Positive electrode active material and lithium secondary battery containing the same

Abstract

The present invention provides a positive electrode active material with improved resistance to strain caused by volume contraction / expansion of unit particles resulting from repeated charging and discharging, and a lithium secondary battery containing the same. [Solution] The positive electrode active material contains a lithium transition metal oxide in the form of a single particle consisting of one unit particle and / or a similar single-particle form in which 30 or fewer unit particles are aggregated. By reducing the number of intergranular cracks and intragranular cracks present in the positive electrode active material, it is possible to enable a lithium secondary battery using the positive electrode active material to exhibit stable electrochemical properties for a long period of time.

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material and a lithium secondary battery containing the same, and more specifically, to a positive electrode active material comprising a lithium transition metal oxide in the form of a single particle consisting of one unit particle and / or a similar single-particle form in which 30 or fewer unit particles are aggregated, wherein the positive electrode active material reduces intergranular cracks and intragranular cracks present in the positive electrode active material, thereby enabling the lithium secondary battery using the positive electrode active material to exhibit stable electrochemical properties for a long period of time, and to a lithium secondary battery containing the same. [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 positive electrode and negative electrode active materials, 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 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 problems such as decreased stability and an increase in the content of unreacted lithium impurities such as LiOH and Li2CO3 on the surface.

[0009] Furthermore, many lithium transition metal oxides have a secondary particle morphology in which multiple primary particles are aggregated, and the specific surface area increases as the number of primary particles constituting the secondary particle increases. As the specific surface area of ​​the lithium transition metal oxide increases, if strain due to random volume contraction / expansion of the primary particles accumulates due to repeated charging and discharging, cracks (intergranular cracks) may occur between adjacent primary particles. Intergranular cracks present within the secondary particles reduce the connectivity between adjacent primary particles and interfere with the lithium ion transport mechanism. In addition, increasing the exposed surface area of ​​the primary particles can lead to an increase in side reactions with the electrolyte, which can rapidly reduce the stability of the positive electrode active material.

[0010] Therefore, recently, in order to solve the above-mentioned problems of grain boundary cracks, attempts have been made to induce the growth of the primary particles through high-temperature firing and reduce the specific surface area of the lithium transition metal oxide. However, when inducing the growth of the primary particles through high-temperature firing, although the tendency of intergranular cracks to decrease is shown, there is a problem that cracks increase in the primary particles. The cracks existing in the primary particles include intragranular cracks existing inside the primary particles and surface cracks existing on the surface of the primary particles. Similar to the intergranular cracks, the surface cracks existing on the surface of the primary particles may increase the exposed surface (surface area) of the primary particles and increase the side reactions with the electrolyte. In addition, when cracks such as intragranular cracks and surface cracks existing in the primary particles increase, the resistance to strain caused by the random volume shrinkage / expansion of the primary particles due to repeated charge and discharge may rapidly weaken. That is, the more cracks exist in the primary particles or the more cracks accumulate in the primary particles due to repeated charge and discharge, the more rapidly the life of the lithium secondary battery using the positive electrode active material decreases. Summary of the Invention Problems to be Solved by the Invention

[0011] In the lithium secondary battery market, while the growth of lithium secondary batteries for electric vehicles is playing a role as a market driver, the demand for the positive electrode active material used in lithium secondary batteries is also continuously increasing.

[0012] For example, conventionally, lithium secondary batteries using lithium iron phosphate (LFP) have been mainly used from the perspective of ensuring safety, etc. Recently, however, the use of nickel-based lithium transition metal oxides with a larger energy capacity per weight compared to LFP has been on the rise (of course, relatively inexpensive LFP may still be used to reduce costs).

[0013] Also, recently, nickel-based lithium transition metal oxides mainly used as the positive electrode active material of high-capacity lithium secondary batteries generally have a ternary composition such as NCM (Ni-Co-Mn) and NCA (Ni-Co-Al), or a quaternary composition such as NCMA (Ni-Co-Mn-Al).

[0014] As described above, many ternary or quaternary lithium transition metal oxides have a secondary particle form in which a plurality of primary particles are aggregated. The present invention aims to improve the energy density of the positive electrode active material and prevent a decrease in life and stability by inducing the growth of primary particles, which are the unit particles constituting the lithium transition metal oxide, so that the lithium transition metal oxide has a single particle form consisting of one unit particle and / or a similar single particle form in which 30 or fewer unit particles are aggregated, thereby providing a positive electrode active material with reduced intergranular cracks in the lithium transition metal oxide.

[0015] In addition, conventionally, when inducing the growth of primary particles, which are the unit particles constituting the lithium transition metal oxide, by a method known in the art, cracks such as intragranular cracks and surface cracks may increase in the lithium transition metal oxide, particularly in the primary particles, which are the unit particles constituting the lithium transition metal oxide.

[0016] Therefore, the present invention aims to provide a positive electrode active material having a single-particle form consisting of one unit particle and / or a similar single-particle form in which 30 or fewer unit particles are aggregated, and having improved resistance to strain caused by volume contraction / expansion of unit particles due to repeated charging and discharging, by suppressing and / or mitigating the formation of cracks within the unit particles, and a lithium secondary battery that can exhibit stable electrochemical properties for a long period of time using the same.

[0017] 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]

[0018] According to one aspect of the present invention for solving the aforementioned technical problems, a positive electrode active material is provided that includes a lithium transition metal oxide capable of lithium intercalation / deintercalation.

[0019] The lithium transition metal oxide can reduce intergranular cracks by having at least one form selected from a single-particle form consisting of one unit particle and a similar single-particle form in which 30 or fewer unit particles are aggregated.

[0020] Furthermore, the lithium transition metal oxide can improve resistance to strain caused by volume contraction / expansion of unit particles due to repeated charging and discharging by suppressing and / or mitigating the formation of cracks such as intragranular cracks and surface cracks in the unit particles.

[0021] For example, the ratio (a1 / a) of the total area (a1) of the cracks present on the surface of the unit particle to the surface area (a) of the unit particle observed from the surface SEM image of the lithium transition metal oxide may be 10% or less, 9% or less, 8% or less, 7.5% or less, or 7.2% or less.

[0022] Further, when the radius of the unit particle measured from the cross-sectional SEM image of the unit particle is r, the internal cracks on the surface part where the distance (d) from the center of the unit particle is (2 / 3)r < d may be more than the internal cracks in the central part where the distance (d) from the center of the unit particle is 0 ≦ d ≦ (2 / 3)r.

[0023] Therefore, the positive electrode active material according to the present invention can improve the long-term stability by reducing the cracks present in the surface part of the unit particle.

[0024] The lithium transition metal oxide contains lithium and a transition metal, and the content of nickel in the transition metal may be 50 mol% or more, 60 mol% or more, 70 mol% or more, 80 mol% or more, or 90 mol% or more. Also, the content of nickel in the transition metal may be 99 mol% or less, 95 mol% or less, or 90 mol% or less.

[0025] The lithium transition metal oxide may be represented by the following Chemical Formula 1. [Chemical Formula 1] Li a Ni 1-(b+c+d+e) Co b Mn c M1 d M2 e O2 In Chemical Formula 1, M1 is at least one selected from Na, K, Mg, Ca, Sr, Ba, and Rb, M2 is at least one selected from 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, 0.85≦a≦1.15, 0≦b≦0.20, 0≦c≦0.20, 0.001≦d≦0.05, 0≦e≦0.10, and 0.5≦1-(b+c+d+e)<1.0.

[0026] The content of alkali metals and alkaline earth metals in the total elements excluding lithium in the lithium transition metal oxide may be 0.1 mol% to 5.0 mol%, 0.5 mol% to 4.0 mol%, or 1.0 mol% to 3.0 mol%.

[0027] The average particle size (D) of the lithium transition metal oxide present in the single-particle form. 50 ) is 1.5 μm or more and 10.0 μm or less, and the average particle size (D) of the lithium transition metal oxide present in the similar single-particle form is 1.5 μm or more and 10.0 μm or less. 50 ) may be between 3.0 μm and 20.0 μm.

[0028] The number of unit particles observed from the surface SEM image of the lithium transition metal oxide existing in the aforementioned similar single-particle morphology may be 20 or less.

[0029] The surface shape of the unit particles observed from the surface SEM image of the lithium transition metal oxide has a long axis and a short axis, and the average length of the long axis of the unit particles may be between 2.0 μm and 10.0 μm.

[0030] The surface shape of the unit particles observed from the surface SEM image of the lithium transition metal oxide has a long axis and a short axis, and the average length of the short axis of the unit particles may be 1.0 μm or more and less than 10.0 μm.

[0031] The positive electrode active material according to the present invention can improve the long-term stability of the positive electrode active material by reducing intragranular cracks present in the surface portion of lithium transition metal oxides in which the growth of unit particles is induced through high-temperature firing.

[0032] Furthermore, according to another aspect of the present invention, a positive electrode containing the positive electrode active material described above is provided.

[0033] Furthermore, according to yet another aspect of the present invention, a lithium secondary battery using the positive electrode described above is provided. [Effects of the Invention]

[0034] According to the present invention, by inducing the growth of primary particles, which are the unit particles constituting the lithium transition metal oxide, such that the lithium transition metal oxide has a single-particle form consisting of one unit particle and / or a similar single-particle form in which 30 or fewer unit particles are aggregated, the intergranular cracks in the lithium transition metal oxide can be reduced. By reducing the intergranular cracks in the lithium transition metal oxide, it is possible to improve the lifespan and stability of a lithium secondary battery using the lithium transition metal oxide as a positive electrode active material.

[0035] Furthermore, the present invention has the advantage that, by suppressing and / or mitigating the formation of cracks within the unit particles, resistance to strain caused by volume contraction / expansion of unit particles due to repeated charging and discharging is improved, thereby enabling lithium secondary batteries using the lithium transition metal oxide as the positive electrode active material to exhibit stable electrochemical properties for a long period of time. [Brief explanation of the drawing]

[0036] [Figure 1] Figure 1 shows a surface SEM image of the positive electrode active material (lithium transition metal oxide) according to Example 1. [Figure 2] Figure 2 shows a surface SEM image of the positive electrode active material (lithium transition metal oxide) according to Example 2. [Figure 3] Figure 3 shows a surface SEM image of the positive electrode active material (lithium transition metal oxide) according to Example 3. [Figure 4] Figure 4 shows a surface SEM image of the positive electrode active material (lithium transition metal oxide) according to Example 4. [Figure 5]Figure 5 shows a surface SEM image of the positive electrode active material (lithium transition metal oxide) according to Example 5. [Figure 6] Figure 6 shows a surface SEM image of the positive electrode active material (lithium transition metal oxide) according to Comparative Example 1. [Figure 7] Figure 7 shows a surface SEM image of the positive electrode active material (lithium transition metal oxide) according to Comparative Example 2. [Figure 8] Figure 8 shows a surface SEM image of the positive electrode active material (lithium transition metal oxide) according to Comparative Example 3. [Modes for carrying out the invention]

[0037] 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.

[0038] (Cathode active material) In this application, the positive electrode active material includes a lithium transition metal oxide capable of reversible intercalation / deintercalation of lithium ions. Here, a lithium transition metal oxide means an oxide in which lithium and at least one transition metal are combined.

[0039] The positive electrode active material according to the present invention comprises a lithium transition metal oxide in the form of a single particle consisting of one unit particle and / or a similar single-particle form in which 30 or fewer unit particles are aggregated. The present invention provides a positive electrode active material that enables a lithium secondary battery using the positive electrode active material to exhibit stable electrochemical properties for a long period of time by reducing the cracks present in the positive electrode active material.

[0040] The lithium transition metal oxide may contain intergranular cracks, intragranular cracks, and / or surface cracks.

[0041] The aforementioned intergranular crack refers to a crack formed between adjacent unit particles (primary particles) (grain boundary) when the lithium transition metal oxide exists in a secondary particle morphology such as a similar single-particle morphology. The aforementioned intergranular crack can occur due to the accumulation of strain caused by random volume contraction / expansion of the unit particles constituting the secondary particles during charging and discharging. The positive electrode active material according to the present invention can suppress and / or mitigate intergranular cracks by reducing the number of unit particles constituting the lithium transition metal oxide.

[0042] The intragranular crack refers to a crack formed within a unit particle constituting the lithium transition metal oxide. The intragranular crack refers to an isolated crack inside the unit particle, and the intragranular crack formed inside the unit particle can be observed from a cross-sectional SEM image of the lithium transition metal oxide.

[0043] The aforementioned surface crack refers to a crack formed on the surface of the unit particle. In particular, the frequency of occurrence of the surface crack may increase as the growth of the unit particle is induced. The surface crack formed on the surface of the unit particle can be observed from surface SEM images of the lithium transition metal oxide.

[0044] When inducing the growth of primary particles, which are the unit particles constituting the lithium transition metal oxide, the number of unit particles constituting the lithium transition metal oxide decreases, thereby suppressing and / or mitigating intergranular cracks. However, as the unit particles become larger, intragranular cracks and surface cracks inevitably occur in the unit particles. As a result, the positive electrode active material according to the present invention is characterized by improving the long-term stability of the positive electrode active material by suppressing and / or mitigating the formation of cracks in the unit particles constituting the lithium transition metal oxide. In particular, the positive electrode active material according to the present invention can improve the lifespan and stability of a lithium secondary battery using the lithium transition metal oxide as a positive electrode active material by reducing the surface cracks observed from surface SEM images of the lithium transition metal oxide.

[0045] The lithium transition metal oxide is a composite metal oxide capable of lithium ion intercalation / deintercalation 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° in the diffraction pattern obtained from XRD analysis.

[0046] The lithium transition metal oxide comprises lithium and a transition metal. The transition metal may include at least one selected from nickel, cobalt, and manganese. The lithium transition metal oxide may further contain aluminum.

[0047] As an example, the lithium transition metal oxide may be a lithium nickel-based transition metal composite oxide containing nickel. As another example, in order to improve the low rate characteristics and lifetime characteristics while maintaining the high reversible capacity of LiNiO2, the lithium nickel-based composite oxide may be a ternary lithium transition metal oxide such as so-called NCM (Ni-Co-Mn) and NCA (Ni-Co-Al), or a quaternary lithium transition metal oxide such as NCMA (Ni-Co-Mn-Al), in which some of the nickel is replaced with cobalt, manganese, and / or aluminum. As yet another example, the lithium nickel-based transition metal composite oxide may be a so-called cobalt-free type lithium transition metal oxide that does not contain cobalt.

[0048] The lithium transition metal oxide may be in a single-particle form consisting of one unit particle and / or a similar single-particle form in which 30 or fewer unit particles are aggregated. That is, the lithium transition metal oxide may exist as a single particle or as a secondary particle formed by the aggregation of multiple single particles.

[0049] In this application, "similar single particle" means a secondary particle formed by the aggregation of secondary particles in which a relatively small number of unit particles have aggregated, and such similar single particle may be a secondary particle formed by the aggregation of 5 or fewer, 10 or fewer, 20 or fewer, or 30 or fewer unit particles. When the lithium transition metal oxide has a secondary particle form in which more than 30 unit particles have aggregated, the lithium transition metal oxide can be called a multi-particle form.

[0050] Furthermore, the number of unit particles observed from the surface SEM image of the lithium transition metal oxide existing in the aforementioned similar single-particle morphology may be 20 or less, 18 or less, or 15 or less. If the number of unit particles observed from the surface SEM image of the lithium transition metal oxide existing in the aforementioned similar single-particle morphology is greater than 30, the lithium transition metal oxide can be called a multi-particle morphology. The aforementioned multi-particle morphology of lithium transition metal oxide may not adequately relax intergranular cracks within the lithium transition metal oxide.

[0051] The multiple single particles constituting the secondary particles may each be referred to as primary particles or unit particles, and the secondary particles may be referred to as bulk or bulk particles.

[0052] When lithium transition metal oxides in single-particle form are called non-aggregates and lithium transition metal oxides in secondary-particle form are called aggregates, the positive electrode active material may contain aggregates and / or non-aggregates. For example, the positive electrode active material may contain only non-aggregate lithium transition metal oxides or only aggregated lithium transition metal oxides. Alternatively, the positive electrode active material may exist as an aggregate of non-aggregate lithium transition metal oxides and aggregated lithium transition metal oxides.

[0053] For convenience, in this application, the unit particle, the single particle, the primary particle, and the secondary particle can all be referred to as lithium transition metal oxide, and since the single particle and the primary particle are all single particles, they will all be referred to as primary particles below.

[0054] The primary particles may have a rod shape, an elliptical shape, and / or an amorphous shape. Furthermore, unless specifically intended in the manufacturing process, primary particles of various shapes may exist within the same cathode active material. The primary 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 grain boundaries.

[0055] Furthermore, the primary particle may mean a single crystallite (grain or crystallite). The unit particle and / or the primary particle may have a single crystal structure containing a single crystallite, or a polycrystalline structure containing multiple crystallites.

[0056] When the lithium transition metal oxide exists in single-particle form, the average particle size (D50 ) can be 2.0 μm or more and 10.0 μm or less. When the average particle diameter (D 50 ) of the single particle is smaller than 2.0 μm, the energy density of the positive electrode active material can decrease. On the other hand, when the growth of the single particle is excessively induced, when the average particle diameter (D 50 ) of the single particle becomes larger than 10.0 μm, it may be difficult to effectively control the intracrystalline cracks in the single particle.

[0057] The average particle diameter (D 50 ) of the similar single particles in which the lithium transition metal oxide exists in a similar single particle form can be 3.0 μm to 20 μm, 4.0 μm to 18 μm, 6.0 μm to 16 μm, or 5.0 μm to 15 μm. When the average particle diameter (D 50 ) of the similar single particle is smaller than 3.0 μm, the energy density of the positive electrode active material may decrease. On the other hand, when the growth of the unit particle is excessively induced, when the average particle diameter (D 50 ) of the similar single particle becomes larger than 20.0 μm, the occurrence frequency of intracrystalline cracks in the similar single particle may increase.

[0058] The average particle diameters (D 50 ) of the single particle and the similar single particle can be measured using the laser diffraction method. For example, after dispersing the positive electrode active material containing the single particle and the similar single particle in a dispersion medium, it is introduced into a commercially available laser diffraction particle size measuring device (for example, Microtrac MT 3000), irradiated with ultrasonic waves of about 28 kHz at an output of 60 W, and after obtaining a volume cumulative particle size distribution graph, it can be measured by obtaining the particle size corresponding to 50% of the volume cumulative amount.

[0059] The surface shape of the unit particles observed from the surface SEM image of the lithium transition metal oxide has a long axis and a short axis, and the average length of the long axis of the unit particles may be 2.0 μm or more and 10.0 μm or less, 2.0 μm or more and 9.0 μm or less, or 2.0 μm or more and 8.5 μm or less. In addition, the average length of the short axis of the unit particles observed from the surface SEM image of the lithium transition metal oxide may be 1.0 μm or more and less than 10.0 μm, 1.0 μm or more and 9.0 μm or less, 1.0 μm or more and 8.0 μm or less, 1.0 μm or more and 7.0 μm or less, 1.0 μm or more and 6.0 μm or less, or 1.0 μm or more and 5.0 μm or less.

[0060] If the growth of the unit particles constituting the lithium transition metal oxide is sufficiently induced, the unit particles may have major axis lengths and minor axis lengths within the ranges described above. If the unit particles do not satisfy the major axis lengths and / or minor axis lengths described above, it may be difficult to sufficiently alleviate the intergranular cracks and / or intragranular cracks and surface cracks within the lithium transition metal oxide.

[0061] As mentioned above, the lithium transition metal oxide is a composite metal oxide having a layered crystalline structure belonging to the R-3m space group. Therefore, the unit particle also has a layered crystalline structure belonging to the R-3m space group.

[0062] The layered crystal structure belonging to the R-3m space group exhibits a crystal structure in which lithium-containing lithium layers and transition metal-containing transition metal layers are arranged alternately. As the nickel content in the lithium transition metal oxide increases, changes in the crystal structure may occur in some regions of the lithium transition metal oxide due to cation mixing (Li⇔Ni) between the lithium layer and the transition metal layer. In particular, when the growth of unit particles is induced through high-temperature firing, changes in the crystal structure may occur on the surface of the unit particles exposed to the outside.

[0063] When the cation mixing described above occurs on the surface of the unit particle, a rock-salt phase region that includes a crystal structure of a rock-salt phase belonging to the Fm-3m space group may be formed in at least a part of the surface portion of the unit particle. In the layered crystal structure, the rock-salt phase may act as an impurity phase, increasing the likelihood of cracks occurring in the portion where the rock-salt phase region is formed. However, the internal cracks in the unit particle do not necessarily occur only in the rock-salt phase region and may also occur in the layered region structurally damaged by high-temperature firing.

[0064] For example, when defined with the radius of the unit particle measured from the cross-sectional SEM image of the unit particle as r, the internal cracks on the surface portion where the distance (d) from the center of the unit particle is (2 / 3)r < d may be more than the internal cracks in the central portion where the distance (d) from the center of the unit particle is 0 ≦ d ≦ (2 / 3)r. The internal cracks on the surface portion and the internal cracks in the central portion can be compared as the area of the cracks observed from the cross-sectional SEM image of the unit particle.

[0065] According to the present invention, the ratio (a1 / a) of the total area (a1) of the cracks present on the surface of the unit particle to the surface area (a) of the unit particle observed from the surface SEM image of the lithium transition metal oxide can be 10% or less, 9% or less, 8% or less, 7.5% or less, 7.2% or less. When the ratio (a1 / a) of the total area (a1) of the cracks present on the surface of the unit particle to the surface area (a) of the unit particle exceeds 10%, the resistance to the strain caused by the random volume contraction / expansion of the primary particles during repeated charge and discharge rapidly weakens, and the long-term stability of the positive electrode active material may rapidly decrease.

[0066] Moreover, cracks may not be present on the surface of the unit particle observed from the surface SEM image of the lithium transition metal oxide. In this case, the ratio (a1 / a) of the total area (a1) of the cracks present in the unit particle to the surface area (a) of the unit particle observed from the surface SEM image of the lithium transition metal oxide is 0%.

[0067] The ratio (a1 / a) of the total crack area (a1) present on the surface of a unit particle to the surface area (a) of the unit particle observed from the surface SEM image of the lithium transition metal oxide can be calculated as the average value of the calculation results for all unit particles observed from the surface SEM image of the lithium transition metal oxide, after quantifying the surface area (a) of the unit particle and the total crack area (a1) respectively using an image analysis program based on digital transformation on the surface SEM image of the lithium transition metal oxide. Furthermore, the ratio (a1 / a) of the total crack area (a1) present on the surface of a unit particle to the surface area (a) of the unit particle observed from the surface SEM image of the lithium transition metal oxide can be defined as the ratio (A1 / A) of the average value (A) of the total crack area (A1) present on the surface of an individual unit particle observed from the surface SEM image of the lithium transition metal oxide to the average value (A) of the surface area of ​​an individual unit particle observed from the surface SEM image of the lithium transition metal oxide.

[0068] The surface shape of the unit particles constituting the lithium transition metal oxide is not necessarily circular; for example, it may have a rod shape, an elliptical shape, and / or an amorphous shape. Therefore, the proportion of cracks present on the surface of the individual unit particles as observed from the surface SEM image can vary depending on the location where the surface of the individual unit particles is exposed.

[0069] When r1 is the length of the major axis measured from the surface shape of the unit particle observed from the surface SEM image of the lithium transition metal oxide, the ratio (a2 / a) of the total area (a2) of intragranular cracks (a2) that exist in the region where the distance (d1) from the outermost edge of the unit particle is d1 ≤ (1 / 4)r1 to the total surface area (a) of the unit particle observed from the surface SEM image of the lithium transition metal oxide, can be 5% or less.

[0070] When the length of the minor axis measured from the surface shape of the unit particle observed from the surface SEM image of the lithium transition metal oxide is denoted as r2, the ratio (a3 / a) of the total area (a3) ​​of intragranular cracks (a3) ​​located within the region where the distance (d2) from the outermost edge of the unit particle is d2 ≤ (1 / 3)r2 to the total surface area (a) of the unit particle observed from the surface SEM image of the lithium transition metal oxide, can be 9% or less.

[0071] In this application, the lithium transition metal oxide comprises lithium and a transition metal, and the nickel content in the transition metal may be 50 mol% or more, 60 mol% or more, 70 mol% or more, 80 mol% or more, or 90 mol% or more. Furthermore, the nickel content in the transition metal may be 99 mol% or less, 95 mol% or less, or 90 mol% or less.

[0072] The lithium transition metal oxide can be represented by the following chemical formula 1. [Chemical formula 1] Li a Ni 1-(b+c+d+e) Co b Mn c M1 d M2 e O2 In the aforementioned chemical formula 1, M1 is at least one selected from Na, K, Mg, Ca, Sr, Ba, and Rb. M2 is at least one selected from 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. 0.85≦a≦1.15, 0≦b≦0.20, 0≦c≦0.20, 0.001≦d≦0.05, 0≦e≦0.10, and 0.5≦1-(b+c+d+e)<1.0.

[0073] In the aforementioned chemical formula 1, 'a', which represents the ratio of lithium to all elements other than lithium in the lithium transition metal oxide, may be 0.85 or more and 1.15 or less, 0.85 or more and 1.10 or less, 0.90 or more and 1.15 or less, or 0.90 or more and 1.04 or less.

[0074] If a in the aforementioned chemical formula 1 is less than 0.85, 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, if a in the aforementioned 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 be generated.

[0075] In the aforementioned chemical formula 1, "1-(b+c+d+e)", which represents the mole fraction of nickel relative to all elements other than lithium in the lithium transition metal oxide, can be 0.50 or greater, 0.60 or greater, 0.70 or greater, 0.80 or greater, or 0.90 or greater.

[0076] The upper and lower limits of the mole fraction of nickel relative to all elements other than lithium in the lithium transition metal oxide can be appropriately selected within the range that satisfies the definition described above. 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.

[0077] In the aforementioned chemical formula 1, b, which represents the mole fraction of cobalt relative to all 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. When the lithium transition metal oxide contains cobalt, b in the aforementioned chemical formula 1 is greater than 0. When the mole fraction of cobalt in the lithium transition metal oxide satisfies the above range, a stable crystal structure can be formed, and good output characteristics can be observed.

[0078] In the aforementioned chemical formula 1, c, which represents the mole fraction of manganese relative to all 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 c in the aforementioned chemical formula 1 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.

[0079] By mixing the lithium transition metal oxide precursor with a raw material containing at least one selected from alkali metals and alkaline earth metals, and then firing the mixture, it is possible to induce the growth of unit particles while simultaneously suppressing and / or mitigating the formation of intragranular cracks within the unit particles. When mixing the lithium transition metal oxide precursor with a raw material containing at least one selected from alkali metals and alkaline earth metals and then firing the mixture, the alkali metal and / or alkaline earth metal may be doped into the lithium transition metal oxide.

[0080] In the above chemical formula 1, M1 means alkali metals and alkaline earth metals doped into the lithium transition metal oxide, and M1 may be at least one selected from Na, K, Mg, Ca, Sr, Ba and Rb, at least one selected from K, Ca, Sr, Ba and Rb, at least one selected from K, Sr, Ba and Rb, or at least one selected from K, Sr and Ba.

[0081] The content of alkali metals and alkaline earth metals in the total elements excluding lithium in the lithium transition metal oxide may be 0.1 mol% to 5.0 mol%, 0.5 mol% to 4.0 mol%, or 1.0 mol% to 3.0 mol%.

[0082] In the above chemical formula 1, M2 represents a dopant other than alkali metals and alkaline earth metals present in the lithium transition metal oxide. The dopant may exist in a doped state within the crystal lattice of the unit particle. Furthermore, the electrochemical properties of the positive electrode active material can be improved by including the dopant.

[0083] If the lithium transition metal oxide contains a dopant, then e in chemical formula 1 is greater than 0. Also, if the lithium transition metal oxide contains a dopant, then e, which represents the mole fraction of the dopant relative to all elements other than lithium in the lithium transition metal oxide, may be 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.

[0084] If the lithium transition metal oxide selectively contains a dopant, the dopant may include at least one selected from 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. 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.

[0085] (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 components will be described below.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] The negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0098] 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.

[0099] 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.

[0100] 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. β Examples 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.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] 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.

[0105] 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.

[0106] 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.

[0107] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0108] 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, mixing the cyclic carbonate and linear carbonate in a volume ratio of about 1:1 to about 1:9 can result in excellent electrolyte performance.

[0109] 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.

[0110] 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.

[0111] 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 material 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, 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.)

[0112] The solid electrolyte, preferably a sulfide-based solid electrolyte, may be amorphous or crystalline, or a mixture of amorphous and crystalline materials.

[0113] 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).

[0114] 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.

[0115] 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.

[0116] 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).

[0117] 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.

[0118] 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.

[0119] 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.

[0120] 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.

[0121] (Manufacturing Example 1: Manufacturing of Cathode Active Material) (Example 1) Spherical Ni 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.

[0122] Next, the hydroxide precursor, LiOH (Li / (Ni+Co+Mn)molar ratio=0.9), and Ba(OH)2 (weighed so that the mole fraction of Ba relative to all elements in the hydroxide precursor was 0.01 (1 mol%)) were mixed and heat-treated at 850°C for 8 hours to obtain a lithium transition metal oxide.

[0123] (Example 2) A positive electrode active material was produced in the same manner as in Example 1, except that the hydroxide precursor, LiOH (Li / (Ni+Co+Mn)molar ratio=1.0), and Ba(OH)2 (weighed so that the mole fraction of Ba relative to all elements in the hydroxide precursor was 0.01 (1 mol%)) were mixed and heat-treated.

[0124] (Example 3) A positive electrode active material was produced in the same manner as in Example 1, except that the hydroxide precursor, LiOH (Li / (Ni+Co+Mn)molar ratio=1.0), Ba(OH)2 (weighed so that the mole fraction of Ba relative to all elements in the hydroxide precursor is 0.01 (1 mol%)), and Co(OH)2 (weighed so that the mole fraction of Co further added to the hydroxide precursor is 0.02 (2 mol%)), was mixed and heat-treated.

[0125] (Example 4) A positive electrode active material was produced in the same manner as in Example 1, except that the hydroxide precursor, LiOH (Li / (Ni+Co+Mn)molar ratio=1.04), Ba(OH)2 (weighed so that the mole fraction of Ba relative to all elements in the hydroxide precursor was 0.01 (1 mol%)), and ZrO2 (weighed so that the mole fraction of Zr relative to all elements in the hydroxide precursor was 0.01 (1 mol%)), was mixed and heat-treated.

[0126] (Example 5) A positive electrode active material was produced in the same manner as in Example 1, except that the hydroxide precursor, LiOH (Li / (Ni+Co+Mn)molar ratio = 1.04), and KOH (weighed so that the mole fraction of K relative to all elements in the hydroxide precursor was 0.01 (1 mol%)) were mixed and heat-treated.

[0127] (Comparative Example 1) A positive electrode active material was produced in the same manner as in Example 1, except that the hydroxide precursor, LiOH (Li / (Ni+Co+Mn)molar ratio=1.04), and Ba(OH)2 (weighed so that the mole fraction of Ba relative to all elements in the hydroxide precursor was 0.01 (1 mol%)) were mixed and heat-treated at 700°C.

[0128] (Comparative Example 2) A positive electrode active material was produced in the same manner as in Example 1, except that the hydroxide precursor and LiOH (Li / (Ni+Co+Mn)molar ratio=1.04) were mixed without using Ba(OH)2 and heat-treated at 850°C.

[0129] (Comparative Example 3) A positive electrode active material was produced in the same manner as in Example 1, except that the hydroxide precursor, LiOH (Li / (Ni+Co+Mn)molar ratio=1.04), and Ba(OH)2 (weighed so that the mole fraction of Ba relative to all elements in the hydroxide precursor was 0.01 (1 mol%)) were mixed and heat-treated.

[0130] (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.

[0131] 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.

[0132] (Experimental Example 1: Surface Analysis of Cathode Active Material) Surface FE-SEM images were obtained by imaging each cathode active material (lithium transition metal oxide) manufactured according to Manufacturing Example 1 using a field emission scanning electron microscope (FE-SEM, JEOL JSM-IT800SHL) under the conditions of an acceleration voltage of 5kV, an emission current of 10μA, a working distance of 8mm, and a detector BED.

[0133] Using an image analysis program based on digital transformation, the surface area of ​​individual unit particles and the total area of ​​cracks present on the surface of each individual unit particle were quantified for all unit particles observed from the FE-SEM surface image, and their average values ​​were calculated. Subsequently, the ratio (a1 / a) of the average value of the total area of ​​cracks present on the surface of each individual unit particle observed from the lithium transition metal oxide surface SEM image to the average value of the surface area of ​​each individual unit particle observed from the FE-SEM surface image was calculated.

[0134] Furthermore, the total area of ​​surface cracks located within the region where the distance (d1) from the outermost edge of the unit particle observed from the FE-SEM surface image is within 1 / 4 of the length of the unit particle's major axis (r1) (d1 ≤ (1 / 4)r1), and the total area of ​​surface cracks located within the region where the distance (d2) from the outermost edge of the unit particle observed from the FE-SEM surface image is within 1 / 3 of the length of the unit particle's minor axis (r2) (d2 ≤ (1 / 3)r2), were quantified, and their average values ​​were calculated.

[0135] Subsequently, the ratio (a2 / a) of the average total area of ​​surface cracks located within the region where the distance from the outermost edge of a unit particle (d1) is within 1 / 4 of the length of the unit particle's major axis (r1) (d1 ≤ (1 / 4)r1) to the average surface area of ​​individual unit particles observed from the FE-SEM surface image was calculated, and the ratio (a3 / a) of the average total area of ​​surface cracks located within the region where the distance from the outermost edge of a unit particle (d2) is within 1 / 3 of the length of the unit particle's minor axis (r2) (d2 ≤ (1 / 3)r2) to the average surface area of ​​individual unit particles observed from the FE-SEM surface image was calculated.

[0136] Furthermore, the total number of unit particles observed from the FE-SEM surface image, the average length of the major axis of all unit particles, and the average length of the minor axis of all unit particles were calculated.

[0137] The results of the surface SEM analysis are shown in Table 1 below. [Table 1]

[0138] (Experimental Example 2: Evaluation of the electrochemical properties of lithium secondary batteries (half-cells)) The lithium secondary battery (half-cell) manufactured in Manufacturing Example 2 was subjected to 30 charge-discharge cycles using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, a voltage range of 2.5V to 4.25V, and a current of 0.5C / 0.1C. The ratio of the discharge capacity at the 30th cycle to the initial discharge capacity (cycle capacity retention) was then measured.

[0139] The measurement results are shown in Table 2 below. [Table 2]

[0140] Referring to the results in Table 2, it can be confirmed that the life retention rate of lithium secondary batteries using the positive electrode active materials of Examples 1 to 5 was improved compared to the life retention rate of lithium secondary batteries using the positive electrode active material of Comparative Example 1, which has a multi-particle form. Through these results, it can be predicted that the decrease in the number of unit particles constituting the lithium transition metal oxide contained in the positive electrode active materials of Examples 1 to 5 reduces the intergranular cracks within the lithium transition metal oxide, thereby improving the life and stability of lithium secondary batteries using the lithium transition metal oxide as a positive electrode active material.

[0141] Furthermore, it can be confirmed that the life retention rate of lithium secondary batteries using the positive electrode active materials from Comparative Examples 2 and 3, in which the number of unit particles was reduced compared to Comparative Example 1, is lower than that of lithium secondary batteries using the positive electrode active materials from Examples 1 to 5.

[0142] Based on the above results, it can be predicted that the lithium transition metal oxides contained in the positive electrode active materials of Examples 1 to 5 improve resistance to strain caused by volume contraction / expansion of unit particles due to repeated charging and discharging by suppressing and / or mitigating the formation of cracks within unit particles, thereby improving the life characteristics of lithium secondary batteries using the lithium transition metal oxides as positive electrode active materials. Furthermore, it can be confirmed that the lithium transition metal oxides contained in the positive electrode active materials of Examples 1 to 5 are also advantageous in improving capacity characteristics when they suppress and / or mitigate the formation of cracks within unit particles.

[0143] 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. It contains lithium transition metal oxides that allow for lithium intercalation / deintercalation, The lithium transition metal oxide has at least one form selected from a single-particle form consisting of one unit particle and a similar single-particle form in which 30 or fewer unit particles are aggregated. A positive electrode active material in which the ratio (a1 / a) of the total crack area (a1) present on the surface of the unit particle to the surface area (a) of the unit particle observed from a surface SEM image of the lithium transition metal oxide is 10% or less.

2. When the radius of the unit particle measured from the cross-sectional SEM image of the unit particle is denoted as r, The positive electrode active material according to claim 1, wherein the number of surface cracks where the distance (d) from the center of the unit particle is (2 / 3)r < d is greater than the number of central cracks where the distance (d) from the center of the unit particle is 0 ≤ d ≤ (2 / 3)r.

3. When r1 is defined as the length of the major axis measured from the surface shape of the unit particle observed from the surface SEM image of the lithium transition metal oxide, The positive electrode active material according to claim 1, wherein the ratio (a2 / a) of the total surface area (a) of the unit particle observed from a surface SEM image of the lithium transition metal oxide to the total surface area (a) of the unit particle, where the distance (d1) from the outermost edge of the unit particle is d1 ≤ (1 / 4)r1, is 5% or less.

4. When the length of the minor axis measured from the surface shape of the unit particle observed from the surface SEM image of the lithium transition metal oxide is denoted as r2, The positive electrode active material according to claim 1, wherein the ratio (a3 / a) of the total surface area (a) of the unit particle observed from a surface SEM image of the lithium transition metal oxide to the total surface area (a) of the unit particle, where the distance (d2) from the outermost edge of the unit particle is d2 ≤ (1 / 3)r2, is 9% or less.

5. The lithium transition metal oxide comprises lithium and a transition metal. The positive electrode active material according to claim 1, wherein the nickel content in the transition metal is 50 mol% or more.

6. The average particle size (D) of the lithium transition metal oxide present in the single-particle form. ５０ The positive electrode active material according to claim 1, wherein the particle size is 2.0 μm or more and 10.0 μm or less.

7. The average particle size (D) of the lithium transition metal oxide present in the aforementioned similar single-particle form. ５０ The positive electrode active material according to claim 1, wherein the particle size is 3.0 μm or more and 20.0 μm or less.

8. The positive electrode active material according to claim 1, wherein the number of unit particles observed from a surface SEM image of the lithium transition metal oxide existing in the aforementioned similar single-particle morphology is 20 or less.

9. The surface shape of the unit particle observed from the surface SEM image of the lithium transition metal oxide has a long axis and a short axis, The positive electrode active material according to claim 1, wherein the average length of the major axis of the unit particle is 2.0 μm or more and 10.0 μm or less.

10. The surface shape of the unit particle observed from the surface SEM image of the lithium transition metal oxide has a long axis and a short axis, The positive electrode active material according to claim 1, wherein the average value of the length of the short axis of the unit particle is 1.0 μm or more and less than 10.0 μm.

11. The positive electrode active material according to claim 1, wherein the content of alkali metals and alkaline earth metals in the total elements excluding lithium in the lithium transition metal oxide is 0.1 mol% or more and 5.0 mol% or less.

12. 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 ａ Ni １－（ｂ＋ｃ＋ｄ＋ｅ） Co ｂ Mn ｃ M1 ｄ M2 ｅ O ２ In the aforementioned chemical formula 1, M1 is at least one selected from Na, K, Mg, Ca, Sr, Ba, and Rb. M2 is at least one selected from 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. 0.85 ≤ a ≤ 1.15, 0 ≤ b ≤ 0.20, 0 ≤ c ≤ 0.20, 0.001 ≤ d ≤ 0.05, 0 ≤ e ≤ 0.10, and 0.5 ≤ 1 - (b + c + d + e) ​​< 1.

0.

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

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

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