Positive electrode active material and lithium secondary battery containing the same

Abstract

The present invention provides a positive electrode active material with improved electrochemical properties, including capacitance characteristics and rate characteristics. [Solution] Instead of coating the surface of the unit particles constituting the lithium transition metal oxide, a cobalt-rich region is formed on the surface of the unit particles in which the distribution of cobalt is uneven. This reduces the conductivity deviation between unit particles caused by the formation of an uneven coating layer, and improves electrochemical properties including capacity characteristics and rate characteristics. A lithium secondary battery containing the same is provided.

Description

[Technical Field]

[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 and a lithium secondary battery containing the same, which reduce conductivity deviations between unit particles due to the formation of a non-uniform coating layer and improve electrochemical properties, including capacity characteristics and rate characteristics, by forming cobalt-rich regions on the surface of unit particles in which cobalt is unevenly distributed, instead of coating the surface of the unit particles constituting the lithium transition metal oxide. [Background technology]

[0002] Batteries store electricity by using electrochemically reactive materials at the positive and negative electrodes. A typical example of such a battery is the lithium-ion secondary battery, which stores electrical energy through the difference in chemical potential that occurs when lithium ions are intercalated / deintercalated at the positive and negative electrodes.

[0003] The lithium secondary battery is manufactured by using materials capable of reversible intercalation / deintercalation of lithium ions as the positive electrode active material and the negative electrode active material, and by filling the space between the positive electrode and the negative electrode with an organic electrolyte or a polymer electrolyte.

[0004] Lithium transition metal oxides are used as positive electrode active materials in lithium secondary batteries, and composite oxides such as LiCoO2, LiMn2O4, LiNiO2, and LiMnO2 are being studied as examples.

[0005] Among the aforementioned positive electrode active materials, LiCoO2 is the most widely used due to its excellent lifespan characteristics and charge / discharge efficiency. However, it has the disadvantage of being expensive due to the resource limitations of cobalt used as a raw material, thus limiting its price competitiveness.

[0006] Lithium manganese oxides such as LiMnO2 and LiMn2O4 have the advantages of excellent thermal safety and low cost, but they have the drawbacks of low capacity and poor high-temperature performance. LiNiO2-based cathode active materials have the advantage of exhibiting high discharge capacity, but they are difficult to synthesize due to the active cation mixing of Li and Ni, and the synthesized cathode active materials have the problem of very poor rate characteristics and lifetime characteristics.

[0007] As a result, in order to improve the low rate characteristics and lifetime characteristics of LiNiO2 while maintaining its high reversible capacity, ternary lithium transition metal oxides such as NCM (Ni-Co-Mn) and NCA (Ni-Co-Al), or quaternary lithium transition metal oxides such as NCMA (Ni-Co-Mn-Al), have been developed in which some of the nickel is replaced with cobalt, manganese, and / or aluminum. Since the reversible capacity decreases as the nickel content in such ternary or quaternary lithium transition metal oxides decreases, research to increase the nickel content in lithium transition metal oxides has been actively conducted recently.

[0008] Incidentally, as the nickel content in lithium transition metal oxides increases, the cation mixing within the crystal structure increases, leading to decreased stability and an increase in the content of unreacted lithium impurities such as LiOH and Li2CO3 on the surface.

[0009] The more lithium impurities remain on the surface of the lithium transition metal oxide, the more likely it is to promote gas generation and swelling in lithium secondary batteries using the lithium transition metal oxide as the positive electrode active material. Furthermore, the more lithium impurities remain on the surface of the lithium transition metal oxide, the more likely it is that the paste composition will gel due to the lithium impurities when using the lithium transition metal oxide to produce a paste for forming a positive electrode active material layer.

[0010] Thus, there are various attempts to reduce the content of lithium impurities remaining on the surface of the lithium transition metal oxide through the surface coating of the lithium transition metal oxide and to reduce side reactions between the surface of the lithium transition metal oxide and the electrolyte.

[0011] In addition, when forming a conductive coating layer on the surface of the lithium transition metal oxide, it can contribute to improving the ion or electron transport ability mediated by the lithium transition metal oxide and improving the rate capability as compared with when forming a non-conductive coating layer on the surface of the lithium transition metal oxide. However, different from the non-conductive coating layer, since the electrochemical properties of the conductive coating layer change drastically depending on the continuity and / or thickness of the coating layer, etc., when a non-uniform coating layer is formed on the surface of the lithium transition metal oxide, or when the coating substance exists in the form of fine particles on the surface of the lithium transition metal oxide, it may be difficult to exhibit stable electrochemical properties for a long time due to the conductivity deviation between particles.

Summary of the Invention

Problems to be Solved by the Invention

[0012] In the market of lithium secondary batteries, while the growth of lithium secondary batteries for electric vehicles plays a role as a market driver, the demand for the positive electrode material used in lithium secondary batteries is also continuously changing.

[0013] For example, conventionally, lithium secondary batteries using LFP have been mainly used from the viewpoint of ensuring safety, etc., but recently, the use of nickel-based lithium transition metal oxides with a larger energy capacity per weight compared to LFP has a tendency to expand.

[0014] As a result, the positive electrode active material used in higher-specification lithium secondary batteries needs to fully satisfy all the appropriate stability and reliability expected even under more severe operating conditions.

[0015] Conventionally, in order to alleviate problems caused by an increase in the nickel content in the lithium transition metal oxide, a non-conductive coating layer or a conductive coating layer has been formed on the surface of the lithium transition metal oxide. However, from the perspective of electrochemical properties, a lithium transition metal oxide having a conductive coating layer formed thereon tends to be preferred over a non-conductive coating layer.

[0016] However, as described above, since the electrochemical properties change significantly depending on the continuity and / or thickness of the conductive coating layer, a non-uniform coating layer may be formed on the surface of the lithium transition metal oxide, or when the coating substance exists in the form of fine particles on the surface of the lithium transition metal oxide, it is often difficult to exhibit stable electrochemical properties over a long period due to the conductivity deviation between the particles. Excellent continuity of the conductive coating layer means that the conductive coating layer entirely covers the surface of the lithium transition metal oxide.

[0017] For example, in order to improve the continuity of the conductive coating layer and reduce the deviation of the conductive coating layer between the particles, the content of the coating raw material substance input in the coating process for the lithium transition metal oxide must be increased. However, when increasing the content of the coating raw material substance input in the coating process for the lithium transition metal oxide, conversely, the deviation of the conductive coating layer between the particles may increase or the conductive coating layer may generally become thicker. When the conductive coating layer becomes excessively thick, it may reduce the ion or electron transport ability mediated by the lithium transition metal oxide. [[ID=u10]]

[0018] The present invention aims to provide a positive electrode active material with improved electrochemical properties, including capacitance characteristics and rate characteristics, by reducing the conductivity deviation between unit particles caused by the formation of a non-uniform coating layer, and by forming a cobalt-rich region on the surface of the unit particles in which cobalt is unevenly distributed, instead of coating the surface of the unit particles constituting the lithium transition metal oxide.

[0019] Furthermore, the present invention aims to provide a positive electrode active material that can improve resistance to volume changes of unit particles due to repeated charging and discharging, thereby preventing surface degradation due to cracks occurring on the surface of unit particles, by forming a cobalt-rich region of a predetermined thickness on the surface of unit particles constituting a lithium transition metal oxide, in which the distribution of cobalt is unevenly distributed.

[0020] Another object of the present invention is to provide a positive electrode comprising a positive electrode active material as defined in this application.

[0021] Another object of the present invention is to provide a lithium secondary battery using the positive electrode as defined in this application.

[0022] The objects of the present invention are not limited to those mentioned above, and other objects and advantages of the present invention not mentioned can be understood from the following description and may be more clearly understood from embodiments of the present invention. It can also be readily seen that the objects and advantages of the present invention can be achieved by the means and combinations set forth in the claims. [Means for solving the problem]

[0023] According to one aspect of the present invention, a positive electrode active material is provided which includes a lithium transition metal oxide containing lithium, nickel, and cobalt, wherein the lithium transition metal oxide has a secondary particle form in which a plurality of unit particles are aggregated, and one or more of the unit particles have a cobalt-rich region on their surface where the distribution of cobalt is unevenly distributed.

[0024] The lithium transition metal oxide is characterized in that, instead of forming a cobalt-containing coating layer on the surface of the unit particles, a cobalt-rich region is formed on the surface of the unit particles in which the distribution of cobalt is uneven.

[0025] The cobalt-rich region is distinct from the cobalt-containing coating layer formed on the surface of the unit particle and / or the secondary particle. The cobalt-containing coating layer covers the outermost surface of the unit particle, while the cobalt-rich region is a region located inside the outermost surface of the unit particle.

[0026] As a result, there are no cobalt-containing oxides having crystallographic properties different from those of the lithium transition metal oxide on the surface of the secondary particles. Furthermore, there are no cobalt-containing oxides having crystallographic properties different from those of the lithium transition metal oxide at the grain boundaries formed by the gaps between adjacent unit particles and / or contact between adjacent unit particles.

[0027] The thickness of the cobalt-rich region is preferably 1 nm to 100 nm, 1 nm to 90 nm, 1 nm to 80 nm, 1 nm to 70 nm, or 1 nm to 60 nm.

[0028] The lithium transition metal oxide can be represented by the following chemical formula 1. [Chemical formula 1] Li w Ni 1-(x+y+z) Co x Mn y M1 z O2 In the aforementioned chemical formula 1, M1 is at least one selected from Na, K, Mg, Ba, B, Ce, Hf, Ta, Cr, F, Al, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, P, Sr, Ge, Nd, Gd, and Cu. 0.95 ≤ w ≤ 1.15, 0 <x≦0.20、0≦y≦0.20、0<z≦0.20である。

[0029] The ratio (I2 / I1) of the average intensity (I2) of the peaks located in the region 2θ = 45.25 ± 0.25° to the maximum intensity (I1) of the peaks located in the region 2θ = 44.5 ± 0.5° in the diffraction spectrum obtained by X-ray diffraction (XRD) analysis of the positive electrode active material using Cu-kα rays may be greater than 0.001 and less than 0.015.

[0030] Furthermore, according to another aspect of the present invention, a positive electrode containing the aforementioned positive electrode active material is provided.

[0031] Furthermore, according to yet another aspect of the present invention, a lithium secondary battery is provided in which the aforementioned positive electrode is used. [Effects of the Invention]

[0032] As described above, according to the present invention, instead of coating the surface of the unit particles constituting the lithium transition metal oxide, a cobalt-rich region in which the distribution of cobalt is unevenly distributed is formed on the surface of the unit particles. This reduces the conductivity deviation between unit particles caused by the formation of an uneven coating layer, and improves electrochemical properties including capacitance characteristics and rate characteristics.

[0033] Furthermore, according to the present invention, by forming a cobalt-rich region with a predetermined thickness on the surface of the unit particles constituting the lithium transition metal oxide, resistance to volume changes of the unit particles due to repeated charging and discharging can be improved, and surface degradation due to cracks occurring on the surface of the unit particles can be prevented. This makes it possible to expect an improvement in the lifespan characteristics of a lithium secondary battery using the lithium transition metal oxide as the positive electrode active material.

[0034] Along with the effects described above, the specific effects of the present invention will be described below while explaining the specific matters for carrying out the invention. [Brief explanation of the drawing]

[0035] [Figure 1]Figure 1 shows the TEM / EDS image of the lithium transition metal oxide according to Example 1. [Figure 2] Figure 2 shows TEM / EDS images of lithium transition metal oxides according to Example 2. [Figure 3] Figure 3 shows TEM / EDS images of lithium transition metal oxides according to Example 3. [Figure 4] Figure 4 shows TEM / EDS images of lithium transition metal oxides according to Comparative Example 1. [Figure 5] Figure 5 shows TEM / EDS images of lithium transition metal oxides according to Comparative Example 2. [Figure 6] Figure 6 shows TEM / EDS images of lithium transition metal oxides according to Comparative Example 3. [Figure 7] Figure 7 shows TEM / EDS images of lithium transition metal oxides according to Comparative Example 4. [Figure 8] Figure 8 shows TEM / EDS images of lithium transition metal oxides according to Comparative Example 5. [Figure 9] Figure 9 shows TEM / EDS images of lithium transition metal oxides according to Comparative Example 6. [Modes for carrying out the invention]

[0036] For the convenience of understanding the present invention, certain terms are defined herein. Unless otherwise defined herein, scientific and technical terms used herein have meanings that are generally understood by those of ordinary skill in the art. Furthermore, unless otherwise specified in the context, singular terms should be understood to include their plural forms, and plural terms to include their singular forms.

[0037] The positive electrode active material and the lithium secondary battery containing the positive electrode active material according to the present invention will be described in more detail below.

[0038] (Cathode active material) The positive electrode active material according to one aspect of the present invention includes a lithium transition metal oxide containing lithium, nickel, and cobalt.

[0039] The lithium transition metal oxide is a composite metal oxide capable of intercalation / deintercalation of lithium ions and has a layered crystal structure belonging to the R-3m space group. The lithium transition metal oxide having a layered crystal structure shows a specific peak in the region where 2θ is 18° to 20° (for example, in the region of 2θ = 18.6 ± 1°) among the diffraction patterns obtained from XRD analysis.

[0040] In one embodiment, in order to improve the low rate characteristics and life characteristics while maintaining the high reversible capacity of LiNiO2, the lithium transition metal oxide may be a ternary or quaternary lithium transition metal oxide such as so-called NCM (Ni-Co-Mn) and NCA (Ni-Co-Al) that further contains manganese and / or aluminum, such as NCMA (Ni-Co-Mn-Al).

[0041] The lithium transition metal oxide can have a composition represented by the following Chemical Formula 1. [Chemical Formula 1] Li w Ni 1-(x+y+z) Co x Mn y M1 z O2 In Chemical Formula 1, M1 is at least one selected from Na, K, Mg, Ba, B, Ce, Hf, Ta, Cr, F, Al, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, P, Sr, Ge, Nd, Gd, and Cu, 0.95 ≦ w ≦ 1.15, 0 < x ≦ 0.20, 0 ≦ y ≦ 0.20, 0 < z ≦ 0.20.

[0042] The ratio w, which represents the proportion of lithium to all elements other than lithium in the lithium transition metal oxide, may be 0.95 or more and 1.15 or less, 0.95 or more and 1.10 or less, 0.98 or more and 1.10 or less, or 1.01 or more and 1.10 or less.

[0043] In the aforementioned chemical formula 1, if w is less than 0.95, the capacity of the positive electrode active material containing the lithium transition metal oxide represented by the aforementioned chemical formula 1 may decrease. On the other hand, in the aforementioned chemical formula 1, if w 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 be generated.

[0044] In the aforementioned chemical formula 1, "1-(x+y+z)", which represents the mole fraction of nickel relative to all metal elements other than lithium in the lithium transition metal oxide, may be between 0.50 and 0.95, 0.60 and 0.95, 0.70 and 0.95, 0.80 and 0.95, or 0.90 and 0.95. When the mole fraction of nickel in the lithium transition metal oxide satisfies the above range, a stable crystal structure can be formed, and a high energy density can be observed.

[0045] In the aforementioned chemical formula 1, x, which represents the mole fraction of cobalt relative to all metal elements other than lithium in the lithium transition metal oxide, may be 0.20 or less, 0.15 or less, or 0.10 or less. When the mole fraction of cobalt in the lithium transition metal oxide satisfies the above range, a stable crystal structure is formed, and good output characteristics can be observed.

[0046] In the aforementioned chemical formula 1, y, which represents the mole fraction of manganese relative to all metal elements other than lithium in the lithium transition metal oxide, may be 0.20 or less, 0.15 or less, 0.10 or less, or 0.05 or less. If the lithium transition metal oxide contains manganese, then in the aforementioned chemical formula 1, y is greater than 0. When the mole fraction of manganese in the lithium transition metal oxide satisfies the above range, a stable crystal structure can be formed.

[0047] In the above chemical formula 1, M1 represents a dopant doped into the lithium transition metal oxide. The lithium transition metal oxide has a layered crystalline structure in which lithium layers containing lithium and transition metal layers containing transition metals are arranged alternately, and the dopant may exist in a doped state within the crystal lattice of the lithium transition metal oxide (at least one of the lithium layers and / or transition metal layers).

[0048] In the aforementioned chemical formula 1, z, which represents the mole fraction of the dopant relative to all metal elements other than lithium in the lithium transition metal oxide, may be 0.20 or less, 0.15 or less, 0.10 or less, 0.05 or less, 0.04 or less, 0.03 or less, 0.02 or less, or 0.01 or less.

[0049] If the lithium transition metal oxide selectively contains a dopant, the dopant may contain at least one selected from Na, K, Mg, Ca, Sr, Ba, Rb, B, Ce, Hf, Ta, Cr, F, Al, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Ge, Nd, Gd, and Cu, preferably at least one selected from Mg, Ca, Ba, B, Al, V, Ti, Fe, Zr, Zn, Si, Nb, Mo, W, and Cu, and more preferably at least one selected from B, Al, Ti, Zr, Nb, Mo, and W. The type, combination, and content of the dopant can be appropriately selected within a range that does not negatively affect the electrochemical properties and stability of the positive electrode active material.

[0050] The lithium transition metal oxide has a secondary particle form in which multiple unit particles are aggregated. These unit particles can be called primary particles.

[0051] The unit particles may have circular, rod-shaped, elliptical, and / or amorphous shapes. Furthermore, unless specifically intended in the manufacturing process, unit particles of various shapes may exist within the same positive electrode active material. Additionally, the unit particles refer to particle units that, when observed at a magnification of 5,000x to 20,000x using a scanning electron microscope, appear to have no visible grain boundaries.

[0052] As the size of the unit particle, the average value of the length in the long axis direction and the length in the short axis direction of the unit particle ([length in the long axis direction + length in the short axis direction] / 2) can be used. As the average size of the unit particle, the arithmetic mean of the sizes of the unit particles present in the secondary particle can be used.

[0053] As an example, the unit particle may have an average particle size of 0.1 μm to 1.0 μm. In this case, the secondary particle may be an aggregate formed by the aggregation of more than 30, 50 or more, 100 or more, or several hundred or more unit particles.

[0054] As another example, the unit particles may have an average particle size of 2.0 μm to 10.0 μm, 2.0 μm to 8.0 μm, 2.0 μm to 6.0 μm, 2.0 μm to 5.0 μm, 2.0 μm to 4.0 μm, or 2.0 μm to 3.0 μm. In this case, the secondary particles may be aggregates formed by the aggregation of 30 or fewer unit particles.

[0055] The average particle size (D) of the secondary particles 50 The average particle size of the secondary particles (D) may be 2.0 μm or more and 16.0 μm or less, 2.0 μm or more and 14.0 μm or less, 2.0 μm or more and 12.0 μm or less, 3.0 μm or more and 16.0 μm or less, 3.0 μm or more and 14.0 μm or less, 3.0 μm or more and 12.0 μm or less, 4.0 μm or more and 16.0 μm or less, 4.0 μm or more and 14.0 μm or less, or 4.0 μm or more and 12.0 μm or less. 50 ) can vary depending on the number and size of the unit particles that make up the secondary particles.

[0056] The average particle size of the lithium transition metal oxide (secondary particles) in the positive electrode active material can be measured using the laser diffraction method. For example, after dispersing the secondary particles in a dispersion medium, they are introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000), and after irradiating them with ultrasound at approximately 28 kHz with 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 used to determine the average particle size (D 50 It can be used as ).

[0057] Unless otherwise defined, the terms "particle surface" as used in this application mean the region relatively close to the "outermost surface" of the particle, and "particle center" means the region relatively closer to the "middle" of the particle than the aforementioned "surface."

[0058] As a result, the "surface portion of the unit particle (primary particle)" refers to the region that is relatively close to the "outermost surface" of the unit particle (primary particle), and the "center portion of the unit particle (primary particle)" refers to the region that is relatively closer to the "middle" of the unit particle (primary particle) than the "surface portion".

[0059] Similarly, the "surface portion of the secondary particle" refers to the region relatively close to the "outermost surface" of the secondary particle, and the "center portion of the secondary particle" refers to the region relatively closer to the "middle" of the secondary particle than the "surface portion."

[0060] In this case, the region within any particle excluding the "particle surface" can be defined as the "particle's central region."

[0061] More specifically, in the present application, when the radius of the lithium transition metal oxide measured from the cross-sectional SEM image of the lithium transition metal oxide is R, a region where the distance (D) from the center of the lithium transition metal oxide satisfies (1 / 2)R < D is defined as the surface portion, and a region where the distance (D) from the center of the lithium transition metal oxide satisfies 0 ≦ D ≦ (1 / 2)R is defined as the center portion. Here, it is assumed that the lithium transition metal oxide is a secondary particle. The outermost contour of the surface portion corresponds to the outermost surface of the secondary particle. Therefore, the surface portion corresponds to the region from the point where the distance (D) from the center of the secondary particle satisfies (1 / 2)R < D to the outermost surface of the secondary particle.

[0062] The secondary particle can have a shape that is not a perfect sphere, such as an elliptical shape. Alternatively, there may be irregularities on the surface of the secondary particle. In this case, the diameter of the secondary particle can be calculated from the average value of the lengths of the major axis and the minor axis of the lithium transition metal oxide measured from the cross-sectional SEM image of the secondary particle. Half of the average value of the lengths of the major axis and the minor axis of the lithium transition metal oxide measured from the cross-sectional SEM image of the secondary particle can be used as the radius (R) of the secondary particle.

[0063] The lithium transition metal oxide has a secondary particle form in which a plurality of unit particles are aggregated, and on the surface portion of one or more of the unit particles, there is a cobalt-rich region where the distribution of cobalt is uneven. The cobalt-rich region may be present in a part or all of the total unit particles constituting the secondary particle, and preferably, the cobalt-rich region is present in all the unit particles constituting the secondary particle.

[0064] The cobalt-rich region is distinct from the cobalt-containing coating layer formed on the surface of the unit particle and / or the secondary particle. The cobalt-containing coating layer covers the outermost surface of the unit particle, while the cobalt-rich region is located inside the outermost surface of the unit particle. The cobalt-containing coating layer can also be distinguished from the cobalt-rich region in that it contains cobalt-containing oxide. In this application, cobalt-containing oxide refers to cobalt oxides such as Co3O4 and lithium-cobalt oxides such as LiCoO2 and LiCo2O4. Furthermore, in this application, cobalt-containing oxide does not have the composition defined by chemical formula 1 and is a different compound from the lithium transition metal oxide defined in this application. As a result, the cobalt-containing oxide has different crystallographic properties from the lithium transition metal oxide.

[0065] For example, the cobalt-containing coating layer formed on the surface of the unit particle has different crystallographic properties from the unit particle. Therefore, in the cross-sectional TEM image of the unit particle, a physical boundary is formed between the unit particle and the cobalt-containing coating layer, or a boundary exists where the crystal structure changes.

[0066] Furthermore, the cobalt-containing coating layer formed on the surface of the unit particles exhibits different crystallographic properties from those of the unit particles. Since the cobalt-containing coating layer contains cobalt-containing oxides such as LiCoO2 and LiCo2O4, when a cobalt-containing coating layer is formed on the surface of the unit particles, peaks specific to cobalt-containing oxides such as LiCoO2 and LiCo2O4 can be observed in the diffraction spectrum obtained through Cu-kα X-ray diffraction (XRD) analysis of the positive electrode active material containing the lithium transition metal oxide.

[0067] In other words, the lithium transition metal oxide according to the present invention has a cobalt-rich region of a predetermined thickness formed on the surface of a unit particle, where the distribution of cobalt is unevenly distributed from the outermost surface of the unit particle toward the center of the unit particle, instead of a cobalt-containing coating layer being formed on the surface of the unit particle. As a result, metal oxides having different crystallographic properties from the lithium transition metal oxide (for example, cobalt-containing oxides such as LiCoO2 and LiCo2O4) do not need to be present on the surface of the secondary particles. Furthermore, metal oxides having different crystallographic properties from the lithium transition metal oxide (for example, cobalt-containing oxides such as LiCoO2 and LiCo2O4) do not need to be present in the gaps between adjacent unit particles or in the grain boundaries formed by contact between adjacent unit particles.

[0068] The cobalt-rich region can be defined through TEM / EDS analysis of the cathode active material containing the lithium transition metal oxide.

[0069] The aforementioned TEM / EDS analysis method is a method for quantitatively analyzing the cobalt (Co) content by mapping the target element, cobalt (Co), onto a cross-sectional FE-TEM image of the lithium transition metal oxide using a known method.

[0070] The average mole fraction of cobalt measured within the cobalt-rich region via TEM / EDS analysis is greater than the average mole fraction of cobalt within the unit particle, and the average mole fraction of cobalt measured within the cobalt-rich region via TEM / EDS analysis may be 1.35 times or more but less than 10 times the mole fraction of cobalt measured at the center of the unit particle.

[0071] If the average mole fraction of cobalt measured within the cobalt-rich region is less than 1.35 times the mole fraction of cobalt measured at the center of the unit particle, it may be difficult to adequately prevent surface degradation due to cracks occurring on the surface of the unit particle by improving its resistance to volume changes caused by repeated charging and discharging. If the average mole fraction of cobalt measured within the cobalt-rich region is more than 10 times the mole fraction of cobalt measured at the center of the unit particle, the surface kinetic properties of the unit particle may decrease, which may lead to a decrease in the rate characteristics of a lithium secondary battery using the lithium transition metal oxide as the positive electrode active material.

[0072] The average mole fraction of cobalt measured within the cobalt-rich region is preferably greater than the mole fraction of cobalt measured at the grain boundaries formed by the contact of adjacent unit particles. If the average mole fraction of cobalt measured within the cobalt-rich region is smaller than the mole fraction of cobalt measured at the grain boundaries formed by the contact of adjacent unit particles, there is a high probability that a cobalt-containing coating layer containing cobalt-containing oxides such as LiCoO2 and LiCo2O4 exists at the grain boundaries formed by the contact of the unit particles.

[0073] According to the present invention, by adjusting the conditions of the secondary heat treatment step and the raw materials used in the secondary step during the manufacturing process of the positive electrode active material, the cobalt-rich region within the unit particle can be formed with a predetermined thickness.

[0074] The thickness of the cobalt-rich region is preferably 1 nm to 100 nm, 1 nm to 90 nm, 1 nm to 80 nm, 1 nm to 70 nm, or 1 nm to 60 nm. If the thickness of the cobalt-rich region is less than 1 nm, it may be difficult to adequately prevent surface degradation due to cracks occurring on the surface of the unit particles by improving resistance to volume changes due to repeated charging and discharging. If the thickness of the cobalt-rich region is greater than 100 nm, the surface kinetic properties of the unit particles may decrease, which may lead to a decrease in the rate characteristics of the lithium secondary battery using the lithium transition metal oxide as the positive electrode active material.

[0075] In the unit particle in which the cobalt-rich region exists, a concentration-maintaining region exists inside the cobalt-rich region in which the cobalt concentration is maintained. The concentration-maintaining region means a region in which the cobalt concentration is maintained within a range of ±5 mol%.

[0076] The concentration maintenance region can be confirmed by mapping the target element, cobalt (Co), onto a cross-sectional FE-TEM image of the lithium transition metal oxide using EDS mapping, and by line scanning the cobalt (Co) mapped along the direction from the outermost surface of the unit particle toward the center of the unit particle, thereby confirming that the cobalt concentration is maintained within a range of ±5 mol%.

[0077] The boundary between the cobalt-rich region and the concentration-maintaining region can be determined by the line-scanning results for cobalt (Co) obtained by TEM / EDS analysis, where the cobalt content, which is relatively high in the region near the outermost surface of the unit particle, converges to the average cobalt concentration within the unit particle. Similarly, the thickness of the cobalt-rich region can be the distance from the outermost surface of the unit particle to the point where the cobalt content converges to the average cobalt concentration within the unit particle.

[0078] For example, if the molar concentration of cobalt at the outermost surface of the unit particle is 30 mol%, and the average cobalt concentration within the unit particle is 5 mol%, then a line-scanning result that linearly shows the cobalt content within the unit particle would show that the cobalt concentration within the unit particle decreases from 30 mol% to 5 mol%, and the point where the molar concentration of cobalt converges to 5 mol% becomes the boundary between the cobalt-rich region and the concentration-maintaining region. The thickness of the cobalt-rich region can be found from the outermost surface of the unit particle to the point where the molar concentration of cobalt converges to 5 mol%.

[0079] The ratio (I2 / I1) of the average intensity (I2) of the peaks located in the region 2θ = 45.25 ± 0.25° to the maximum intensity (I1) of the peaks located in the region 2θ = 44.5 ± 0.5° within the diffraction spectrum can be used to infer the content of a Co-containing impurity (LCO) phase on the surface of the lithium transition metal oxide or the possibility of a cobalt-containing coating layer being present on the surface of the lithium transition metal oxide.

[0080] The ratio (I2 / I1) of the average intensity (I2) of the peaks located in the region 2θ = 45.25 ± 0.25° to the maximum intensity (I1) of the peaks located in the region 2θ = 44.5 ± 0.5° in the diffraction spectrum obtained by X-ray diffraction (XRD) analysis of the positive electrode active material using Cu-kα rays is greater than 0.001 and less than 0.015, greater than 0.001 and 0.013 or less, and 0.00 Preferably, the value is greater than 1 and less than or equal to 0.011, greater than 0.001 and less than or equal to 0.010, greater than 0.001 and less than or equal to 0.0097, 0.002 or more and less than or equal to 0.0097, 0.003 or more and less than or equal to 0.0097, 0.004 or more and less than or equal to 0.0097, 0.005 or more and less than or equal to 0.0097, 0.006 or more and less than or equal to 0.0097, 0.007 or more and less than or equal to 0.0097, or 0.0076 or more and less than or equal to 0.0097.

[0081] Since the intensity of the peaks located within the region 2θ = 45.25 ± 0.25° can be greater than 0 due to the presence of peaks located within the region 2θ = 44.5 ± 0.5°, the peak intensity ratio (I2 / I1) may be greater than 0.001.

[0082] Furthermore, the peak intensity ratio (I(003) / I(104)) of the maximum peak intensity (I(003)) corresponding to the (003) crystal plane to the maximum peak intensity (I(004)) corresponding to the (104) crystal plane, calculated from the diffraction spectrum obtained through X-ray diffraction (XRD) analysis of the positive electrode active material using Cu-kα rays, can be between 1.5 and 2.0.

[0083] The aforementioned peak intensity ratio (I(003) / I(104)) can be calculated as the ratio of the maximum peak intensity (I(003)) corresponding to the (003) crystal plane (2θ=18.6±1.0° region) to the maximum peak intensity (I(104)) corresponding to the (104) crystal plane (2θ=44.5±1.0° region).

[0084] Generally, I(003) / I(104), calculated from the diffraction spectrum obtained through X-ray diffraction (XRD) analysis using Cu-kα rays on the positive electrode active material, is an indirect indicator of how the lithium layer and the transition metal layer are regularly separated and form layers within the crystal structure of the lithium transition metal oxide. For example, a larger I(003) / I(104) indicates less cation mixing (cation mixing of lithium and nickel) between the lithium layer and the transition metal layer.

[0085] As defined in this application, a positive electrode active material having a cobalt-rich region on the surface of a unit particle in which the distribution of cobalt is unevenly distributed can form a stable crystal structure if the I(003) / I(104) calculated from the diffraction spectrum obtained through X-ray diffraction (XRD) analysis of the positive electrode active material using Cu-kα rays is within the range of 1.5 to 2.0.

[0086] Furthermore, the difference between 2θ(110), where a peak corresponding to the (110) crystal plane is observed, and 2θ(108), where a peak corresponding to the (108) crystal plane is observed, calculated from the diffraction spectrum obtained through X-ray diffraction (XRD) analysis of the positive electrode active material using Cu-kα rays, can be 0.3° to 0.5°.

[0087] The aforementioned 2θ(110)-2θ(108) can be calculated as the difference between 2θ(110), where a peak corresponding to the (110) crystal plane (2θ=64.6°~64.8° region) is observed, and 2θ(108), where a peak corresponding to the (108) crystal plane (2θ=64.3°~64.5° region) is observed, as calculated from the diffraction spectrum.

[0088] Similar to I(003) / I(104) described above, the degree of separation between the peak corresponding to the (110) crystal plane and the peak corresponding to the (108) crystal plane can be used as an indicator of the regularity of the crystal structure in a positive electrode active material that has a cobalt-rich region on the surface of a unit particle where the distribution of cobalt is unevenly distributed. If the regularity of the crystal structure of a positive electrode active material that has a cobalt-rich region on the surface of a unit particle where the distribution of cobalt is unevenly distributed decreases, the peak corresponding to the (110) crystal plane and the peak corresponding to the (108) crystal plane may not be sufficiently separated and may overlap to appear as a single peak. As defined in this application, a positive electrode active material that has a cobalt-rich region on the surface of a unit particle where the distribution of cobalt is unevenly distributed can form a stable crystal structure if 2θ(110)-2θ(108), calculated from the diffraction spectrum obtained through X-ray diffraction (XRD) analysis of the positive electrode active material using Cu-kα rays, is within the range of 0.3° to 0.5°.

[0089] (Lithium-ion secondary battery) According to another aspect of the present invention, a positive electrode can be provided that includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. Here, the positive electrode active material layer may include positive electrode active materials according to various embodiments of the present invention. Therefore, since the positive electrode active material is the same as that described above, for convenience, a detailed explanation will be omitted, and only the remaining undescribed configurations will be described below.

[0090] The positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector may also have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to increase the adhesion strength of the positive electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, nonwoven fabric, etc.

[0091] The positive electrode active material layer may be manufactured by applying a positive electrode slurry composition, which includes a conductive material and, if necessary, a binder, together with the positive electrode active material, to the positive electrode current collector.

[0092] In this case, the positive electrode active material may be present in an amount of 80 wt% to 99 wt%, more specifically, 85 wt% to 98.5 wt%, relative to the total weight of the positive electrode active material layer. When present within this content range, excellent capacity characteristics can be observed, but the material is not necessarily limited to this range.

[0093] The conductive material is used to impart conductivity to the electrodes and can be used without particular limitations as long as it has electronic conductivity without causing chemical changes in the battery it is configured in. 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 powders or metal fibers such as copper, nickel, aluminum, and silver, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, or conductive polymers such as polyphenylene derivatives. One of these may be used alone or a mixture of two or more. The conductive material may be included in an amount of 0.1 wt% to 15 wt% relative to the total weight of the positive electrode active material layer.

[0094] The binder plays a role in improving the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, of which one or more may be used. The binder may be included in an amount of 0.1 to 15 wt% relative to the total weight of the positive electrode active material layer.

[0095] The positive electrode may be manufactured by a conventional positive electrode manufacturing method, except for the use of the positive electrode active material. Specifically, it may be manufactured by coating a positive electrode slurry composition, prepared by dissolving or dispersing the positive electrode active material and selectively a binder and conductive material in a solvent, onto a positive electrode current collector, followed by drying and rolling.

[0096] The solvent may be any solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and one or more of these may be used individually or in mixtures of two or more. The amount of solvent used should be such that it dissolves or disperses the cathode active material, conductive material, and binder, and then provides a viscosity that allows for excellent thickness uniformity during coating for cathode manufacturing, taking into consideration the coating thickness and production yield of the slurry.

[0097] In another embodiment, the positive electrode may be manufactured by casting the positive electrode slurry composition onto a separate support, peeling it off the support, and then laminating the resulting film onto the positive electrode current collector.

[0098] Furthermore, according to yet another aspect of the present invention, an electrochemical element including the aforementioned positive electrode may be provided. Specifically, the electrochemical element may be a battery, a capacitor, or the like, and more specifically, a lithium secondary battery.

[0099] The lithium secondary battery may specifically include a positive electrode, a negative electrode positioned opposite the positive electrode, and a separator membrane and electrolyte interposed between the positive and negative electrodes. Here, since the positive electrode is as described above, for convenience, a detailed explanation will be omitted, and only the remaining components not mentioned above will be described in detail below.

[0100] The lithium secondary battery may further selectively include a battery container for housing the electrode assembly comprising the positive electrode, the negative electrode, and the separator membrane, and a sealing member for sealing the battery container.

[0101] The negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0102] The negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. 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. The negative electrode current collector may also typically have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, fine irregularities can be formed on the surface of the current collector to strengthen the bonding force of the negative electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, and nonwoven fabric.

[0103] The negative electrode active material layer may be manufactured by applying a negative electrode slurry composition, which includes the negative electrode active material together with a conductive material and, if necessary, a selective binder, to the negative electrode current collector.

[0104] As the 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 lithium-doped and dedoped metal oxides such as (0<β<2), SnO2, vanadium oxide, and lithium vanadium oxide, or composites containing the metallic compound and carbonaceous material, such as Si-C composites or Sn-C composites. One or more of these mixtures may be used. A metallic lithium thin film may also be used as the negative electrode active material. Furthermore, low-crystalline carbon and high-crystalline carbon may all be used as the carbon material. Typical examples of low-crystalline carbon include soft carbon and hard carbon, while typical examples of high-crystalline carbon include amorphous, plate-like, flaky, 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.

[0105] The aforementioned negative electrode active material may be present in an amount of 80 wt% to 99 wt% based on the total weight of the negative electrode active material layer.

[0106] The binder may be added in an amount of 0.1 wt% to 10 wt% based on the total weight of the negative electrode active material layer, as a component that assists in bonding between the conductive material, active material, and current collector. 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.

[0107] The conductive material may be added as a component to further improve the conductivity of the negative electrode active material, 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 a conductive material is not particularly limited as long as it has conductivity without inducing a chemical change 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, and thermal black, conductive fibers such as carbon fibers and metal fibers, metal powders such as carbon fluoride, aluminum, and nickel powder, conductive whiskers such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and conductive materials such as polyphenylene derivatives may be used.

[0108] In one embodiment, the negative electrode active material layer may be manufactured by coating a negative electrode slurry composition, which is prepared by dissolving or dispersing a negative electrode active material and a binder and conductive material selectively in a solvent, onto a negative electrode current collector and then drying it, or by casting the negative electrode slurry composition onto a separate support, peeling it off the support, and then laminating the resulting film onto the negative electrode current collector.

[0109] On the other hand, in the lithium secondary battery, the separation membrane separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. It can be used without particular limitations as long as it is a membrane typically used in lithium secondary batteries, and it is especially preferable that it has low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity. Specifically, porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof may be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, to ensure heat resistance or mechanical strength, coated separation membranes containing ceramic components or polymeric substances may be used, and may be selectively used as single-layer or multi-layer structures.

[0110] Furthermore, the electrolytes used in the present invention include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0111] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0112] The organic solvent can be any solvent that serves as a medium through which ions involved in the electrochemical reaction of the battery can move, without any particular limitations. Specifically, the organic solvent may be an ester solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; an ether solvent such as dibutyl ether or tetrahydrofuran; a ketone solvent such as cyclohexanone; an aromatic hydrocarbon solvent such as benzene or fluorobenzene; dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate (propylene Carbonate solvents such as carbonate (PC), alcohol solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, and may include a double-bonded aromatic ring or ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, or sulfolanes may be used. Among these, carbonate solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred. In this case, the electrolyte performance can be improved by mixing the cyclic carbonate and the linear carbonate in a volume ratio of about 1:1 to about 1:9.

[0113] The lithium salt may be any compound capable of providing lithium ions for use in a lithium secondary battery, without any particular limitations. Specifically, the lithium salt may be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3), LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. The concentration of the lithium salt is preferably within the range of 0.1 M to 2.0 M. When the concentration of the lithium salt falls within this range, the electrolyte has appropriate conductivity and viscosity, so it can exhibit excellent electrolyte performance and lithium ions can move effectively.

[0114] In the present invention, when the electrolyte used is a solid electrolyte, a solid inorganic electrolyte such as a sulfide-based solid electrolyte, oxide-based solid electrolyte, nitride-based solid electrolyte, or halogen-based solid electrolyte may be used, and preferably a sulfide-based solid electrolyte may be used.

[0115] As the material for the sulfide-based solid electrolyte, a solid electrolyte containing Li, element X (where 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 sulfide-based solid electrolyte materials include Li2S-P2S5, Li2S-P2S-LiX (where 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, Li2S-P2S5-ZmSn (where m and n are integers and Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, and Li2S-SiS2-Li p MO q(Here, p and q are integers, and M is P, Si, Ge, B, Al, Ga, or In.)

[0116] The solid electrolyte, preferably a sulfide-based solid electrolyte, may be amorphous or crystalline, or a mixture of amorphous and crystalline materials.

[0117] As a material for oxide-based solid electrolytes, Li7La3Zr2O 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 PO 4-x N x (LiPON), Li 2+2x Zn 1-x Examples include GeO4 (LISICON).

[0118] The aforementioned solid electrolyte may be arranged as a separate layer (solid electrolyte layer) between the positive electrode and the negative electrode. Furthermore, the solid electrolyte may be included in a portion of the positive electrode active material layer of the positive electrode independently of the solid electrolyte layer, or in a portion of the negative electrode active material layer of the negative electrode independently of the solid electrolyte layer.

[0119] In addition to the electrolyte components, the electrolyte may further contain one or more additives for the purpose of improving battery life characteristics, suppressing the decrease in battery capacity, and improving battery discharge capacity, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexalic acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive may be present in an amount of 0.1 wt% to 5 wt% relative to the total weight of the electrolyte.

[0120] As described above, the lithium secondary battery containing the positive electrode active material according to the present invention exhibits excellent discharge capacity, output characteristics, and life characteristics in a stable manner, making it useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in the electric vehicle field, such as hybrid electric vehicles (HEVs).

[0121] The external shape of the lithium secondary battery according to the present invention is not particularly limited, but it may be cylindrical, rectangular, pouch-shaped, or coin-shaped using a can. Furthermore, the lithium secondary battery can be used not only as a battery cell used as a power source for small devices, but may also be preferably used as a unit battery in medium-to-large battery modules containing multiple battery cells.

[0122] According to yet another aspect of the present invention, a battery module and / or a battery pack including the lithium secondary battery as a unit cell can be provided.

[0123] The battery module or battery pack can be used as a power source for one or more medium-to-large devices, including power tools, electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs), or power storage systems.

[0124] The present invention will be described in more detail below with reference to examples. However, these examples are for illustrative purposes only and should not be construed as limiting the scope of the present invention.

[0125] (Manufacturing Example 1: Manufacturing of Cathode Active Material) (Example 1) (a) Spherical Ni by co-precipitation method 0.90 Co 0.05 Mn 0.05 (OH)2 hydroxide precursors were synthesized. Specifically, in a 90L reactor, 50 wt% NaOH and 18 wt% NH4OH were added to a 1.5M aqueous solution of composite transition metal sulfuric acid, which was prepared by mixing nickel sulfate, cobalt sulfate, and manganese sulfate in a molar ratio of 90:5:5. The pH in the reactor was maintained at 12.0, the reactor temperature was maintained at 45°C, and inert gas N2 was added to the reactor to prevent oxidation of the produced precursor. After the reaction was complete, the precursor was separated, washed with deionized water, and dried at 120°C for 10 hours to obtain Ni 0.90 Co 0.05 Mn 0.05 (OH)2 hydroxide precursor was obtained.

[0126] (b) The hydroxide precursor obtained in step (a) above was subjected to a primary heat treatment at 720°C for 8 hours in a LiOH (Li / (Ni+Co+Mn)molar ratio = 1.065; where (Ni+Co+Mn) is a value calculated based on the composition of the hydroxide precursor obtained in step (a) above) atmosphere to obtain an intermediate product.

[0127] (c) The intermediate product obtained in step (b) above was weighed with Co(OH)2 (weighed so that the mole fraction of Co relative to all metal elements excluding lithium in the intermediate product was 0.02 (2.0 mol%)), ZrO2 (weighed so that the mole fraction of Zr relative to all metal elements excluding lithium in the intermediate product was 0.0005 (0.05 mol%)), and TiO2 (weighed so that the mole fraction of Ti relative to all elements in the hydroxide precursor was 0.001 (0.1 mol%)), and the mixture was subjected to secondary heat treatment in an O2 atmosphere at 680°C for 5 hours to obtain the final product (lithium transition metal oxide).

[0128] (Example 2) The positive electrode active material was manufactured in the same manner as in Example 1, except that the secondary heat treatment temperature in step (c) was set to 740°C.

[0129] (Example 3) A positive electrode active material was produced in the same manner as in Example 1, except that the intermediate product obtained in step (b) was weighed with Co(OH)2 (weighed so that the mole fraction of Co relative to all metal elements excluding lithium in the intermediate product was 0.01 (1.0 mol%)), ZrO2 (weighed so that the mole fraction of Zr relative to all metal elements excluding lithium in the intermediate product was 0.0005 (0.05 mol%)), and TiO2 (weighed so that the mole fraction of Ti relative to all elements in the hydroxide precursor was 0.001 (0.1 mol%)), and a secondary heat treatment was performed in an O2 atmosphere at 680°C for 5 hours.

[0130] (Comparative Example 1) The positive electrode active material was manufactured in the same manner as in Example 1, except that the secondary heat treatment temperature in step (c) was set to 500°C.

[0131] (Comparative Example 2) The positive electrode active material was manufactured in the same manner as in Example 1, except that the secondary heat treatment temperature in step (c) was set to 650°C.

[0132] (Comparative Example 3) The positive electrode active material was manufactured in the same manner as in Example 1, except that the secondary heat treatment temperature in step (c) was set to 800°C.

[0133] (Comparative Example 4) The positive electrode active material was manufactured in the same manner as in Example 1, except that the secondary heat treatment temperature in step (c) was set to 900°C.

[0134] (Comparative Example 5) The cathode active material was produced in the same manner as in Example 1, except that the intermediate product obtained in step (b) was not mixed with Co(OH)2, ZrO2, and TiO2, and was subjected to secondary heat treatment at 680°C for 5 hours in an O2 atmosphere.

[0135] (Comparative Example 6) A positive electrode active material was produced in the same manner as in Example 1, except that the intermediate product obtained in step (b) was weighed with Co(OH)2 (weighed so that the mole fraction of Co relative to all metal elements excluding lithium in the intermediate product was 0.04 (4.0 mol%)), ZrO2 (weighed so that the mole fraction of Zr relative to all metal elements excluding lithium in the intermediate product was 0.0005 (0.05 mol%)), and TiO2 (weighed so that the mole fraction of Ti relative to all elements in the hydroxide precursor was 0.001 (0.1 mol%)), and a secondary heat treatment was performed in an O2 atmosphere at 680°C for 5 hours.

[0136] (Manufacturing Example 2: Manufacturing of Lithium-ion Rechargeable Batteries (Half-Cells)) A cathode slurry was prepared by dispersing 94 wt% of each of the cathode active materials produced according to Production Example 1, 3 wt% of carbon black, and 3 wt% of PVDF binder in 30 g of N-methyl-2-pyrrolidone (NMP). The cathode slurry was uniformly coated onto a 15 μm thick aluminum thin film and vacuum-dried at 135°C to produce a cathode for a lithium secondary battery.

[0137] A half-cell was manufactured using a lithium foil as the counter electrode for the positive electrode, a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) as the separation membrane, and an electrolyte containing ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:7 with LiPF6 present at a concentration of 1.15 M.

[0138] (Experimental Example 1. TEM / EDS analysis of positive electrode active material) Whether or not there are cobalt-rich regions in which the distribution of cobalt within the unit particles constituting the lithium transition metal oxide is unevenly distributed in each positive electrode active material manufactured according to Manufacturing Example 1 was analyzed by EDS (Energy Dispersive X-ray Spectroscopy) mapping onto the cross-sectional TEM image of the lithium transition metal oxide.

[0139] TEM / EDS analysis was performed using the following method. First, the lithium transition metal oxides contained in each positive electrode active material manufactured according to Manufacturing Example 1 were selected, and after cross-sectional treatment of the lithium transition metal oxides using FIB (Ga-ion source), cross-sectional images were taken with TEM (transmission electron microscope). Next, the target element, cobalt (Co), was mapped through EDS mapping onto the cross-sectional TEM images, and the degree of uneven distribution of cobalt, the thickness of the cobalt-rich region, the average cobalt content c1 (mol%) within the cobalt-rich region, and the average cobalt content c2 (mol%) measured at the center of the unit particle were measured. The results of the TEM / EDS analysis are shown in Figures 1 to 9.

[0140] The results of the above analysis are shown in Table 1 below.

[0141] [Table 1]

[0142] Referring to Figures 4, 5, and 9, which are TEM / EDS images of lithium transition metal oxides from Comparative Examples 1, 2, and 6, it was confirmed that cobalt-containing oxides were present in the form of fine particles on the surface of the lithium transition metal oxides from Comparative Examples 1, 2, and 6.

[0143] In Comparative Example 1, it is expected that the cobalt mixed in step (c) did not penetrate into the unit particles, and that a large amount of cobalt formed a cobalt-containing coating layer containing cobalt-containing oxides on the surface of the unit particles. In Comparative Example 2, some of the cobalt mixed in step (c) penetrated into the unit particles, forming cobalt-rich regions, but it can be confirmed that a large amount of cobalt formed a cobalt-containing coating layer containing cobalt-containing oxides on the surface of the unit particles.

[0144] In Comparative Examples 3 and 4, it can be confirmed that the cobalt mixed in step (c) diffuses uniformly within the unit particles, preventing the formation of cobalt-rich regions on the surface of the unit particles where the cobalt distribution is uneven. In Comparative Example 6, the cobalt content mixed in step (c) was excessively high, causing some of the cobalt to penetrate into the unit particles and form cobalt-rich regions. However, it can be confirmed that a large amount of cobalt formed a cobalt-containing coating layer containing cobalt-containing oxides on the surface of the unit particles.

[0145] (Experimental Example 2. XRD Analysis of Cathode Active Material) X-ray diffraction (XRD) analysis was performed on each of the positive electrode active materials produced according to Production Example 1 to analyze the crystallographic properties of the lithium transition metal oxide contained in the positive electrode active material.

[0146] Specifically, the XRD analysis was performed using a Bruker D8E Endeavor diffractometer with Cu-Kα radiation (1.540598 Å). From the diffraction spectrum obtained through X-ray diffraction (XRD) analysis using Cu-kα rays on the positive electrode active material, the full width at half-maximum (FWHM) of the peak corresponding to the (10⁴) crystal plane (2θ = 44.5 ± 1.0° region) of the R-3m layered crystal structure was calculated.

[0147] Furthermore, the difference between the peak intensity ratio (I(003) / I(104)) of the peak corresponding to the (003) crystal plane (2θ=18.6±1.0° region) and the peak intensity corresponding to the (104) crystal plane (I(104)) calculated from the diffraction spectrum was calculated, and 2θ(110) where the peak corresponding to the (110) crystal plane (2θ=64.6°~64.8° region) is observed, and 2θ(108) where the peak corresponding to the (108) crystal plane (2θ=64.3°~64.5° region) is observed was calculated.

[0148] The peaks in the region 2θ = 44.5 ± 0.5° within the diffraction spectrum obtained by X-ray diffraction (XRD) analysis of the positive electrode active material using Cu-kα rays are specific to the (104) crystal plane of the lithium transition metal oxide having an R-3m layered crystal structure, and exhibit the strongest peak intensity in the region 2θ = 44.5 ± 1.0°. When cobalt-containing oxides such as LiCoO2 and LiCo2O4 are present as impurities, a peak split occurs in the region 2θ = 44.5 ± 1.0° from the diffraction spectrum, allowing observation of a peak in the region 2θ = 45.25 ± 0.25°.

[0149] Within the region 2θ = 45.25 ± 0.25°, peaks specific to the (104) crystal plane of LiCoO2 and peaks specific to the (400) crystal plane of LiCo2O4 can be observed. Furthermore, since the peaks specific to the (104) crystal plane of LiCoO2 and the peaks specific to the (400) crystal plane of LiCo2O4 may overlap and not be precisely separated, it is preferable that when calculating the I2 / I1 peak intensity ratio described later, I2 is considered as the average intensity of the peaks present within the region 2θ = 45.25 ± 0.25°.

[0150] The stronger the peak intensity within the region 2θ = 45.25 ± 0.25°, the higher the content of the Co-containing impurity (LCO) phase on the surface of the lithium transition metal oxide, which means there is a high probability that a cobalt-containing coating layer exists on the surface of the lithium transition metal oxide.

[0151] When a peak in the region 2θ = 45.25 ± 0.25° is observed from the diffraction spectrum, it indicates the presence of an LCO peak. To indirectly indicate the possibility of a Co-containing impurity (LCO) phase content on the surface of the lithium transition metal oxide or the presence of a cobalt-containing coating layer on the surface of the lithium transition metal oxide, the ratio (I2 / I1) of the average intensity (I2) of the peaks in the region 2θ = 45.25 ± 0.25° to the maximum intensity (I1) of the peaks in the region 2θ = 44.5 ± 0.5° in the diffraction spectrum was calculated.

[0152] The average crystallite size of the lithium transition metal oxide was calculated from a straight line obtained by plotting the diffraction angle θ (rad) and full width at half maximum β (rad) in the region 2θ = 10° to 120° on a coordinate plane with sinθ on the horizontal axis and βcosθ on the vertical axis, using Rietveld refinement on the diffraction spectrum obtained through X-ray diffraction (XRD) analysis using Cu-kα rays on the positive electrode active material.

[0153] Furthermore, the lattice strain ε, expressed as Δd / d, was calculated as 1 / 4 of the slope of the straight line obtained by plotting the diffraction angle θ (rad) and full width at half maximum β (rad) in the region 2θ = 10° to 120° on a coordinate plane with sinθ on the horizontal axis and βcosθ on the vertical axis through Rietveld Refinement of the diffraction spectrum (or it can be expressed as β / 4tanθ).

[0154] The results of the above analysis are shown in Table 2 below.

[0155] [Table 2]

[0156] Referring to Table 2 above, when Co-containing impurities (LCO) are present in the lithium transition metal oxide, it can be confirmed from the diffraction spectrum that a peak split within the region 2θ = 44.5 ± 1.0° (more specifically, a peak within the region 2θ = 45.25 ± 0.25°) exists. Furthermore, when a peak split within the region 2θ = 44.5 ± 1.0° exists in the diffraction spectrum, it can be confirmed that the full width at half maximum of the peak corresponding to the (10⁴) crystal plane in the diffraction spectrum is greater than 0.158°.

[0157] Furthermore, as confirmed by TEM / EDS analysis, the I2 / I1 intensity ratio measured from the positive electrode active material in Comparative Examples 1, 2, and 6, where cobalt-containing oxide (LCO) is present on the surface of the lithium transition metal oxide (unit particle), exceeds 0.015. In other words, regardless of the presence or absence of cobalt-rich regions within the unit particle, if cobalt-containing oxide is present as an impurity on the surface of the unit particle and / or the lithium transition metal oxide, the I2 / I1 intensity ratio measured from the positive electrode active material may exceed 0.015.

[0158] Generally, lattice strain, which is one of the indicators showing the regularity of crystallites such as the crystallite size or the distance between adjacent crystallites constituting the lithium transition metal oxide, tends to increase as the cobalt-rich regions within the unit particles are formed. Referring to the results of Comparative Examples 3 and 4, as confirmed by TEM / EDS analysis, it can be seen that when the cobalt-rich regions within the unit particles are not formed and the material diffuses uniformly, the lattice strain has a value of less than 0.00040. Referring to the results of Comparative Example 6, it can be seen that as the cobalt content forming the cobalt-rich regions within the unit particles becomes excessive, the lattice strain has a value of greater than 0.00060.

[0159] (Experimental Example 3. Evaluation of the electrochemical properties of lithium secondary batteries (half-cells)) For lithium secondary batteries (half-cells) manufactured in Manufacturing Example 2, the initial charge capacity, initial discharge capacity, initial efficiency, and 2.0C / 0.1C rate characteristics were measured through charge-discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, a voltage range of 3.0V to 4.3V, and a discharge rate of 2.0C / 0.1C.

[0160] Furthermore, the same lithium secondary battery (half-cell) was subjected to 50 charge-discharge cycles using an electrochemical analyzer (Toyo, Toscat-3100) under conditions of room temperature (25°C) and high temperature (45°C), voltage range 3.0V~4.3V, and 1C / 1C. The ratio of the discharged capacity at the 50th cycle to the initial capacity (cycle capacity retention) was then measured.

[0161] The measurement results are shown in Table 3 below.

[0162] [Table 3]

[0163] Comparing the results of Examples 1 to 3 with Comparative Examples 1 and 5, it can be confirmed that when a cobalt-rich region is formed on the surface of a unit particle, there is no decrease in capacity characteristics or initial efficiency, and rate characteristics and capacity retention rates at room temperature and high temperature can all be improved.

[0164] Comparing the results of Examples 1 to 3 with Comparative Examples 3 and 4, it can be confirmed that when cobalt is uniformly diffused inside the unit particle, but instead a cobalt-rich region with unevenly distributed cobalt is formed on the surface of the unit particle, the discharge capacity, rate characteristics, and capacity retention at room temperature and high temperature are all improved.

[0165] Furthermore, comparing the results of Examples 1 to 3 with those of Comparative Examples 2 and 6, it can be confirmed that while some of the cobalt mixed in step (c) penetrated into the unit particles and formed cobalt-rich regions, when a large amount of cobalt formed a cobalt-containing coating layer containing cobalt-containing oxides on the surface of the unit particles, the improvement in rate characteristics and capacity retention was insufficient.

[0166] Although embodiments of the present invention have been described above, any person with ordinary skill in the art can modify and change the present invention in various ways, such as by adding, changing, deleting, or adding components, without departing from the spirit of the invention as described in the claims, and this can also be said to be within the scope of the rights of the present invention.

Claims

1. A positive electrode active material comprising a lithium transition metal oxide containing lithium, nickel, and cobalt, The lithium transition metal oxide has a secondary particle form in which multiple unit particles are aggregated, On the surface of one or more of the unit particles, there are cobalt-rich regions where the distribution of cobalt is unevenly distributed. The thickness of the cobalt-rich region is 1 nm to 100 nm. A positive electrode active material in which, through X-ray diffraction (XRD) analysis using Cu-kα rays on the positive electrode active material, the ratio (I2 / I1) of the average intensity of the peaks located in the region of 2θ = 45.25 ± 0.25° to the maximum intensity of the peaks located in the region of 2θ = 44.5 ± 0.5° to the maximum intensity of the peaks located in the region of 2θ = 44.5 ± 0.5° is greater than 0.001 and less than 0.

015.

2. The positive electrode active material according to claim 1, wherein the average mole fraction of cobalt measured within the cobalt-rich region is greater than the average mole fraction of cobalt within the unit particle.

3. The positive electrode active material according to claim 1, wherein the average mole fraction of cobalt measured within the cobalt-rich region is 1.35 times or more and less than 10 times the mole fraction of cobalt measured at the center of the unit particle.

4. The lithium transition metal oxide is represented by the following chemical formula 1, and is the positive electrode active material according to claim 1. [Chemical formula 1] Li ｗ Ni １－（ｘ＋ｙ＋ｚ） Co ｘ Mn ｙ M1 ｚ O ２ [Here, in the chemical formula 1, M1 is at least one selected from Na, K, Mg, Ba, B, Ce, Hf, Ta, Cr, F, Al, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, P, Sr, Ge, Nd, Gd, and Cu. 0.95 ≤ w ≤ 1.15, 0 < x ≤ 0.20, 0 ≤ y ≤ 0.20, 0 < z ≤ 0.

20.

5. The positive electrode active material according to claim 1, wherein the average particle size of the unit particles is 0.1 μm to 1.0 μm.

6. The positive electrode active material according to claim 1, wherein the average particle size of the secondary particles is 2.0 μm to 16.0 μm.

7. The positive electrode active material according to claim 1, wherein no cobalt-containing oxide is present on the surface of the secondary particles.

8. The positive electrode active material according to claim 1, wherein cobalt-containing oxides are not present in the grain boundaries formed by gaps between adjacent unit particles or contact between adjacent unit particles.

9. The positive electrode active material according to claim 8, wherein the average mole fraction of cobalt measured within the cobalt-rich region is greater than the mole fraction of cobalt measured at the grain boundaries formed by the contact of adjacent unit particles.

10. The positive electrode active material according to claim 1, wherein the peak intensity ratio (I(003) / I(104)) of the maximum peak intensity corresponding to the (003) crystal plane (I(003)) to the maximum peak intensity corresponding to the (104) crystal plane (I(104)), calculated from the diffraction spectrum obtained by X-ray diffraction (XRD) analysis of the positive electrode active material using Cu-kα rays, is 1.5 to 2.

0.

11. The positive electrode active material according to claim 1, wherein the difference between 2θ(110), where a peak corresponding to the (110) crystal plane is observed and 2θ(108), where a peak corresponding to the (108) crystal plane is observed, calculated from the diffraction spectrum obtained by X-ray diffraction (XRD) analysis of the positive electrode active material using Cu-kα rays, is 0.3° to 0.5°.

12. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 11.

13. A lithium secondary battery using the positive electrode described in claim 12.

Images