases and the number of primary particles constituting the lithium transition metal oxide decreases, intergranular cracks can be reduced. However, there is a problem of becoming more susceptible to intragranular cracks, and as the size of the primary particles increases, the intragranular lithium ion diffusion path becomes longer, which may lead to a decrease in kinetic properties.

[0013] Therefore, it is an important task to improve mechanical reliability by securing the particle fracture strength of Mid-Ni type lithium transition metal oxides having a polycrystalline structure with an average particle size of less than 1 μm of primary particles.

[0014] Under this technical background, the present invention aims to provide a positive electrode active material comprising a Mid-Ni type lithium transition metal oxide having a polycrystalline structure, which suppresses the degradation of the lifespan characteristics of a lithium secondary battery by enhancing the particle strength and crystallinity of the lithium transition metal oxide.

[0015] In addition, another objective of the present invention is to provide a lithium secondary battery using the positive active material defined herein.

[0016] The objects of the present invention are not limited to those mentioned above, and other unmentioned objects and advantages of the present invention may be understood from the following description and will be more clearly understood by the embodiments of the present invention. Furthermore, it will be readily apparent that the objects and advantages of the present invention can be realized by the means and combinations thereof set forth in the claims.

[0017]

[0018] The present invention for solving the aforementioned technical problem includes the following invention.

[0019] [1] A positive electrode active material comprising a lithium transition metal oxide having a nickel (Ni) content of 50 mol% or more and 70 mol% or less among all metal elements excluding lithium, wherein the manganese (Mn) content in the lithium transition metal oxide is greater than the cobalt (Co) content, and the lithium transition metal oxide has the form of secondary particles formed by the aggregation of multiple primary particles having an average particle size of less than 1 μm, and the compressive fracture strength of the positive electrode active material is 12.2 kgf / mm 2 Above 20.0 kgf / mm 2 Lee Ha-in, positive active material.

[0020] [2] The lithium transition metal oxide described above is a positive electrode active material described in [1], having the form of secondary particles aggregated from a plurality of primary particles having an average particle size of 0.02 μm to 0.8 μm.

[0021] [3] The lithium transition metal oxide described above is a positive active material described in [1] or [2], having the form of secondary particles with an average particle size of 3 μm to 20 μm.

[0022] [4] The average crystallite size of the lithium transition metal oxide calculated by Rietveld Refinement on the diffraction spectrum obtained by X-ray diffraction analysis using Cu-Kα rays for the above positive active material is 110 nm or more and less than 148 nm, as described in any one of [1] to [3].

[0023] [5] The above lithium transition metal oxide is a positive electrode active material described in any one of [1] to [4], wherein the content of cobalt (Co) among all metal elements excluding lithium is less than 20 mol%.

[0024] [6] The above lithium transition metal oxide is a positive electrode active material described in any one of [1] to [5], wherein the content of manganese (Mn) among all metal elements excluding lithium is 20 mol% or more and less than 50 mol%.

[0025] [7] A positive active material described in any one of [1] to [6], wherein the particle breakage ratio calculated by Equation 1 by measuring the change in average particle size of secondary particles before and after 4.5 ton press of the positive active material is 0.20 or less.

[0026] [Equation 1]

[0027] Particle breakage ratio = (D50 press 전 - D50 press 후 ) / D50 press 전

[0028] [8] A positive active material described in any one of [1] to [7], in which at least one dopant is doped into the crystal lattice of the lithium transition metal oxide.

[0029] [9] The above dopant is at least one selected from W, Y and Zr, the positive active material described in [8].

[0030]

[0010] The above lithium transition metal oxide is an anode active material described in any one of [1] to [7] having an average composition represented by the following chemical formula 1:

[0031] [Chemical Formula 1]

[0032] Li a Ni 1-(b+c+d) Co b Mn c M1 d O2

[0033] In the above chemical formula 1,

[0034] M1 is at least one selected from Na, K, Mg, Ba, B, Ca, Ce, Hf, Ta, Cr, F, Al, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, W, Mo, P, Sr, Ge, Nd, Gd and Cu, and

[0035] 0.95≤a≤1.15, 0≤b<0.20, 0.20≤c<0.50, 0 <d≤0.10, 0.5≤1-(b+c+d)≤0.70이다.

[0036] In addition, the present invention includes other embodiments as follows.

[0037]

[0011] A positive electrode comprising a positive electrode active material described in any one of [1] to

[0010] .

[0038]

[0012] A lithium secondary battery using the positive electrode described in

[0011] .

[0039]

[0040] Although the cathode active material according to the present invention comprises a Mid-Ni type lithium transition metal oxide having a polycrystalline structure with an average primary particle size of less than 1 μm, it can improve particle fracture strength by improving the structural stability of the particles and strengthening crystallinity through doping with a specific dopant. Accordingly, the problem of reduced mechanical strength, which has been pointed out as a problem of conventional Mid-Ni type lithium transition metal oxides, can be effectively improved.

[0041] By improving the particle fracture strength of the lithium transition metal oxide constituting the cathode active material, the occurrence of fracture or cracking of secondary particles can be suppressed, thereby improving the stability of the electrode structure during the charge / discharge cycle of the lithium secondary battery. Accordingly, the lifespan characteristics of the lithium secondary battery are improved, and an excellent capacity retention rate can be secured even under repeated charge / discharge conditions.

[0042] In addition to the effects described above, the specific effects of the present invention are described together with the specific details for implementing the invention below.

[0043]

[0044] Figure 1 is an SEM image confirming particle breakage of the positive active material according to PAM4 before (a) and after (b) a 4.5-ton press according to the method described in Experimental Example 1.

[0045] Figure 2 is an SEM image confirming particle breakage of the positive active material according to PAM1 before (a) and after (b) a 4.5-ton press according to the method described in Experimental Example 1.

[0046]

[0047] For convenience, specific terms are defined herein to facilitate a better understanding of the present invention. Unless otherwise defined herein, scientific and technical terms used in this invention shall have the meanings generally understood by those skilled in the art. Furthermore, unless specifically indicated in the context, terms in their singular form shall be understood to include their plural form, and terms in their plural form shall be understood to include their singular form.

[0048]

[0049] positive electrode active material

[0050] In the present invention, the positive electrode active material comprises a lithium transition metal oxide capable of reversible intercalation / deintercalation of lithium ions. Here, a lithium transition metal oxide refers to an oxide formed by the complex of lithium and a metal element.

[0051] The above lithium transition metal oxide is a complex metal oxide capable of lithium ion intercalation / deintercalation and has a layered crystal structure belonging to the R-3m space group. The above lithium transition metal oxide having a layered crystal structure exhibits a specific peak in the region where 2θ is 18° to 20° (e.g., the region where 2θ = 18.6 ± 1°) among the rotation patterns obtained from XRD analysis.

[0052] The lithium transition metal oxide comprises at least lithium and a transition metal. The transition metal may comprise at least one, at least two, or all selected from nickel (Ni), cobalt (Co), and manganese (Mn).

[0053] Preferably, the lithium transition metal oxide may be a lithium nickel-based composite oxide containing nickel (Ni). Additionally, the lithium transition metal oxide may be a lithium nickel-based composite oxide containing nickel (Ni) and cobalt (Co).

[0054] In order to improve low rate characteristics and lifetime characteristics while maintaining the high reversible capacity of LiNiO2, the lithium nickel-based composite oxide may be a lithium transition metal oxide of the ternary type, such as so-called NCM (Ni-Co-Mn) and NCA (Ni-Co-Al), or the quaternary type, such as NCMA (Ni-Co-Mn-Al), in which a portion of nickel (Ni) is substituted with cobalt (Co), manganese (Mn), and / or aluminum (Al). The ternary or quaternary type lithium transition metal oxide may further include dopants other than nickel (Ni), cobalt (Co), manganese (Mn), and aluminum (Al).

[0055] The lithium transition metal oxide defined herein is a Mid-Ni type lithium transition metal oxide with a relatively low nickel (Ni) content. In the present invention, a lithium transition metal oxide in which the nickel (Ni) content among the transition metals is 70 mol% or less is defined as a Mid-Ni type lithium transition metal oxide, and a lithium transition metal oxide in which the nickel (Ni) content among the transition metals is greater than 70 mol% (e.g., 75 mol% or more or 80 mol% or more) is defined as a High-Ni type lithium transition metal oxide.

[0056] The above lithium transition metal oxide may have an average composition represented by the following chemical formula 1. The average composition of the above lithium transition metal oxide can be measured according to a known ICP analysis method using an inductively coupled plasma spectrometer (ICP).

[0057] [Chemical Formula 1]

[0058] Li a Ni 1-(b+c+d) Co b Mn c M1 d O2

[0059] In the above chemical formula 1,

[0060] M1 is at least one selected from Na, K, Mg, Ba, B, Ca, Ce, Hf, Ta, Cr, F, Al, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, W, Mo, P, Sr, Ge, Nd, Gd and Cu, and

[0061] 0.95≤a≤1.15, 0≤b<0.20, 0.20≤c<0.50, 0 <d≤0.10, 0.5≤1-(b+c+d)≤0.70이다.

[0062] a, representing the ratio of lithium to all elements other than lithium in the lithium transition metal oxide (Li / Ni+Co+Mn+M1), may be 0.95 or more, 0.96 or more, 0.97 or more, 0.98 or more, 0.99 or more, 1.00 or more, 1.01 or more, 1.02 or more, 1.03 or more, 1.04 or more, or 1.05 or more. Additionally, a may be 1.15 or less, 1.14 or less, 1.13 or less, 1.12 or less, 1.11 or less, 1.10 or less, 1.09 or less, 1.08 or less, 1.07 or less, 1.06 or less, or 1.05 or less. The upper and lower limits of the mole fraction of lithium (Li) to all elements other than lithium in the lithium transition metal oxide may be appropriately selected within a range that satisfies the definition described above. When the mole fraction of lithium (Li) among the above lithium transition metal oxides satisfies the above range, a stable crystal structure can be formed.

[0063] If a in the above chemical formula 1 is less than 0.95, the capacity of the positive electrode active material containing the lithium transition metal oxide represented by the above chemical formula 1 may decrease. On the other hand, if a in the above chemical formula 1 is greater than 1.15, phase separation may occur due to the excess lithium and manganese present in the lithium transition metal oxide, and an impurity phase belonging to a space group other than the R-3m space group may occur.

[0064] In the above chemical formula 1, 1-(b+c+d), representing the ratio of nickel (Ni) to all elements other than lithium in the lithium transition metal oxide (Ni / Ni+Co+Mn+M1), may be 0.50 or more, 0.51 or more, 0.52 or more, 0.53 or more, 0.54 or more, 0.55 or more, 0.56 or more, 0.57 or more, 0.58 or more, 0.59 or more, or 0.60 or more. Additionally, 1-(b+c+d) may be 0.70 or less, 0.69 or less, 0.68 or less, 0.67 or less, 0.66 or less, or 0.65 or less. The upper and lower limits of the mole fraction of nickel (Ni) to all elements other than lithium in the lithium transition metal oxide may be appropriately selected within a range that satisfies the definition described above. When the mole fraction of nickel (Ni) among the above lithium transition metal oxides satisfies the above range, a stable Mid-Ni type crystal structure can be formed.

[0065] When the nickel (Ni) content in the above lithium transition metal oxide exceeds 70 mol%, the lithium-nickel cation mixing within the crystal structure increases, which lowers the stability of the layered structure and increases the tendency for structural strain and oxygen release during the charge / discharge process, which may increase crystal lattice stress. These factors can lead to particle cracking and grain boundary weakening, thereby reducing the mechanical strength of the particles.

[0066] On the other hand, if the nickel (Ni) content in the lithium transition metal oxide is less than 50 mol%, phase separation may occur due to the presence of an excess amount of transition metals other than nickel (Ni) (e.g., manganese, etc.), and as a result, impurity phases such as a spinel phase or a rock-salt phase that do not belong to the R-3m space group may be formed. These impurity phases cause a degradation of electrochemical properties, which can lead directly to performance deterioration such as reduced capacity and reduced output.

[0067] Accordingly, the lithium transition metal oxide according to the present invention has a Mid-Ni composition of 50 mol% or more and 70 mol% or less, thereby ensuring a balance of structural stability and energy density while simultaneously improving particle strength and electrochemical properties.

[0068] In the above chemical formula 1, b, representing the ratio of cobalt (Co) to all elements other than lithium in the lithium transition metal oxide (Co / Ni+Co+Mn+M1), may be 0 or more, 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, or 0.05 or more. Additionally, b may be less than 0.20, 0.19 or less, 0.18 or less, 0.17 or less, 0.16 or less, 0.15 or less, less than 0.15, 0.14 or less, 0.13 or less, 0.12 or less, 0.11 or less, 0.10 or less, 0.09 or less, 0.08 or less, 0.07 or less, 0.06 or less, or 0.05 or less. The upper and lower limits of the mole fraction of cobalt (Co) relative to all elements other than lithium in the above lithium transition metal oxide can be appropriately selected within a range that satisfies the definition described above. When the mole fraction of cobalt (Co) in the above lithium transition metal oxide satisfies the above range, a stable crystal structure can be formed and good output characteristics can be exhibited.

[0069] In the above chemical formula 1, c, representing the ratio of manganese (Mn) to the total elements other than lithium in the lithium transition metal oxide (Mn / Ni+Co+Mn+M1), may be 0.20 or more, 0.21 or more, 0.22 or more, 0.23 or more, 0.24 or more, 0.25 or more, 0.26 or more, 0.27 or more, 0.28 or more, 0.29 or more, or 0.30 or more. Additionally, c may be less than 0.50, 0.45 or less, 0.40 or less, 0.35 or less, 0.34 or less, 0.33 or less, or 0.32 or less. When the mole fraction of manganese (Mn) in the lithium transition metal oxide satisfies the above ranges, a stable crystal structure can be formed.

[0070] In addition, from the perspective of improving the particle strength of a Mid-Ni type lithium transition metal oxide having a polycrystalline structure, it is desirable that the manganese (Mn) content in the lithium transition metal oxide be greater than the cobalt (Co) content.

[0071] Manganese (Mn) plays a role in providing structural stability to the particles of lithium transition metal oxides and improving the stability of the crystal lattice. Accordingly, if the manganese (Mn) content in the composition of the lithium transition metal oxide is sufficiently secured, internal stresses caused by lattice shrinkage and expansion resulting from repeated charge / discharge processes can be alleviated, and as a result, the mechanical reliability of the particles can be improved.

[0072] However, in the case of so-called High-Ni type lithium transition metal oxides with a nickel (Ni) content exceeding 70 mol%, the improvement in particle strength is not clearly evident even if the manganese (Mn) content is greater than the cobalt (Co) content. This is because, in High-Ni compositions, the lattice deformation and oxygen release tendencies occurring during the charge / discharge process act dominantly, so the influence of the relative ratio of manganese and cobalt on particle strength is limited.

[0073] On the other hand, in the Mid-Ni type, where the nickel (Ni) content is 50 mol% or more and 70 mol% or less, if the manganese (Mn) content is greater than the cobalt (Co), the crystal lattice stabilization effect is relatively large, and the mechanical strength of the particles can be improved.

[0074] In the above chemical formula 1, M1 represents a dopant that can be incorporated into the crystal structure of the lithium transition metal oxide.

[0075] The above lithium transition metal oxide has a layered crystal structure in which a lithium layer containing lithium and a transition metal layer containing a transition metal are alternately arranged, and at least one dopant may be incorporated into at least one crystal layer among the lithium layer and the transition metal layer, preferably the transition metal layer.

[0076] In the above chemical formula 1, d, representing the mole fraction of the dopant (M1 / Ni+Co+Mn+M1) relative to the total elements other than lithium in the lithium transition metal oxide, may be 0.10 or less, 0.09 or less, 0.08 or less, 0.07 or less, 0.06 or less, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, 0.01 or less, 0.009 or less, 0.008 or less, 0.007 or less, 0.006 or less, 0.005 or less, 0.004 or less, 0.003 or less, 0.002 or less, 0.0015 or less, or 0.001 or less. Additionally, d may be 0.0005 or more, or 0.001 or more. When the mole fraction of the dopant within the crystal lattice of the lithium transition metal oxide satisfies the above range, the likelihood of the crystallographic features of the lithium transition metal oxide intended in this invention being expressed increases.

[0077] The lithium transition metal oxide may comprise at least one selected from Na, K, Mg, Ba, B, Ca, Ce, Ca, Hf, Ta, Cr, F, Al, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, W, Mo, P, Sr, Ge, Nd, Gd, and Cu, preferably at least one selected from W, Mg, Y, Zr, Ca, and V, more preferably at least one selected from W, Y, and Zr. Additionally, the lithium transition metal oxide may comprise at least one dopant selected from W, Y, and Zr and at least one dopant selected from Na, K, Mg, Ba, B, Ca, Ce, Ca, Hf, Ta, Cr, F, Al, V, Ti, Fe, Zn, Si, Nb, Ga, Sn, Mo, P, Sr, Ge, Nd, Gd, and Cu.

[0078] When a dopant capable of causing local lattice distortion within the crystal lattice of the above lithium transition metal oxide is doped, it can provide a stabilization effect at the grain boundaries by partially inhibiting the growth of crystallites and effectively improve the mechanical strength of secondary particles.

[0079] Accordingly, when at least one dopant selected from W, Y, and Zr is doped into the crystal lattice of the lithium transition metal oxide, or when at least one dopant selected from W, Y, and Zr and at least one dopant selected from Na, K, Mg, Ba, B, Ca, Ce, Ca, Hf, Ta, Cr, F, Al, V, Ti, Fe, Zn, Si, Nb, Ga, Sn, Mo, P, Sr, Ge, Nd, Gd, and Cu are doped, the growth of crystallites of the lithium transition metal oxide can be suppressed, thereby improving the mechanical strength of the secondary particles.

[0080] The above lithium transition metal oxide has the form of secondary particles in which multiple primary particles are aggregated.

[0081] The primary particles may have a rod shape, an elliptical shape, and / or an irregular shape. Additionally, primary particles of various shapes may exist within the same cathode active material unless specifically intended during the manufacturing process. Furthermore, the primary particles refer to particle units that do not appear to have grain boundaries when observed using a scanning electron microscope at a magnification of 5,000 to 20,000 times.

[0082] The average particle size (D) of the above primary particles 50 ) may be 0.02 μm or more and less than 1 μm, 0.03 μm or more and less than 1 μm, 0.04 μm or more and less than 1 μm, 0.05 μm or more and less than 1 μm, 0.06 μm or more and less than 1 μm, 0.07 μm or more and less than 1 μm, 0.09 μm or more and less than 1 μm, 0.1 μm or more and less than 1 μm, 0.02 μm or more and 0.8 μm or less, 0.03 μm or more and 0.8 μm or less, 0.04 μm or more and 0.8 μm or less, 0.05 μm or more and 0.5 μm or less, 0.06 μm or more and 0.8 μm or less, 0.07 μm or more and 0.8 μm or less, 0.09 μm to 0.8 μm or less, or 0.1 μm to 0.8 μm or less.

[0083] The average particle size (D) of the above primary particles 50 ) can be calculated as the average value of the length in the major axis direction and the length in the minor axis direction of the primary particle ([major axis length + minor axis length] / 2). The average particle size of the primary particle can be calculated as the average value of the particle sizes of all primary particles observed from surface SEM images and / or cross-sectional SEM images of the lithium transition metal oxide according to a known method and / or the average value of measurements for 100 or more arbitrarily selected primary particles.

[0084] If the average particle size of the above primary particles is smaller than 0.02 μm, the specific surface area of ​​the positive active material containing a lithium transition metal oxide with a polycrystalline structure becomes excessively large, which may lead to a decrease in stability due to adverse reactions with the electrolyte.

[0085] On the other hand, if the average particle size of the primary particles is 1 μm or more, intergranular cracks may be reduced, but they become more susceptible to intragranular cracks, and as the lithium ion diffusion path becomes longer, problems may arise in which kinetic properties deteriorate.

[0086] The average particle size of the above secondary particles can be measured using the laser diffraction method. For example, after dispersing the secondary particles in a dispersion medium, they can be introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000), irradiated with ultrasound of about 28 kHz at an output of 60 W, obtained a volume cumulative particle size distribution graph, and then measured by determining the particle size corresponding to 50% of the volume cumulative amount.

[0087] Average particle size (D) of the above lithium transition metal oxide 50 ) may be 3 μm or more and 20 μm or less, preferably 10 μm or more and 18 μm or less. The average particle size of the secondary particles (D 50 If ) is less than 3 μm, the compressive density of the electrode may decrease, and if it exceeds 20 μm, the dispersibility within the electrode and crack resistance during the charging and discharging process may decrease.

[0088] As described above, the lithium transition metal oxide defined in the present invention can effectively improve the mechanical strength of secondary particles by partially suppressing the growth of crystallites through doping with a dopant capable of causing local lattice distortion within the crystal lattice, thereby providing a stabilization effect at the grain boundaries.

[0089] The average crystallite size of the lithium transition metal oxide calculated by Rietveld Refinement on the diffraction spectrum obtained through X-ray diffraction analysis using Cu-Kα rays for the above positive active material may be 110 nm or more and less than 148 nm, preferably 110 nm or more and less than 147 nm, more preferably 115 nm or more and less than 147 nm, and even more preferably 117 nm or more and less than 146 nm.

[0090] The above average crystallite size can be quantitatively analyzed through X-ray diffraction analysis (XRD) by Cu-Kα X-rays on the lithium transition metal oxide. Specifically, the above average crystallite size can be calculated through Rietveld analysis of the X-ray diffraction spectrum in the 2θ=10°~120° region obtained through XRD analysis of the cathode active material containing the lithium transition metal oxide.

[0091] If the above average crystallite size is less than 110 nm, the crystallite size becomes excessively small, which may reduce the mechanical cohesiveness of the primary particles forming the polycrystalline structure. Consequently, stress caused by volume changes during the charging and discharging process is concentrated at the grain boundaries, which may lead to an increase in cracking or fragmentation of secondary particles and a decrease in the stability of the electrode structure. Furthermore, if the crystallite size is excessively small, the specific surface area may increase unnecessarily, which may promote side reactions with the electrolyte.

[0092] Conversely, if the average crystallite size is 148 nm or larger, it becomes difficult to provide a sufficient stabilization effect at the grain boundaries, which may have an adverse effect on improving the mechanical strength of secondary particles.

[0093] Therefore, when the range of average crystallite size defined in the present invention is satisfied, the structural stability and electrochemical properties of the polycrystalline Mid-Ni type lithium transition metal oxide can be secured in balance.

[0094] The particle fracture strength defined herein refers to a strength indicator for a secondary particle unit formed by the aggregation of multiple primary particles, rather than a primary particle unit. In other words, it is not the strength of a single crystal grain, but a value calculated based on the point where the entire secondary particle cracks or fractures when subjected to a compressive load.

[0095] Generally, positive electrode active materials having a single-crystal structure do not have internal grain boundaries, so they exhibit strong mechanical cohesion and high fracture strength of the entire secondary particle. On the other hand, in a polycrystalline structure, multiple primary particles aggregate to form secondary particles, so the cohesion at the grain boundaries within the particles is relatively weak. In addition, sliding phenomena may occur at the grain boundaries due to volume changes that occur repeatedly during the repetitive charging and discharging process, which inevitably leads to a decrease in the fracture strength of the secondary particle.

[0096] Nevertheless, the positive electrode active material according to the present invention can improve the fracture strength of secondary particles by partially suppressing the growth of crystallites in a polycrystalline Mid-Ni type lithium transition metal oxide to control crystallite size and promoting stabilization at grain boundaries.

[0097] The compressive fracture strength of the positive electrode active material (secondary particle) according to the present invention is 12.2 kgf / mm 2 Above 20.0 kgf / mm 2 Below, preferably 12.25 kgf / mm 2 Above 17.0 kgf / mm 2 Below, more preferably 12.26 kgf / mm 2Above 15.53 kgf / mm 2 It may be less than.

[0098] The compressive fracture strength of the above secondary particle is 12.2 kgf / mm 2 If it is less than that, the mechanical cohesion of the secondary particles is insufficient, so cracking or crushing may easily occur during the charging / discharging process.

[0099] On the other hand, the polycrystalline structure has the advantage that stress can be relieved at grain boundaries, allowing strain to be distributed throughout the entire particle during charging and discharging. However, the compressive fracture strength of the aforementioned secondary particle is 20.0 kgf / mm 2 If it exceeds [value], the grains become structurally too dense, which reduces flexibility due to electrode expansion / contraction, and consequently increases the likelihood of intragranular cracking.

[0100] In addition, the anode active material according to the present invention can effectively suppress intragranular cracking by securing grain boundary crack resistance and stress relaxation behavior.

[0101] The effect of suppressing cracks within the particles of the anode active material can be quantitatively evaluated by measuring the change in the average particle size of the secondary particles before and after 4.5 ton press for the anode active material (secondary particles) according to the present invention, and by calculating the particle breakage ratio according to Equation 1 below.

[0102] [Equation 1]

[0103] Particle breakage ratio = (D50 press 전 - D50 press 후 ) / D50 press 전

[0104] The higher the particle breakage ratio calculated according to Equation 1 above, the more easily the secondary particles crack or break under external compressive loads, which indicates that the inter-particle bonding strength and internal stress relaxation characteristics are insufficient. Conversely, the lower the particle breakage ratio calculated according to Equation 1 above, the more stably the structure of the secondary particles can be maintained even when an external load is applied.

[0105] To improve the mechanical reliability and lifetime characteristics of the positive electrode active material, it is preferable that the particle breakage ratio calculated according to Equation 1 for the positive electrode active material be 0.20 or less.

[0106] Generally, as the compressive fracture strength of the aforementioned secondary particles increases, the grain fracture ratio calculated according to Equation 1 above may decrease. However, even if the compressive fracture strength of the secondary particles is at a similar level, if grain boundary crack resistance and stress relaxation behavior are not sufficiently secured, the grain fracture ratio may increase.

[0107] In addition, if the compressive fracture strength of secondary particles is excessively high, internal stress may not be sufficiently relieved, and the vulnerability to intragranular cracks may increase, leading to an increase in the particle fracture rate.

[0108] Therefore, 12.2 kgf / mm 2 Above 20.0 kgf / mm 2When the particle breakage ratio calculated according to Equation 1 is 0.20 or less while having the compressive fracture strength below, the stability of the grain boundary, stress relaxation characteristics, and intergranular bonding strength are generally improved, thereby ensuring excellent mechanical stability.

[0109]

[0110] lithium secondary battery

[0111] According to another aspect of the present invention, an anode may be provided comprising an anode current collector and an anode active material layer formed on the anode current collector. Herein, the anode active material layer may include an anode active material according to various embodiments of the present invention. Accordingly, since the anode active material is the same as previously described, a specific description is omitted for convenience, and only the remaining unmentioned components will be described below.

[0112] The above positive current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery, and 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. In addition, the above positive current collector may typically have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesion of the positive active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0113] The above positive active material layer can be manufactured by applying a positive slurry composition, which includes a conductive material and optionally a binder together with the positive active material, to the positive current collector.

[0114] At this time, the positive active material may be included in an amount of 80 wt% to 99 wt%, more specifically 85 wt% to 98.5 wt%, based on the total weight of the positive active material layer. Excellent capacity characteristics may be exhibited when included within the above-mentioned content range, but it is not necessarily limited thereto.

[0115] The above conductive material is used to impart conductivity to the electrode, and in the battery being constructed, it may be used without special limitations as long as it possesses electronic conductivity without causing chemical changes. 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, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, or silver; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxide such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one of these alone or a mixture of two or more may be used. The above conductive material may be included in an amount of 0.1 wt% to 15 wt% based on the total weight of the positive electrode active material layer.

[0116] The above binder serves to improve adhesion between positive active material particles and adhesion between the positive active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these alone or a mixture of two or more may be used. The above binder may be included in an amount of 0.1 wt% to 15 wt% based on the total weight of the positive active material layer.

[0117] The above-described anode can be manufactured according to a conventional anode manufacturing method, except for using the above-described anode active material. Specifically, it can be manufactured by applying an anode slurry composition, prepared by dissolving or dispersing the above-described anode active material and optionally a binder and a conductive material in a solvent, onto an anode current collector, and then drying and rolling.

[0118] The above solvent may be a solvent commonly used in the relevant technical field, and may include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one of these alone or a mixture of two or more may be used. The amount of the above solvent used is sufficient if it has a viscosity that dissolves or disperses the anode active material, conductive material, and binder, taking into account the coating thickness of the slurry and the manufacturing yield, and subsequently provides excellent thickness uniformity when coated for anode manufacturing.

[0119] In addition, in another embodiment, the anode may be manufactured by casting the anode slurry composition onto a separate support and then laminating the film obtained by peeling off from the support onto an anode current collector.

[0120] In addition, according to another aspect of the present invention, an electrochemical device comprising the anode described above may be provided. Specifically, the electrochemical device may be a battery, a capacitor, etc., and more specifically, a lithium secondary battery.

[0121] Specifically, the above lithium secondary battery may include a positive electrode, a negative electrode positioned opposite to the positive electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode. Here, since the positive electrode is the same as previously described, a detailed description is omitted for convenience, and only the remaining components not previously mentioned are described in detail below.

[0122] The above lithium secondary battery may optionally further include a battery container housing the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member sealing the battery container.

[0123] The above cathode may include a cathode current collector and a cathode active material layer located on the cathode current collector.

[0124] The above-mentioned negative current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy may be used. In addition, the above-mentioned negative current collector may typically have a thickness of 3 μm to 500 μm, and, similar to the positive current collector, fine irregularities may be formed on the surface of the current collector to strengthen the bonding strength of the negative active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0125] The above-mentioned cathode active material layer can be manufactured by applying a cathode slurry composition, which includes a conductive material and optionally a binder together with the above-mentioned cathode active material, to the above-mentioned cathode current collector.

[0126] As the above-mentioned negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples 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, or Al alloys; and SiO₂ βExamples include metal oxides capable of doping and dedoping lithium, such as (0 < β < 2), SnO2, vanadium oxide, and lithium vanadium oxide; or composites comprising the metal compound and carbonaceous material, such as Si-C composites or Sn-C composites, and any one or more of these may be used. Additionally, a metallic lithium thin film may be used as the negative electrode active material. Furthermore, the carbon material may include low-crystallinity carbon and high-crystallinity carbon. Representative examples of low-crystallinity carbon include soft carbon and hard carbon, while representative examples of high-crystallinity carbon include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch-derived cokes.

[0127] The above negative electrode active material may be included in an amount of 80 wt% to 99 wt% based on the total weight of the negative electrode active material layer.

[0128] The above binder is a component that assists in the bonding between the conductive material, the active material, and the current collector, and can typically be added in an amount of 0.1 wt% to 10 wt% based on the total weight of the negative active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0129] The above conductive material is a component for further improving the conductivity of the negative electrode active material, and may be added in an amount of 10 wt% or less, preferably 5 wt% or less, based on the total weight of the negative electrode active material layer. Such conductive material is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fiber or metal fiber; metal powder such as carbon fluoride, aluminum, or nickel powder; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxide such as titanium oxide; conductive materials such as polyphenylene derivatives may be used.

[0130] In one embodiment, the negative active material layer may be manufactured by applying a negative slurry composition, prepared by dissolving or dispersing a negative active material and optionally a binder and a conductive material in a solvent, onto a negative current collector and drying it, or by casting the negative slurry composition onto a separate support and then laminating the film obtained by peeling it off from the support onto a negative current collector.

[0131] Meanwhile, in the above-mentioned lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a pathway for the movement of lithium ions. Any separator typically used in lithium secondary batteries may be used without special limitations, and it is particularly desirable that it has low resistance to the movement of electrolyte ions and excellent electrolyte moisture retention capacity. Specifically, a porous polymer film, such as a porous polymer film made from a polyolefin-based polymer like ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used. Furthermore, a coated separator containing ceramic components or polymer materials may be used to ensure heat resistance or mechanical strength, and may optionally be used in a single-layer or multi-layer structure.

[0132] In addition, the electrolytes used in the present invention may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which are usable when manufacturing lithium secondary batteries, but are not limited to these.

[0133] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0134] The above organic solvent may be used without special restrictions as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, the above organic solvent may include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether or tetrahydrofuran; ketone-based solvents such as cyclohexanone; and aromatic hydrocarbon-based solvents such as benzene and fluorobenzene. Carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol-based solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double bond, a directional ring, or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes may be used. Among these, a carbonate-based solvent is preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery, and a low-viscosity linear carbonate-based compound (e.g., ethylmethyl carbonate, dimethyl carbonate or diethyl carbonate, etc.) is more preferred.In this case, using a mixture of cyclic carbonate and chain carbonate in a volume ratio of about 1:1 to about 1:9 can result in excellent performance of the electrolyte.

[0135] The above lithium salt can be used without special restrictions as long as it is a compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the above lithium salt is LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2) 2. LiCl, LiI, or LiB(C2O4)2, etc., may be used. It is preferable to use the lithium salt within a concentration range of 0.1M to 2.0M. When the concentration of the lithium salt falls within the above range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0136] When the electrolyte used in the present invention is a solid electrolyte, for example, solid inorganic electrolytes such as sulfide-based solid electrolytes, oxide-based solid electrolytes, nitride-based solid electrolytes, halide-based solid electrolytes, etc. may be used, and preferably, sulfide-based solid electrolytes may be used.

[0137] As a material for a sulfide-based solid electrolyte, a solid electrolyte containing Li, an element X (wherein X is at least one selected from P, As, Sb, Si, Ge, Sn, B, Al, Ga, and In) and S may be used. Examples of the above-mentioned sulfide-based solid electrolyte materials include Li2S-P2S5, Li2S-P2S-LiX (wherein X is a halogen element such as I or Cl), Li2S-P2S5-Li2O, Li2S-P2S5-Li2O-LiI, Li2S-SiS2, Li2S-SiS2-LiI, Li2S-SiS2-LiBr, Li2S-SiS2-LiCl, Li2S-SiS2-B2S3-LiI, Li2S-SiS2-P2S5-LiI, Li2S-B2S3, and Li2S-P2S5-Z m S n (Here, m and n are integers, and Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (Here, p and q are integers, and M is P, Si, Ge, B, Al, Ga, or In) etc.

[0138] The solid electrolyte, preferably a sulfide-based solid electrolyte, may be amorphous or crystalline, or may be in a mixed state of amorphous and crystalline.

[0139] Li7La3Zr2O is a material for oxide-based solid electrolytes. 12 , Li 7-x La3Zr 1-x Nb x O 12 , Li 7-3x La3Zr2Al x O 12 , Li 3x La 2 / 3-x TiO3, Li 1+x Al x Ti 2-x (PO4)3, Li 1+x Al x Ge 2-x (PO4)3, Li3PO4, Li 3+x PO4-x N x (LiPON), Li 2+2x Zn 1-x There are GeO4 (LISICON), etc.

[0140] The aforementioned solid electrolyte may be disposed as a separate layer (solid electrolyte layer) between the anode and the cathode. Additionally, the solid electrolyte may be partially included within the anode active material layer of the anode independently of the solid electrolyte layer, or the solid electrolyte may be partially included within the cathode active material layer of the cathode independently of the solid electrolyte layer.

[0141] In addition to the above electrolyte components, the above electrolyte may further include one or more additives for the purpose of improving the lifespan characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery, such as, for example, haloalkylene carbonate-based compounds like difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, triamide hexaphosphate, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive may be included in an amount of 0.1 wt% to 5 wt% based on the total weight of the electrolyte.

[0142] As described above, since the lithium secondary battery containing the positive electrode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and lifespan characteristics, it is useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in the field of electric vehicles such as hybrid electric vehicles (HEVs).

[0143] The external shape of the lithium secondary battery according to the present invention is not subject to any particular limitations, but may be cylindrical, prismatic, pouch, or coin-shaped using a can. In addition, the lithium secondary battery can be used not only as a battery cell used as a power source for small devices, but can also preferably be used as a unit cell in a medium-to-large battery module containing a plurality of battery cells.

[0144] According to another aspect of the present invention, a battery module comprising the lithium secondary battery as a unit cell and / or a battery pack comprising the same may be provided.

[0145] The battery module or the battery pack may be used as a power source for one or more medium-to-large devices, including a power tool; an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a power storage system.

[0146] The present invention will be described in more detail below through examples. However, these examples are intended solely to illustrate the present invention and should not be interpreted as limiting the scope of the present invention.

[0147]

[0148] Preparation Example 1. Preparation of positive electrode active material

[0149] PAM1

[0150] (a) Spherical Ni by the known co-precipitation method 0.60 Co 0.10 Mn 0.30(OH)2 hydroxide precursors were synthesized. In a 90L reactor, 25 wt% NaOH (aq) and 30 wt% NH4OH (aq) were added to a 1.5 M complex transition metal sulfuric acid aqueous solution prepared by mixing nickel sulfate, cobalt sulfate, and manganese sulfate in a molar ratio of 60:10:30. The pH inside the reactor was maintained at 11.5, and the reactor temperature was maintained at 60°C. An inert gas, N2, was introduced into the reactor to prevent the oxidation of the prepared precursors. After completing the synthesis reaction under stirring, washing and dehydration were performed using a filter press (F / P) to [prepare] Ni 0.60 Co 0.10 Mn 0.30 (OH)2 hydroxide precursor was obtained.

[0151] (b) A mixture was prepared by mixing the hydroxide precursor obtained in step (a) with Li2CO3. Li2CO3 was mixed so that the Li / (Ni+Co+Mn) molar ratio based on the intermediate product was 1.03. Subsequently, the mixture was heated from room temperature to 920°C over 6 hours under an air atmosphere, then heat-treated at 920°C for 8 hours, and naturally cooled to obtain a positive electrode active material containing a lithium transition metal oxide.

[0152]

[0153] PAM2

[0154] (a) Spherical Ni by the known co-precipitation method 0.60 Co 0.10 Mn 0.30(OH)2 hydroxide precursors were synthesized. In a 90L reactor, 25 wt% NaOH (aq) and 30 wt% NH4OH (aq) were added to a 1.5 M complex transition metal sulfuric acid aqueous solution prepared by mixing nickel sulfate, cobalt sulfate, and manganese sulfate in a molar ratio of 60:10:30. The pH inside the reactor was maintained at 11.5, and the reactor temperature was maintained at 60°C. An inert gas, N2, was introduced into the reactor to prevent the oxidation of the prepared precursors. After completing the synthesis reaction under stirring, washing and dehydration were performed using a filter press (F / P) to [prepare] Ni 0.60 Co 0.10 Mn 0.30 (OH)2 hydroxide precursor was obtained.

[0155] (b) A first mixture was prepared by mixing the hydroxide precursor obtained in step (a), Li2CO3, and WO3. Li2CO3 was mixed so that the Li / (Ni+Co+Mn+W) molar ratio based on the intermediate product was 0.85. WO3 was mixed so that it was 0.2 mol% relative to the total metal elements excluding lithium in the first mixture. Subsequently, the first mixture was heated from room temperature to 920°C over 6 hours under an air atmosphere, then heat-treated at 920°C for 8 hours, and cooled naturally to obtain an intermediate product in the form of a lithium transition metal oxide.

[0156] (c) A second mixture was prepared by mixing the intermediate product obtained in step (b) with LiOH. The LiOH mixed with the intermediate product was used such that the Li / (Ni+Co+Mn) molar ratio based on the lithium transition metal oxide in the final product was 1.03. Subsequently, the second mixture was heated from room temperature to 920°C over 6 hours under an air atmosphere, then heat-treated at 920°C for 8 hours, and cooled naturally to obtain a final product containing lithium transition metal oxide.

[0157]

[0158] PAM3

[0159] A positive electrode active material was prepared in the same way as PAM2, except that 0.1 mol% of Y2O3 was used instead of WO3 in step (b) above.

[0160]

[0161] PAM4

[0162] A positive electrode active material was prepared in the same way as PAM2, except that 0.1 mol% of ZrO2 was used instead of WO3 in step (b) above.

[0163]

[0164] PAM5

[0165] A positive electrode active material was prepared in the same way as PAM2, except that 0.1 mol% of TiO2 was used instead of WO3 in step (b) above.

[0166]

[0167] PAM6

[0168] A positive electrode active material was prepared in the same way as PAM2, except that 0.2 mol% of Al2O3 was used instead of WO3 in step (b) above.

[0169]

[0170] PAM7

[0171] The composition of the precursor is Ni 0.60 Co 0.20 Mn 0.20 The positive active material was prepared in the same way as PAM1, except that it was made of (OH)2.

[0172]

[0173] PAM8

[0174] The composition of the precursor is Ni 0.80 Co 0.10 Mn 0.10 The positive active material was prepared in the same way as PAM1, except that it was made of (OH)2.

[0175]

[0176] PAM9

[0177] A positive electrode active material was prepared in the same way as PAM2, except that the heat treatment temperature of step (b) above was set to 1,020℃.

[0178]

[0179] Preparation Example 2. Preparation of a lithium secondary battery (half-cell)

[0180] A positive electrode slurry was prepared by dispersing 94 wt% of each positive electrode active material prepared according to Preparation Example 1, 3 wt% of carbon black, and 3 wt% of PVDF binder in 30 g of N-methyl-2-pyrrolidone (NMP). The positive electrode slurry was uniformly coated onto an aluminum thin film with a thickness of 15 μm and vacuum dried at 135°C to produce a positive electrode for a lithium secondary battery.

[0181] A half-cell was prepared using a lithium foil as the counter electrode for the above anode, a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) as the separator, and an electrolyte in which LiPF6 is present at a concentration of 1.15 M in a solvent mixed with ethylene carbonate and ethyl methyl carbonate in a volume ratio of 3:7.

[0182]

[0183] Experimental Example 1. Analysis of particle characteristics of the cathode active material

[0184] (1) Measurement of the average size of primary particles: A cross-section of the cathode active material prepared according to Preparation Example 1 was observed using a scanning electron microscope (SEM). Multiple primary particles constituting the secondary particles were identified from the observed cross-sectional SEM image, and the lengths of the major axis and minor axis were measured for each primary particle. The average value of the measured major and minor axis lengths was defined as the size of each primary particle, and the same measurement was repeated for at least 100 randomly selected primary particles. The final average size of the primary particles was calculated by averaging the sizes of the above 100 or more primary particles.

[0185] (2) Measurement of the average crystallite size of lithium transition metal oxide: X-ray diffraction (XRD) analysis was performed on each positive electrode active material prepared according to Example 1 to calculate the average crystallite size of the lithium transition metal oxide contained in the positive electrode active material.

[0186] XRD analysis was performed using a Bruker D8 Advance diffractometer with Cu-Kα radiation (1.540598 Å). The average crystallite size was calculated from a straight line obtained by plotting the diffraction angle θ (rad) and full width β (rad) within the 2θ=10°~120° region on a coordinate plane with sinθ on the horizontal axis and βcosθ on the vertical axis through Rietveld Refinement of the diffraction spectrum obtained through X-ray diffraction (XRD) analysis using Cu-kα rays of the anode active material.

[0187] (3) Measurement of particle breakdown strength of the positive electrode active material: The compressive breakdown strength of the secondary particles constituting the positive electrode active material was measured through a single particle compression test.

[0188] Specifically, individual secondary particles were selected from each cathode active material prepared according to Preparation Example 1 under an optical microscope and a compressive load was applied using a micro-indenter. The load at the point where the secondary particle cracked or fractured was recorded, and the load value was corrected by the particle diameter to convert it into compressive fracture strength. The above measurement was repeated for at least 30 particles, and the final particle fracture strength was calculated as the average of each individual measurement value.

[0189] (4) Particle breakage ratio of the positive active material after 4.5 ton press: For each positive active material prepared according to Preparation Example 1, the particle breakage ratio was measured by measuring the change in the average particle size of the secondary particles before and after the 4.5 ton press. The average particle size of the secondary particles before and after the press was defined as the particle size corresponding to 50% of the volume accumulation in the volume accumulation particle size distribution obtained using a laser diffraction particle size measuring device (Microtrac MT 3000).

[0190] Particle breakage ratio = (D50 press 전 - D50 press 후 ) / D50 press 전

[0191] The above measurement results are shown in Table 1 below.

[0192] Classification Primary average particle size (μm) Average crystallite size (nm) Compressive fracture strength (kgf / mm²) 2 Particle Breakage Ratio PAM 10.5 148 12.00.213 PAM 20.31 17 12.26 0.197 PAM 30.5 140 13.5 0.179 PAM 40.5 146 15.5 30.168 PAM 50.6 155 11.5 40.308 PAM 60.5 144 12.00.240 PAM 70.41 15 12.48 0.213 PAM 8 2.02 10 24.00.408 PAM 9 2.118 6 20.00.312

[0193]

[0194] Referring to the results in Table 1, it was confirmed that when the growth of crystallites of the lithium transition metal oxide was suppressed and the grain boundaries were stabilized (PAM2 to PAM4), improved compressive fracture strength and a lower grain breakage rate were exhibited compared to PAM1.

[0195] The above results are expected to be due to the fact that local lattice distortion within the crystal lattice in PAM2 to PAM4 was induced, thereby strengthening the bonding force at the grain boundaries and providing a stress relaxation effect.

[0196] Meanwhile, it was confirmed that the dopants (Ti, Al) used in PAM5 and PAM6 did not induce sufficient lattice distortion within the crystal lattice, resulting in minimal improvement in compressive fracture strength and grain breakage rate.

[0197] In addition, in the case of PAM7, the internal stress relaxation effect was insufficient because the manganese (Mn) content in the composition of the lithium transition metal oxide was not sufficiently secured. Consequently, although the compressive fracture strength of the secondary particles was slightly improved, the grain boundary crack resistance and stress relaxation behavior were not sufficiently secured, resulting in an increase in the particle fracture rate.

[0198] In the case of PAM8, which has a High-Ni composition rather than a Mid-Ni composition, crystallite growth was promoted despite using the same dopant as PAM2, resulting in an increase in primary grain and average crystallite size. Consequently, intergranular cracks decreased, improving the compressive fracture strength of secondary grains; however, it was observed that the grain fracture rate increased due to increased susceptibility to intragranular cracks.

[0199] Similarly, in the case of PAM9, in which the growth of primary grains and crystallites was induced by increasing the heat treatment temperature during firing, intergranular cracks were reduced and the compressive fracture strength of secondary grains was improved, but it was confirmed that the grain breakage rate increased due to the decrease in mechanical reliability caused by the occurrence of intragranular cracks.

[0200]

[0201] Experimental Example 2. Evaluation of Electrochemical Characteristics of a Lithium Secondary Battery (Half-Cell)

[0202] For the lithium secondary battery (half-cell) prepared in Example 2, the initial discharge capacity and the ratio of the discharge capacity after 40 cycles to the initial capacity (cycle capacity retention rate) were measured through charge / discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, a voltage range of 2.7V to 4.45V, and a discharge rate of 0.05 / 0.05C.

[0203] The above measurement results are shown in Table 2 below.

[0204] Category Initial Discharge Capacity (mAh / g) Cycle Capacity Retention Rate (%) PAM1 20 1.5 9 3.0 PAM2 20 1.4 9 5.3 PAM3 20 1.3 9 6.9 PAM4 20 1.6 9 4.9 PAM5 20 1.2 8 9.1 PAM6 20 1.2 9 0.7 PAM7 19 2.8 8 4.0 PAM8 19 1.0 6 0.0 PAM9 18 9.0 8 8.4

[0205]

[0206] Referring to the results in Table 2, it was confirmed that when the growth of crystallites of the lithium transition metal oxide was suppressed and the grain boundaries were stabilized (PAM2 to PAM4), the cycle capacity retention rate was improved while maintaining a similar level of initial discharge capacity compared to PAM1 in the pristine state.

[0207] In other words, it was confirmed that even if the growth of crystallites of the lithium transition metal oxide is inhibited and grain boundaries are stabilized, there is no significant decrease in the initial discharge capacity; rather, lifespan characteristics can be effectively improved by securing excellent grain strength and structural stability.

[0208] On the other hand, when the compressive fracture strength of secondary particles is insufficient or the particle breakage rate increases due to a lack of stress relaxation within the lattice, the electrode structure collapses or electrochemical contact instability occurs as charge / discharge cycles are repeated, showing a tendency for the lifespan characteristics to deteriorate rapidly.

[0209]

[0210] Although embodiments of the present invention have been described above, those skilled in the art may modify and change the present invention in various ways by adding, changing, deleting, or adding components, etc., without departing from the spirit of the present invention as described in the claims, and such modifications and changes are also to be included within the scope of the rights of the present invention.

Claims

1. A positive electrode active material comprising a lithium transition metal oxide having a nickel (Ni) content of 50 mol% or more and 70 mol% or less among all metal elements excluding lithium, wherein The content of manganese (Mn) among the above lithium transition metal oxides is greater than the content of cobalt (Co), and The above lithium transition metal oxide has the form of secondary particles in which a plurality of primary particles having an average particle size of less than 1 μm are aggregated, and The compressive fracture strength of the above positive electrode active material is 12.2 kgf / mm 2 Above 20.0 kgf / mm 2 Lee Ha-in, Positive active material.

2. In Paragraph 1, The above lithium transition metal oxide has the form of secondary particles in which a plurality of primary particles having an average particle size of 0.02 μm to 0.8 μm are aggregated. Positive active material.

3. In Paragraph 1, The above lithium transition metal oxide has the form of secondary particles having an average particle size of 3 μm to 20 μm, Positive active material.

4. In Paragraph 1, The average crystallite size of the lithium transition metal oxide calculated by Rietveld Refinement on the diffraction spectrum obtained through X-ray diffraction analysis using Cu-Kα rays for the above-mentioned cathode active material is 110 nm or more and less than 148 nm, Positive active material.

5. In Paragraph 1, The above lithium transition metal oxide has a cobalt (Co) content of less than 20 mol% among the total metal elements excluding lithium, Positive active material.

6. In Paragraph 1, The above lithium transition metal oxide has a manganese (Mn) content of 20 mol% or more and less than 50 mol% among the total metal elements excluding lithium, Positive active material.

7. In Paragraph 1, A particle breakage ratio calculated by Equation 1, which is 0.20 or less by measuring the change in average particle size of secondary particles before and after a 4.5-ton press for the above-mentioned positive electrode active material, Positive active material. [Equation 1] Particle breakage ratio = (D50 press 전 - D50 press 후 ) / D50 press 전 8. In Paragraph 1, At least one dopant doped into the crystal lattice of the lithium transition metal oxide, Positive active material.

9. In Paragraph 8, The above dopant is at least one selected from W, Y, and Zr, Positive active material.

10. In Paragraph 1, The above lithium transition metal oxide has an average composition represented by the following chemical formula 1, Cathode active material: [Chemical Formula 1] Li a Ni 1-(b+c+d) Co b Mr c M1 d O2 In the above chemical formula 1, M1 is at least one selected from Na, K, Mg, Ba, B, Ca, Ce, Hf, Ta, Cr, F, Al, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, W, Mo, P, Sr, Ge, Nd, Gd and Cu, and 0.95≤a≤1.15, 0≤b<0.20, 0.20≤c<0.50, 0 <d≤0.10, 0.5≤1-(b+c+d)≤0.70이다.

11. A positive electrode comprising a positive electrode active material according to any one of claims 1 to 10.

12. A lithium secondary battery using a positive electrode according to Paragraph 11.

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