Positive electrode active material and lithium secondary battery comprising the same

Abstract

To provide a positive electrode active material for a lithium secondary battery which can simultaneously improve electrochemical properties and stability, which are in a mutual trade-off relationship.SOLUTION: A positive electrode active material comprises a lithium composite oxide enabling lithium intercalation and deintercalation, where the lithium composite oxide has a surface part with a step structure.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material for a lithium secondary battery that can simultaneously improve electrochemical properties and stability, which are in a trade-off relationship, by introducing a step structure into the surface of a lithium composite oxide that constitutes the positive electrode active material, and a lithium secondary battery including the same. [Background technology]

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

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

[0004] Representative materials used as positive electrode active materials for lithium secondary batteries include lithium composite oxides, such as LiCoO2, LiMn2O4, LiNiO2, LiMnO2, and oxides of Ni, Co, Mn, and Al as disclosed in Korean Patent Publication No. 10-2015-0069334 (published June 23, 2015).

[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 a drawback in that it is expensive due to the limited availability of cobalt as a raw material, limiting its price competitiveness.

[0006] Lithium manganese oxides such as LiMnO2 and LiMn2O4 have the advantages of excellent thermal safety and low cost, but suffer from low capacity and poor high-temperature characteristics. Also, LiNiO2-based positive electrode active materials exhibit high discharge capacity, but are difficult to synthesize due to the problem of cation mixing between Li and transition metals, which results in significant problems with rate characteristics.

[0007] Furthermore, depending on the degree of cation mixing, a large amount of Li by-products is generated. Most of these Li by-products contain LiOH and Li2CO3, which can cause gelation during the preparation of the positive electrode paste and gas generation during repeated charge / discharge cycles after electrode fabrication. Furthermore, the residual Li2CO3 among these Li by-products increases the swelling phenomenon of the cell, thereby reducing its lifespan.

[0008] To address these shortcomings, demand for high-Ni cathode active materials with a Ni content of 50% or more has begun to grow as cathode active materials for secondary batteries. However, while these high-Ni cathode active materials exhibit high capacity characteristics, they also suffer from structural instability due to Li / Ni cation mixing caused by the increased Ni content in the cathode active material. This structural instability of the cathode active material can cause rapid degradation of lithium secondary batteries not only at high temperatures but also at room temperature.

[0009] Therefore, it is necessary to develop a positive electrode active material that can overcome these problems of high-Ni positive electrode active materials. Summary of the Invention [Problem to be solved by the invention]

[0010] In a positive electrode active material for a lithium secondary battery, a certain trade-off relationship can be established between some indicators indicating the electrochemical properties of the positive electrode active material and some indicators indicating the stability of the positive electrode active material. Therefore, if the capacity characteristics of the positive electrode active material are excessively improved, the stability of the particles constituting the positive electrode active material may be reduced, and stable charge / discharge performance may not be achieved.

[0011] Therefore, an object of the present invention is to provide a positive electrode active material that can maintain the high electrochemical properties of conventional positive electrode active materials for lithium secondary batteries, particularly high-Ni type positive electrode active materials, while eliminating the low structural stability.

[0012] In particular, an object of the present invention is to provide a positive electrode active material for lithium secondary batteries that can simultaneously improve electrochemical properties and stability, which are in a trade-off relationship, by introducing a step structure into the surface of a lithium composite oxide that constitutes the positive electrode active material.

[0013] Another object of the present invention is to provide a lithium secondary battery using a positive electrode containing the lithium composite oxide defined in the present application. [Means for solving the problem]

[0014] A technology has been proposed to improve stability by reducing surface side reactions of the positive electrode active material by making light elements, typically boron, present in an oxide form in the positive electrode active material.

[0015] However, although the light elements can reduce surface side reactions of the positive electrode active material, they can also increase the content of lithium-containing impurities on the surface depending on their state of existence, thereby causing a deterioration in the electrochemical properties of the positive electrode active material.

[0016] Under these circumstances, the present inventors have confirmed that when a step structure is introduced into the surface of a lithium composite oxide capable of lithium intercalation and deintercalation, electrochemical properties and stability, which are in a trade-off relationship, can be simultaneously improved.

[0017] Thus, according to one aspect of the present invention, there is provided a cathode active material including a lithium composite oxide capable of lithium intercalation and deintercalation, in which the lithium composite oxide has a surface portion with a step structure.

[0018] In this case, the step structure formed on the surface of the lithium composite oxide may be a terrace-step-kink structure.

[0019] In one embodiment, the lithium composite oxide is a lithium composite oxide containing at least nickel and cobalt, and may further contain at least one selected from manganese and aluminum.

[0020] The lithium composite oxide may be represented by the following Chemical Formula 1: [Chemical formula 1] Li w Ni 1-(x+y+z) Co x M1 y M2 z O2

[0021] (where, M1 is at least one selected from Mn and Al; M2 is at least one selected from Mn, Ba, Ce, Hf, Ta, Cr, F, Mg, Al, Cr, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Sr, Ge, Nd, Gd, B, and Cu; M1 and M2 are different elements, 0.5≦w≦1.5, 0≦x≦0.50, 0 <y≦0.20、0≦z≦0.20である)

[0022] In addition, the positive electrode active material may include secondary particles formed by agglomeration of a plurality of the lithium composite oxide as primary particles, and a coating layer containing an oxide represented by the following Chemical Formula 2 may be formed on at least a portion of the surfaces of the primary particles and the secondary particles: [Chemical formula 2] Li a M3 b O c

[0023] (where, M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, Ce, V, Ba, Ta, Sn, Hf, Gd, and Nd; 0≦a≦8, 0 <b≦8、2≦c≦13である)

[0024] Even when a coating layer containing an oxide represented by the following chemical formula 2 is formed on at least a portion of the surfaces of the primary particles and the secondary particles, the coating layer may have a surface portion with a stepped structure, similar to the lithium composite oxide. Furthermore, the stepped structure formed on the surface portion of the coating layer may be a terrace-step-kink structure.

[0025] According to another aspect of the present invention, there is provided a positive electrode including the positive electrode active material described above.

[0026] According to yet another aspect of the present invention, there is provided a lithium secondary battery using the above-described positive electrode. [Effects of the Invention]

[0027] The lithium composite oxide constituting the cathode active material according to the present invention has a step structure on the surface thereof, unlike conventional lithium composite oxides which mostly have a nearly spherical surface. The step structure structurally stabilizes the lithium composite oxide and simultaneously suppresses side reactions with the electrolyte.

[0028] In particular, by forming a coating layer on the surface of the lithium composite oxide as described above, it is possible to suppress a decrease in the charge amount due to deintercalation of lithium and reduce the possibility of side reactions with the electrolyte. [Brief explanation of the drawings]

[0029] [Figure 1] 3A and 3B are diagrams schematically illustrating a process of introducing a step structure into a surface of a lithium composite oxide included in a positive electrode active material according to an embodiment of the present invention. [Figure 2] 1 is an SEM image of the surface of the lithium composite oxide contained in the positive electrode active material according to Example 1. [Figure 3] 1 is an SEM image of the surface of a lithium composite oxide contained in a positive electrode active material according to Comparative Example 1. [Figure 4] 10 is an SEM image of the surface of the lithium composite oxide contained in the positive electrode active material according to Example 3. [Figure 5] 10 is an SEM image of the surface of the lithium composite oxide contained in the positive electrode active material according to Comparative Example 2. [Figure 6] 10 is an SEM image of the surface of the lithium composite oxide contained in the positive electrode active material according to Example 4. [Figure 7] 10 is an SEM image of the surface of the lithium composite oxide contained in the positive electrode active material according to Comparative Example 3. [Figure 8] 10 is an SEM image of the surface of the lithium composite oxide contained in the positive electrode active material according to Comparative Example 4. [Figure 9]10 is an SEM image of the surface of the lithium composite oxide contained in the positive electrode active material according to Example 5. [Figure 10] 10 is an SEM image of the surface of the lithium composite oxide contained in the positive electrode active material according to Comparative Example 5. [Figure 11] 10 is an SEM image of the surface of the lithium composite oxide contained in the positive electrode active material according to Comparative Example 6. [Figure 12] 1 is a TEM image of the surface of the lithium composite oxide contained in the positive electrode active material according to Example 1, showing each crystal plane forming the surface of the lithium composite oxide. FIG. [Figure 13] 1 is a graph showing an XPS depth profile of a lithium composite oxide contained in a positive electrode active material according to Example 1. [Figure 14] 14 is a graph showing a B1s XPS spectrum for a region where boron (B) is detected in the XPS depth profile of FIG. 13. [Figure 15] 3 is a diagram showing the interplanar distances of crystal planes in regions A and B in a TEM image of the lithium composite oxide contained in the positive electrode active material according to Example 1. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0030] Hereinafter, positive electrode active materials and lithium secondary batteries including the same according to various embodiments of the present invention will be described in more detail.

[0031] positive electrode active material A positive electrode active material according to one embodiment of the present invention includes a lithium composite oxide capable of lithium intercalation and deintercalation and having a surface portion with a step structure.

[0032] The lithium composite oxide is an oxide having a layered structure containing at least one transition metal in addition to lithium, and may contain at least nickel and cobalt. The lithium composite oxide may further contain at least one selected from manganese and aluminum in addition to nickel and cobalt.

[0033] The lithium composite oxide may be represented by the following Chemical Formula 1: [Chemical formula 1] Li w Ni 1-(x+y+z) Co x M1 y M2 z O2

[0034] (where, M1 is at least one selected from Mn and Al; M2 is at least one selected from Mn, Ba, Ce, Hf, Ta, Cr, F, Mg, Al, Cr, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Sr, Ge, Nd, Gd, B, and Cu; M1 and M2 are different elements, 0.5≦w≦1.5, 0≦x≦0.50, 0 <y≦0.20、0≦z≦0.20である)

[0035] As represented by Chemical Formula 1, the lithium composite oxide may further contain a dopant in addition to nickel, cobalt, manganese, and / or aluminum. In Chemical Formula 1, the dopant is represented by M2, and M2 is an element different from M1. That is, when M1 is Mn, M2 is at least one dopant other than Mn; when M1 is Al, M2 is at least one dopant other than Al; and when M1 is Mn and Al, M2 is at least one dopant other than Mn and Al.

[0036] Furthermore, the lithium composite oxide represented by Chemical Formula 1 may be a high-Ni type lithium composite oxide having a Ni content of 50% or more, preferably 60% or more, and more preferably 80% or more. The Ni content in the lithium composite oxide can be calculated by the following Formula 1 and can be expressed as "1-(x+y+z)" in Chemical Formula 1. For example, when the lithium composite oxide is a lithium composite oxide represented by NCM811, "1-(x+y+z)" in Chemical Formula 1 is 0.80. [Formula 1] Ni content=[Ni(mol%) / Ni(mol%)+Co(mol%)+M1(mol%)+M2(mol%)]*100

[0037] The positive electrode active material may include secondary particles formed by aggregation of a plurality of the lithium composite oxide primary particles.

[0038] Here, the primary particle refers to a single crystal grain (grain or crystallite), and the secondary particle refers to an aggregate formed by aggregation of a plurality of primary particles. The primary particle may have a rod-like, elliptical, and / or irregular shape. Voids and / or grain boundaries may exist between the primary particles constituting the secondary particle.

[0039] For example, the primary particles may be separated from adjacent primary particles within the secondary particle to form internal voids. Furthermore, the primary particles may contact the internal voids without forming grain boundaries with adjacent primary particles, thereby forming the surface present within the secondary particle. Meanwhile, the surface of the primary particles present at the outermost surface of the secondary particle, exposed to the outside air, forms the surface of the secondary particle.

[0040] The average major axis diameter of the primary particles is in the range of 0.1 μm to 5 μm, preferably 0.1 μm to 2 μm, so that the optimum density of the cathode manufactured using the cathode active material according to various embodiments of the present invention can be realized. Also, the average particle size of the secondary particles may be 1 μm to 30 μm, depending on the number of agglomerated primary particles.

[0041] Meanwhile, the lithium composite oxide included in the cathode active material according to an embodiment of the present invention has a step structure on the surface thereof, and the step structure may be present on the surface of the primary particles and / or the secondary particles, more preferably on the surface of the secondary particles.

[0042] The step-like structure formed on the surface of the lithium composite oxide does not simply refer to voids present on the surface of the lithium composite oxide or curvatures on the surface formed by aggregation of a plurality of the lithium composite oxides, but should be understood as a step-like structure formed overall on the surface of the primary particles and / or the secondary particles.

[0043] The stepped structure may be defined as a terrace-step structure, which may further include a kink structure.

[0044] FIG. 1 is a diagram schematically illustrating a process of introducing a step structure into the surface of a lithium composite oxide included in a positive electrode active material according to an embodiment of the present invention.

[0045] The stepped structure is a structure formed by stacking multiple planes that are horizontal or nearly horizontal to the surface of the primary particles and / or secondary particles, and the area of ​​the planes decreases from the portion relatively close to the surface of the primary particles and / or secondary particles toward the portion relatively far from the surface, so that the structure in which multiple planes are stacked can form a stepped structure.

[0046] Referring to FIG. 1, the terraces t and steps s constituting the stepped structure may be periodically repeated, and the surface area of ​​the terraces t farther from the surface of the primary particles and / or the secondary particles is formed smaller than that of the terraces t closer to the surface of the primary particles and / or the secondary particles, thereby forming the stepped structure on the surface of the primary particles and / or the secondary particles.

[0047] That is, the terraces t and the steps s may be periodically repeated, resulting in a structure similar to a so-called staircase shape present on the surface of the primary particles and / or the secondary particles. The direction in which the terraces t and the steps s are periodically repeated from the surface of the primary particles and / or the secondary particles may follow the long and short axis directions of the lithium composite oxide, or may be non-directional.

[0048] 1, the stepped structure may further include a kink k. The kink k refers to a recessed or protruding region of the terrace t and / or the step s in the terrace-step structure, and the position and direction of the kink k formed on the terrace t and / or the step s may also be omnidirectional.

[0049] The above-described step-like structure can simultaneously improve the electrochemical properties and stability, which are in a trade-off relationship, or at least can contribute to improving the stability while minimizing the deterioration of the electrochemical properties, or improving the electrochemical properties while minimizing the deterioration of the stability.

[0050] The crystallographic structure of the lithium composite oxide and the directionality of the step structure formed on the surface of the lithium composite oxide will be described below.

[0051] Referring to FIG. 12 showing a TEM image of a surface portion of a lithium composite oxide included in a cathode active material according to an embodiment of the present invention, the step-like structure formed on the surface portion of the primary particles and / or the secondary particles may be arranged to intersect with an extension direction of a (003) crystal plane of the lithium composite oxide.

[0052] The (003) crystal plane is a specific crystal plane originating from the lithium composite oxide and is formed along one direction inside the lithium composite oxide, while the step-like structure present on the surface portion of the lithium composite oxide is formed so as to intersect with the extension direction of the (003) crystal plane.

[0053] In this case, the step structure being arranged to intersect with the extension direction of the (003) crystal plane means that at least the step structure is present so as to intersect with the extension direction of the (003) crystal plane in a direction perpendicular or nearly perpendicular to the extension direction of the (003) crystal plane.

[0054] In this way, the step-like structure is formed to intersect with the extension direction of the (003) crystal plane of the lithium composite oxide, which can contribute to improving the diffusion ability of lithium ions through the lithium composite oxide.

[0055] The step structure formed on the surface of the lithium composite oxide may be formed along the extension direction of at least one crystal plane selected from the group consisting of {01x}, {02y}, and {03z} planes among the crystal planes present on the surface of the lithium composite oxide, or along a plane parallel or nearly parallel to the extension direction (where x, y, and z are each independently an integer between 0 and 12).

[0056] In this case, the step-like structure being formed along the extension direction of a specific crystal plane means that the terrace-steps of the step-like structure are formed along the extension direction of a specific crystal plane formed on the surface of the lithium composite oxide, or are formed to form a dihedral angle of 90° or more with respect to the extension direction of the specific crystal plane.

[0057] That is, the stepped structure may be formed such that any crystal plane belonging to at least one family of planes selected from the family of {01x}, {02y}, and {03z} planes among the crystal planes present on the surface portion of the lithium composite oxide has a predetermined dihedral angle with another adjacent crystal plane. In this case, the dihedral angle that any crystal plane belonging to at least one family of planes selected from the family of {01x}, {02y}, and {03z} planes has with another adjacent crystal plane may be 90° or more.

[0058] For example, crystal planes belonging to the {01x} plane family include the (011), (012), and (014) crystal planes, crystal planes belonging to the {02y} plane family include the (021) and (022) crystal planes, and crystal planes belonging to the {03z} plane family include the (003) and (030) crystal planes.

[0059] For reference, even if the plane group and crystal plane are not specifically specified in the present application, the stepped structure defined in the present application can be formed on the surface portion of the lithium composite oxide.

[0060] On the other hand, the step structure formed on the surface of the lithium composite oxide may have a periodically repeated shape. Specifically, the step structure may be formed so that it periodically repeats a step structure that forms a predetermined dihedral angle with at least one crystal plane selected from the group of {01x}, {02y}, and {03z} planes of the lithium composite oxide (where x, y, and y are each independently an integer between 0 and 12).

[0061] Similarly, even if the plane family and crystal plane are not specifically specified in the present application, the step-like structure may be formed by periodically repeating step structures that form a predetermined dihedral angle from the plane family and crystal plane that are mainly formed on the surface portion of the lithium composite oxide.

[0062] When the lithium ion diffusion path in the lithium composite oxide constituting the positive electrode active material is formed in a direction parallel to the extension direction of the (003) crystal plane and at the same time, the diffusion and desorption of lithium ions from the inside to the outside of the lithium composite oxide are oriented toward the (012), (014), and (022) crystal planes, which are relatively free, the electrochemical properties of the positive electrode active material containing the lithium composite oxide can be expected to be improved.

[0063] Meanwhile, the provided step structure may be formed non-directionally regardless of the orientation of the crystal plane measured from the inside and surface of the lithium composite oxide.

[0064] In a further embodiment, a coating layer containing an oxide represented by the following Chemical Formula 2 may be formed on at least a portion of the surface of the primary particles and / or the secondary particles. [Chemical formula 2] Li a M3 b O c

[0065] (where, M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, Ce, V, Ba, Ta, Sn, Hf, Gd, and Nd; 0≦a≦8, 0 <b≦8、2≦c≦13である)

[0066] The coating layer is selectively present on at least a portion of the surface of the primary particles and / or the secondary particles, and the coating layer can be defined as a region on the surface of the primary particles and / or the secondary particles where the oxide represented by Chemical Formula 2 is present.

[0067] The coating layer may coat at least a portion of the surface of the primary particles and / or the secondary particles continuously or discontinuously, and when the coating layer is discontinuous, it may exist in the form of islands. The coating layer may also exist in the form of a solid solution that does not form a boundary with the primary particles and / or the secondary particles, but this is not necessarily the case.

[0068] On the other hand, even when the coating layer is present on the surface of the lithium composite oxide, it is preferable that the step structure present on the surface of the lithium composite oxide is maintained as it is.

[0069] If the coating layer is present on the surface of the lithium composite oxide, the coating layer may also have a surface portion with a stepped structure, in which case the stepped structure follows the stepped structure present on the surface portion of the lithium composite oxide.

[0070] Similarly, when the stepped structure present on the surface of the lithium composite oxide is a terrace-step-kink structure further including a kink structure, the surface of the coating layer also has a terrace-step-kink structure.

[0071] Meanwhile, at least one of the oxides included in the coating layer may be a boron-containing oxide such as a borate-based compound or an LBO (lithium borate)-based compound, which may be represented by the following Chemical Formula 3: [Chemical formula 3] Li d B e O f

[0072] (where, 0≦d≦8, 0 <e≦8、2≦f≦13である)

[0073] Non-limiting examples of the boron-containing oxide include B2O3, Li2O-B2O3, LiBO2, Li3BO3, Li3BO5, Li2B4O7, Li2B2O7, and Li2BO 13 , Li6B4O9, etc.

[0074] The boron-containing oxide provides a surface protection effect for the lithium composite oxide and suppresses surface side reactions, thereby improving the stability of the cathode active material. The coating layer further includes an oxide other than the boron-containing oxide, thereby reducing lithium-containing impurities present on the surface of the lithium composite oxide and simultaneously acting as a diffusion path for lithium ions, thereby improving the electrochemical properties of the cathode active material.

[0075] The oxide is a composite oxide of lithium and an element represented by M3, or an oxide of M3, and the oxide is, for example, Li a W b O c , Li a Zr b O c , Li a Ti b O c , Li a Ni b O c , Li a Co b O c , Li a Al b O c , Co b O c , Al b O c , W b O c , Zr b O c or Ti b O c However, the above-mentioned examples are merely given for the sake of convenience to facilitate understanding, and the oxides defined in the present application are not limited to the above-mentioned examples.

[0076] The oxide may be an oxide in which lithium and at least two elements represented by M3 are combined, or may further contain an oxide in which lithium and at least two elements represented by M3 are combined. The oxide in which lithium and at least two elements represented by M3 are combined may be, for example, Li a (W / Ti) b O c , Li a (W / Zr) b O c , Li a (W / Ti / Zr) b O c , Li a (W / Ti / B) b O c It may be, but is not necessarily limited to, the above.

[0077] Lithium secondary battery According to another aspect of the present invention, there is provided a positive electrode including a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, wherein the positive electrode active material layer may include the lithium composite oxide according to any of the various embodiments of the present invention described above as a positive electrode active material. Therefore, a specific description of the lithium composite oxide will be omitted, and only the remaining components not described above will be described below. For convenience, the lithium composite oxide described above will be referred to as the positive electrode active material below.

[0078] 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, baked carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. The positive electrode current collector may typically have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to enhance the adhesive strength of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0079] The positive electrode active material layer may be prepared by coating a positive electrode slurry composition containing the positive electrode active material, a conductive material, and optionally a binder, on the positive electrode current collector.

[0080] In this case, the positive electrode active material may be included in an amount of 80 to 99 wt %, more specifically, 85 to 98.5 wt %, based on the total weight of the positive electrode active material layer, and excellent capacity characteristics can be exhibited when the amount is within this range, but the amount is not necessarily limited thereto.

[0081] The conductive material is used to impart conductivity to the electrode and can be any material that provides electronic conductivity without causing chemical changes in the resulting battery. Specific examples include graphite (e.g., natural graphite or artificial graphite); carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon-based materials (e.g., carbon fiber); metal powder or metal fiber (e.g., copper, nickel, aluminum, or silver); conductive whiskers (e.g., zinc oxide or potassium titanate); conductive metal oxides (e.g., titanium oxide); and conductive polymers (e.g., polyphenylene derivatives). These materials may be used alone or in combination. The conductive material may be present in an amount of 0.1 to 15 wt % based on the total weight of the positive electrode active material layer.

[0082] The binder serves to improve adhesion between positive electrode active material particles and 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, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination. The binder may be included in an amount of 0.1 to 15 wt % based on the total weight of the positive electrode active material layer.

[0083] The positive electrode may be manufactured by a conventional method for manufacturing a positive electrode, except for using the positive electrode active material. Specifically, the positive electrode may be manufactured by dissolving or dispersing the positive electrode active material and, optionally, a binder and a conductive material in a solvent to prepare a positive electrode slurry composition, coating the composition on a positive electrode current collector, and then drying and rolling the composition.

[0084] The solvent may be a solvent commonly used in the art, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and may be used alone or in combination. The amount of the solvent used is sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, and to provide a viscosity that allows excellent thickness uniformity when the slurry is applied to produce a positive electrode, taking into consideration the coating thickness of the slurry and the production yield.

[0085] In another embodiment, the positive electrode may be manufactured by casting the positive electrode slurry composition on a separate support, peeling it off from the support, and laminating the resulting film on a positive electrode current collector.

[0086] According to yet another aspect of the present invention, there may be provided an electrochemical device including the above-described positive electrode. The electrochemical device may be, for example, a battery or a capacitor, and more specifically, a lithium secondary battery.

[0087] Specifically, the lithium secondary battery may include a positive electrode, a negative electrode facing the positive electrode, and a separator and an electrolyte interposed between the positive electrode and the negative electrode. Here, since the positive electrode is as described above, detailed description thereof will be omitted for convenience, and only the remaining components not described above will be described in detail below.

[0088] The lithium secondary battery may optionally further include a battery container that houses an electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0089] The negative electrode may include a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector.

[0090] 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, or the like, or an aluminum-cadmium alloy may be used. The negative electrode current collector may typically have a thickness of 3 μm to 500 μm. As with the positive electrode current collector, the surface of the current collector may be formed with fine irregularities to strengthen the binding force of the negative electrode active material. The negative electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0091] The negative electrode active material layer may be prepared by coating a negative electrode slurry composition containing the negative electrode active material, a conductive material, and optionally a binder, on the negative electrode current collector.

[0092] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. 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, and Al alloys; and SiO. β Examples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide, or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. One or a mixture of two or more of these may be used. A thin film of metallic lithium may also be used as the negative electrode active material. Both low-crystalline carbon and high-crystalline carbon may also be used as the carbon material. Typical low-crystalline carbons include soft carbon and hard carbon, while typical high-crystalline carbons include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-fired carbons such as petroleum or coal tar pitch-derived cokes.

[0093] The negative electrode active material may be contained in an amount of 80 to 99 wt % based on the total weight of the negative electrode active material layer.

[0094] The binder, which aids in bonding between the conductive material, active material, and current collector, may typically be added in an amount of 0.1 to 10 wt% based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0095] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 10 wt % or less, preferably 5 wt % or less, based on the total weight of the negative electrode active material layer. Such a conductive material is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. Examples of such a conductive material include graphite such as natural graphite and artificial graphite, carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, conductive fibers such as carbon fiber and metal fiber, metal powders such as carbon fluoride, aluminum, and nickel powder, conductive whiskey such as zinc oxide and potassium titanate, conductive metal oxides such as titanium oxide, and conductive materials such as polyphenylene derivatives.

[0096] In one embodiment, the negative electrode active material layer may be prepared by coating a negative electrode slurry composition prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material, in a solvent on a negative electrode current collector and drying the coating. Alternatively, the negative electrode slurry composition may be cast on a separate support, peeled from the support, and then laminating the resulting film on the negative electrode current collector.

[0097] Meanwhile, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without limitation. It is particularly preferable that the separator has low resistance to ion migration and excellent electrolyte humidification ability. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can be used, and it can be selectively used in a single-layer or multi-layer structure.

[0098] Furthermore, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the production of lithium secondary batteries, but are not limited to these.

[0099] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0100] The organic solvent may be any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone, ether solvents such as dibutyl ether and tetrahydrofuran, ketone solvents such as cyclohexanone, aromatic hydrocarbon solvents such as benzene and fluorobenzene, dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of suitable solvents include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, which may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constants, which can enhance the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred. In this case, the cyclic carbonate and the chain carbonate may be mixed in a volume ratio of about 1:1 to about 1:9 to produce excellent electrolyte performance.

[0101] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries without particular limitation. 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. The lithium salt is preferably used at a concentration in the range of 0.1 to 2.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0102] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purposes of improving battery life characteristics, suppressing battery capacity reduction, improving battery discharge capacity, etc. In this case, the additives may be contained in an amount of 0.1 to 5 wt % based on the total weight of the electrolyte.

[0103] As described above, a lithium secondary battery including the cathode active material according to the present invention stably exhibits excellent discharge capacity, output characteristics, and life characteristics, and is therefore useful in the fields of portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0104] The external shape of the lithium secondary battery according to the present invention is not particularly limited, and may be a cylindrical shape using a can, a square shape, a pouch shape, a coin shape, etc. Furthermore, the lithium secondary battery can be used not only as a battery cell used as a power source for a small device, but also as a unit battery for a medium- to large-sized battery module including a plurality of battery cells.

[0105] According to yet another aspect of the present invention, there may be provided a battery module including the lithium secondary battery as a unit cell and / or a battery pack including the same.

[0106] The battery module or the battery pack can be used as a power source for one or more medium- to large-sized devices, such as a power tool, an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV), or a power storage system.

[0107] Production Example 1. Production of positive electrode active material Example 1 Ni was synthesized using a precursor solution containing nickel sulfate, cobalt sulfate, and manganese sulfate in a molar ratio of 98:1:1. 0.98 Co 0.01 Mn 0.01 The nickel composite precursor of (OH)2 was prepared.

[0108] The nickel composite precursor was mixed with LiOH (Li / metal ratio = 1.06), and then the mixture was heated to 790°C at a rate of 2°C per minute in a calcination furnace while maintaining an O2 atmosphere, and calcined for 10 hours. The mixture was then crushed to obtain a lithium composite oxide.

[0109] Next, the lithium composite oxide was washed with distilled water containing 3 mol% of Co(OH)2, filtered, and dried.

[0110] Next, the dried lithium composite oxide was mixed with LiOH and H3BO3, and then the mixture was placed in a firing furnace, maintained in an O2 atmosphere, and heated to 720°C at a rate of 4°C per minute for 10 hours for secondary firing. After that, the mixture was naturally cooled to obtain the final product, the positive electrode active material.

[0111] Before the secondary firing, LiOH was mixed so that its concentration was 4 mol % and H3BO3 was mixed so that its concentration was 0.1 mol % in the entire composition.

[0112] Example 2 A positive electrode active material was prepared in the same manner as in Example 1, except that the content of H3BO3 was mixed to be 0.3 mol% in the total composition before the secondary firing.

[0113] Comparative Example 1 A positive electrode active material was prepared in the same manner as in Example 1, except that, before the secondary firing, a lithium composite oxide was mixed with LiOH instead of mixing the lithium composite oxide with LiOH and H3BO3. Before the secondary firing, LiOH was mixed so that its content in the total composition was 4 mol%.

[0114] Example 3 Ni was synthesized using a precursor solution in which nickel sulfate and cobalt sulfate were mixed in a molar ratio of 96.0:4.0. 0.96 Co 0.04 The nickel composite precursor of (OH)2 was prepared.

[0115] The nickel composite precursor was mixed with LiOH (Li / metal ratio = 1.04) and 1.4 mol% of Al(OH)3, and then the mixture was heated to 720°C at a rate of 2°C per minute in a furnace while maintaining an O2 atmosphere, and subjected to primary firing for 10 hours. The mixture was then crushed to obtain a lithium composite oxide.

[0116] Next, the lithium composite oxide was washed with distilled water containing 3 mol% of Co(OH)2, filtered, and dried.

[0117] Next, the dried lithium composite oxide was mixed with LiOH and H3BO3, and then the mixture was placed in a furnace with an O2 atmosphere, heated to 720°C at a rate of 4°C per minute, and subjected to secondary baking for 10 hours. After that, the mixture was naturally cooled to obtain the final product, the positive electrode active material.

[0118] Before the secondary firing, LiOH was mixed to account for 5 mol % of the total composition, and H3BO3 was mixed to account for 0.25 mol % of the total composition.

[0119] Comparative Example 2 A positive electrode active material was prepared in the same manner as in Example 3, except that the lithium composite oxide was mixed with LiOH before the secondary firing, instead of mixing the lithium composite oxide with LiOH and H3BO3. Before the secondary firing, LiOH was mixed so that its content in the total composition was 5 mol%.

[0120] Example 4 Ni was synthesized using a precursor solution in which nickel sulfate and cobalt sulfate were mixed in a molar ratio of 92.0:8.0. 0.92 Co 0.08 The nickel composite precursor of (OH)2 was prepared.

[0121] The nickel composite precursor was mixed with LiOH (Li / metal ratio = 1.04) and 1.4 mol% of Al(OH)3, and then the mixture was heated to 750°C at a rate of 2°C per minute in a furnace while maintaining an O2 atmosphere, and subjected to primary firing for 10 hours. The mixture was then crushed to obtain a lithium composite oxide.

[0122] Next, the lithium composite oxide was mixed with LiOH, Co3O4, and H3BO3, and then the mixture was placed in a firing furnace, maintained in an O2 atmosphere, and heated at 4°C per minute to 720°C for 10 hours for secondary firing. After that, the mixture was naturally cooled to obtain the final product, the positive electrode active material.

[0123] Before the secondary firing, LiOH was mixed to a concentration of 4 mol % in the entire composition, Co3O4 to a concentration of 3 mol % in the entire composition, and H3BO3 to a concentration of 0.25 mol % in the entire composition.

[0124] Comparative Example 3 A positive electrode active material was prepared in the same manner as in Example 4, except that before the secondary firing, the lithium composite oxide was mixed with LiOH and H3BO3 instead of mixing the lithium composite oxide with LiOH, Co3O4, and H3BO3.

[0125] Comparative Example 4 A positive electrode active material was prepared in the same manner as in Example 4, except that before the secondary firing, the lithium composite oxide was mixed with LiOH and Co3O4 instead of mixing the lithium composite oxide with LiOH, Co3O4, and H3BO3.

[0126] Example 5 Ni was synthesized using a precursor solution in which nickel sulfate and cobalt sulfate were mixed in a molar ratio of 92.0:8.0. 0.92 Co 0.08 The nickel composite precursor of (OH)2 was prepared.

[0127] The nickel composite precursor was mixed with LiOH (Li / metal ratio = 1.04) and 1.4 mol% Al(OH)3, and then the mixture was heated to 750°C at 2°C per minute in an O2 atmosphere in a calciner for 10 hours, and then crushed to obtain a lithium composite oxide.

[0128] Next, the lithium composite oxide was mixed with Co3O4 and H3BO3, and then the mixture was placed in a firing furnace, maintained in an O2 atmosphere, and heated at 4°C per minute to 720°C for a secondary firing period of 10 hours. After that, the mixture was naturally cooled to obtain the final product, the positive electrode active material.

[0129] Before the secondary firing, Co3O4 was mixed to account for 3 mol % of the total composition, and H3BO3 was mixed to account for 0.25 mol % of the total composition.

[0130] Comparative Example 5 A positive electrode active material was prepared in the same manner as in Example 5, except that before the secondary firing, the lithium composite oxide was mixed with H3BO3 instead of mixing the lithium composite oxide with Co3O4 and H3BO3.

[0131] Comparative Example 6 A positive electrode active material was prepared in the same manner as in Example 5, except that before the secondary firing, the lithium composite oxide was mixed with Co3O4 instead of mixing the lithium composite oxide with Co3O4 and H3BO3.

[0132] Manufacturing Example 2: Manufacturing of lithium secondary batteries A positive electrode slurry was prepared by dispersing 94 wt% of each positive electrode active material prepared in Preparation Example 1, 3 wt% of artificial graphite, and 3 wt% of PVDF binder in 3.5 g of N-methyl-2-pyrrolidone (NMP). The positive electrode slurry was applied to a 20 μm-thick aluminum (Al) thin film as a positive electrode current collector, dried, and roll-pressed to prepare a positive electrode. The loading level of the positive electrode was 7 mg / cm. 2 and the electrode density is 3.2 g / cm 3 It was.

[0133] A coin battery was fabricated by a commonly known manufacturing process using a lithium foil as a counter electrode for the positive electrode, a porous polyethylene film (Celgard 2300, thickness: 25 μm) as a separator, and a liquid electrolyte of 1.15 M LiPF in a solvent of ethylene carbonate and ethyl methyl carbonate mixed in a volume ratio of 3:7.

[0134] Experimental Example 1: Structural analysis of positive electrode active material (1) Cross-sectional SEM analysis of the positive electrode active material An SEM image of the lithium composite oxide contained in the positive electrode active material prepared in Preparation Example 1 was obtained using an FE-SEM device.

[0135] 2 and 3 are SEM images of the surface of the lithium composite oxide contained in the positive electrode active materials according to Example 1 and Comparative Example 1, respectively; FIGS. 4 and 5 are SEM images of the surface of the lithium composite oxide contained in the positive electrode active materials according to Example 3 and Comparative Example 2, respectively; FIGS. 6 to 8 are SEM images of the surface of the lithium composite oxide contained in the positive electrode active materials according to Example 4, Comparative Example 3, and Comparative Example 4, respectively; and FIGS. 9 to 11 are SEM images of the surface of the lithium composite oxide contained in the positive electrode active materials according to Example 5, Comparative Example 5, and Comparative Example 6, respectively.

[0136] Generally, it can be seen that a step-like structure is introduced to the surface of the lithium composite oxide contained in the positive electrode active materials according to Examples 1 to 5. It can also be seen that the step-like structure introduced to the surface of the lithium composite oxide is not simply due to bending formed by primary particles present on the surface of secondary particles.

[0137] The stepped structure introduced into the surface of the lithium composite oxide contained in the positive electrode active materials according to Examples 1 to 5 can also be seen as a terrace-step structure, and a terrace-step-kink structure in which kinks are partially formed can be confirmed.

[0138] On the other hand, it can be seen that the surface of the lithium composite oxide contained in the positive electrode active materials according to Comparative Examples 1 to 6 has almost no step structure.

[0139] (2) TEM analysis of the positive electrode active material In Preparation Example 1, a TEM image of the lithium composite oxide included in the positive electrode active material according to Example 1 was obtained, and the direction of the lithium ion diffusion path formed in the lithium composite oxide was confirmed from the TEM image.

[0140] In addition, the diffraction pattern obtained by FFT (fast Fourier transform) of the HR-TEM image was indexed to identify the crystal plane of the lithium composite oxide along which the lithium ion diffusion pathway formed within the lithium composite oxide was directed.

[0141] FIG. 12 is a TEM image of the surface of the lithium composite oxide contained in the positive electrode active material according to Example 1, showing each crystal plane forming the surface of the lithium composite oxide.

[0142] Referring to FIG. 12, it can be seen that the stepped structure formed on the surface of the lithium composite oxide contained in the cathode active material according to Example 1 is a stepped structure in which specific crystal planes belonging to the family of {01x}, {02y}, and {03z} planes form a dihedral angle of 90 degrees or more with adjacent crystal planes.

[0143] Furthermore, when the lithium ion diffusion path in the lithium composite oxide constituting the positive electrode active material is formed in a direction parallel to the extension direction of the (003) crystal plane and at the same time, the diffusion and desorption of lithium ions from the inside to the outside of the lithium composite oxide are oriented toward the (012), (014), and (022) crystal planes, which are relatively free, the electrochemical properties of the positive electrode active material containing the lithium composite oxide can be expected to be improved.

[0144] (3) Analysis of the surface coating layer The composition of the surface portion of the lithium composite oxide was analyzed by performing XPS analysis on the positive electrode active material of Example 1. The XPS analysis was performed using a Quantum 2000 (Physical Electronics, Inc.) (accelerating voltage: 0.5 to 15 keV, 300 W, energy resolution: approximately 1.0 eV, minimum analysis area: 10 microns, sputter rate: 0.1 nm / min).

[0145] FIG. 13 is a graph showing an XPS depth profile of the lithium composite oxide contained in the positive electrode active material according to Example 1. Referring to FIG. 13, it can be seen that boron (B) was detected only in the outermost surface portion of the lithium composite oxide (secondary particles).

[0146] Referring to FIG. 14, which is a graph showing the B1s XPS spectrum for the region where boron (B) was detected in the XPS depth profile of FIG. 13, it can be seen that a B1s peak specific to lithium borate was detected in the region of about 191.4 eV.

[0147] Furthermore, referring to FIG. 15 showing the interplanar distance of crystal planes in regions A and B in a TEM image of the lithium composite oxide contained in the positive electrode active material according to Example 1, in region A, the interplanar distance of 4.729 Å was measured, which corresponds to the interplanar distance of the (003) crystal planes of a general NCA or NCM type lithium composite oxide.

[0148] On the other hand, in region B corresponding to the outermost surface of the lithium composite oxide, the interplanar distance of about 2.570 Å, which is different from that in region A (ie, the interplanar distance of the (003) crystal plane), was measured.

[0149] 13 and 14, it can be confirmed that any oxide containing B is present on the outermost surface of the lithium composite oxide. In this case, among the oxides containing B, compounds having an interplanar distance of approximately 2.570 Å between crystal planes are LiBO2, Li3BO5, and Li2B4O7, and therefore it can be expected that LiBO2, Li3BO5, and / or Li2B4O7 are present on the outermost surface of the lithium composite oxide.

[0150] Meanwhile, the same experimental results were obtained for the positive electrode active materials according to Examples 2 to 5, and it was confirmed that a coating layer containing lithium borate was formed on the outermost surface of the lithium composite oxide contained in the positive electrode active material.

[0151] Experimental Example 2: Evaluation of the electrochemical properties of lithium secondary batteries The lithium secondary battery (coin cell) manufactured in Manufacturing Example 2 was subjected to a charge-discharge experiment using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, a voltage range of 3.0V to 4.3V, and a discharge rate of 0.1C to 5.0C to measure the charge capacity, discharge capacity, and charge-discharge efficiency.

[0152] In addition, the same lithium secondary battery was charged and discharged 50 times at 25°C and 1C / 1C within a driving voltage range of 3.0V to 4.4V, and the ratio of the discharge capacity at the 50th cycle to the initial capacity (cycle capacity retention) was measured.

[0153] The measurement results are shown in Table 1 below.

[0154] [Table 1]

[0155] Referring to the results in Table 1, it can be seen that the charge / discharge efficiency and cycle capacity retention of the examples using the cathode active material including the lithium composite oxide having a stepped structure formed on the surface thereof are generally higher than those of the comparative examples.

[0156] Although the embodiments of the present invention have been described above, a person having ordinary knowledge in the art may modify and change the present invention in various ways by adding, changing, deleting or adding components within the scope of the concept of the present invention as set forth in the claims, and this can also be said to be included in the scope of the present invention.

Claims

1. A positive electrode active material comprising a lithium composite oxide capable of lithium intercalation and deintercalation, The lithium composite oxide is a positive electrode active material having a surface with a step structure.

2. The positive electrode active material according to claim 1 , wherein the lithium composite oxide contains at least nickel and cobalt.

3. The positive electrode active material according to claim 2 , wherein the lithium composite oxide further contains at least one selected from manganese and aluminum.

4. 2. The positive electrode active material according to claim 1, wherein the step-like structure formed on the surface of the lithium composite oxide is provided so as to intersect with an extension direction of a (003) crystal plane of the lithium composite oxide.

5. 2. The positive electrode active material according to claim 1, wherein the stepped structure formed on the surface portion of the lithium composite oxide is formed such that any crystal plane belonging to at least one family of planes selected from the family of {01x}, {02y}, and {03z} planes among crystal planes present on the surface portion of the lithium composite oxide has a predetermined dihedral angle with another adjacent crystal plane. (wherein x, y, and z are each independently an integer between 0 and 12).

6. 6. The positive electrode active material according to claim 5, wherein a dihedral angle formed by any crystal plane belonging to at least one family of planes selected from the family of {01x}, {02y}, and {03z} planes relative to another adjacent crystal plane is 90° or more.

7. 6. The positive electrode active material according to claim 5, wherein a structure formed such that any crystal plane belonging to at least one family of planes selected from the family of {01x}, {02y}, and {03z} planes among crystal planes present on the surface of the lithium composite oxide has a predetermined dihedral angle with another adjacent crystal plane is periodically repeated on the surface of the lithium composite oxide.

8. 2. The positive electrode active material according to claim 1, wherein the stepped structure is a terrace-step-kink structure.

9. The positive electrode active material according to claim 1 , wherein the lithium composite oxide is represented by the following chemical formula 1: [Chemical formula 1] Li ｗ Ni １－（ｘ＋ｙ＋ｚ） Co ｘ M1 ｙ M2 ｚ O ２ (where, M1 is at least one selected from Mn and Al; M2 is at least one selected from Mn, Ba, Ce, Hf, Ta, Cr, F, Mg, Al, Cr, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Sr, Ge, Nd, Gd, B, and Cu; M1 and M2 are different elements, 0.5≦w≦1.5, 0≦x≦0.50, 0<y≦0.20, 0≦z≦0.20)

10. the positive electrode active material includes secondary particles formed by aggregation of a plurality of the lithium composite oxide primary particles, The cathode active material according to claim 1 , wherein a coating layer containing an oxide represented by the following Chemical Formula 2 is formed on at least a portion of the surfaces of the primary particles and the secondary particles: [Chemical formula 2] Li ａ M3 ｂ O ｃ (where, M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, Ce, V, Ba, Ta, Sn, Hf, Gd, and Nd; 0≦a≦8, 0<b≦8, 2≦c≦13)

11. The positive electrode active material according to claim 10 , wherein the oxide comprises a boron-containing oxide represented by the following Chemical Formula 3: [Chemical formula 3] Li ｄ b ｅ O ｆ (where, 0≦d≦8, 0<e≦8, 2≦f≦13)

12. a positive electrode current collector; a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material according to claim 1 ; a positive electrode comprising:

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

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