Positive electrode active material for secondary battery and lithium secondary battery including the same

Abstract

The present invention provides a positive electrode active material for a secondary battery, the positive electrode active material comprising a nickel-based lithium composite transition metal oxide containing nickel (Ni), wherein the lithium composite transition metal oxide satisfies the following relational expressions 1 and relational expression 2. [Relational expression 1] 80 nm ≤ crystallite sizeFWHM ≤ 150 nm; [Relational expression 2] Δ size (|crystallite sizeIB – crystallite sizeFWHM|) ≤ 20; wherein, in relational expressions 1 and 2, the crystallite sizeFWHM is the crystallite size calculated from X-ray diffraction (XRD) data using the full width at half maximum (FWHM) method, and the crystallite sizeIB is the crystallite size calculated from XRD data using the integral breadth (IB) method.

Description

Technical Field

[0001] Cross - Reference to Related Applications

[0002] This application claims the priority of Korean Patent Application No. 10 - 2019 - 0013809, filed on February 1, 2019, the disclosure of which is incorporated herein by reference.

[0003] The present invention relates to a positive electrode active material for a secondary battery and a lithium secondary battery including the positive electrode active material. Background Art

[0004] In recent years, with the rapid popularization of battery - powered electronic devices such as mobile phones, laptop computers, and electric vehicles, the demand for lightweight secondary batteries with relatively high capacity and small volume has increased rapidly. In particular, since lithium secondary batteries are lightweight and have a high energy density, they have received much attention as a driving power source for portable devices. Therefore, research and development work on improving the performance of lithium secondary batteries has been actively carried out.

[0005] In a lithium secondary battery, in a state where an organic electrolyte solution or a polymer electrolyte solution is filled between a positive electrode and a negative electrode formed of active materials capable of inserting and extracting lithium ions, electric energy is generated through oxidation and reduction reactions when lithium ions are inserted / extracted between the positive electrode and the negative electrode.

[0006] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2 or LiMn2O4, etc.), or lithium iron phosphate compound (LiFePO4) has been used as a positive electrode active material for lithium secondary batteries. In addition, as a method of improving low - thermal stability while maintaining the excellent reversible capacity of LiNiO2, lithium composite metal oxides (hereinafter simply referred to as "NCM - based lithium transition metal oxides" or "NCA - based lithium transition metal oxides") have been developed, in which a part of nickel (Ni) is replaced by cobalt (Co) or manganese (Mn) / aluminum (Al). However, due to the insufficient capacity characteristics of conventionally developed NCM - based / NCA - based lithium transition metal oxides, the application of NCM - based / NCA - based lithium transition metal oxides is limited.

[0007] Therefore, to improve the capacity performance, research has been conducted to increase the content of nickel (Ni) in NCM-based / NCA-based lithium transition metal oxides. However, for high-Ni cathode active materials with a high nickel content, although they can achieve a higher capacity compared to layered cathode active materials with different compositions, their surfaces are unstable, structural degradation occurs during charge and discharge, and the lattice constant changes significantly, that is, a large volume change occurs in the unit cell. Since this volume change causes cracks in the cathode active material and these cracks become severe during charge and discharge, these cracks act as voids that the electrolyte solution cannot reach or reduce the conductivity. To prevent this, the performance of high-Ni cathode active materials has been improved by coating or doping. However, coating / doping may incur additional costs and it may be difficult to uniformly coat / dope the surface or lattice of the cathode active material. Summary of the Invention

[0008] [Technical Problem]

[0009] One aspect of the present invention provides a cathode active material that can minimize cracks in the cathode active material that occur during charge and discharge, increase the capacity retention rate as the number of cycles progresses, and suppress the resistance increase rate by optimizing the crystallite size and reducing the strain in the nickel-based lithium composite transition metal oxide cathode active material.

[0010] [Technical Solution]

[0011] According to one aspect of the present invention, there is provided a cathode active material for a secondary battery, the cathode active material comprising a nickel (Ni)-containing nickel-based lithium composite transition metal oxide, wherein the lithium composite transition metal oxide satisfies the following relational expressions 1 and 2.

[0012] [Relational Expression 1]

[0013] 80 nm ≤ crystallite size FWHM ≤ 150 nm

[0014] [Relational Expression 2]

[0015] Δ size (|crystallite size IB – crystallite size FWHM |) ≤ 20

[0016] In relational expressions 1 and 2, "crystallite size FWHM " is the crystallite size calculated from X-ray diffraction (XRD) data using the full width at half maximum (FWHM) method, and "crystallite size IB " is the crystallite size calculated from XRD data using the integral breadth (IB) method.

[0017] According to another aspect of the present invention, there are provided a positive electrode including the positive electrode active material and a lithium secondary battery.

[0018] [Advantageous Effects]

[0019] According to the present invention, in the nickel-based lithium composite transition metal oxide positive electrode active material, it is possible to minimize cracks in the positive electrode active material that occur during charge and discharge, increase the capacity retention rate as the number of cycles progresses, and suppress the resistance increase rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a graph for evaluating the life characteristics of a secondary battery using the positive electrode active materials of Examples and Comparative Examples of the present invention. DETAILED DESCRIPTION

[0021] Hereinafter, the present invention will be described in more detail to allow a clearer understanding of the present invention. In this case, it should be understood that the words or terms used in the specification and claims should not be construed as having the meanings defined in a common dictionary, and it should be further understood that, based on the principle that the inventor can appropriately define the meanings of the words or terms to best explain the present invention, these words or terms should be construed as having meanings consistent with their meanings in the context of the relevant field and technical idea of the present invention.

[0022] <Positive active material>

[0023] The positive electrode active material for a secondary battery of the present invention is a nickel-based lithium composite transition metal oxide containing nickel (Ni), wherein the lithium composite transition metal oxide satisfies the following relational expressions 1 and 2.

[0024] [Relational Expression 1]

[0025] 80 nm ≤ crystallite size FWHM ≤ 150 nm

[0026] [Relational Expression 2]

[0027] Δ size (|crystallite size IB – crystallite size FWHM |) ≤ 20

[0028] In Relational Expressions 1 and 2, the crystallite size FWHM is the crystallite size calculated from X-ray diffraction (XRD) data using the full width at half maximum (FWHM) method, and the crystallite size IB is the crystallite size calculated from XRD data using the integral breadth (IB) method.

[0029] The positive electrode active material of the present invention is a nickel-based lithium composite transition metal oxide containing nickel (Ni). Preferably, the lithium composite transition metal oxide contains nickel (Ni) and may further contain at least one of cobalt (Co), manganese (Mn), and aluminum (Al). In addition, the lithium composite transition metal oxide may be a high-Ni lithium composite transition metal oxide, wherein the content of nickel (Ni) among the metals other than lithium is 60 mol% or more, more preferably the content of nickel (Ni) is 70 mol% or more, for example, 80 mol% or more. As described above, since the content of nickel (Ni) among the metals other than lithium is 60 mol% or more, high capacity can be ensured.

[0030] In addition, the lithium composite transition metal oxide according to an embodiment of the present invention may be represented by the following formula 1.

[0031] [Formula 1]

[0032] Li a Ni 1-(x+y+z) Co x M’ y M” z O 2+δ

[0033] In Formula 1, M’ is at least one of Mn and Al, M” is at least one of barium (Ba), calcium (Ca), zirconium (Zr), titanium (Ti), magnesium (Mg), tantalum (Ta), niobium (Nb), and molybdenum (Mo), 1.0 ≤ a ≤ 1.5, x < y, 0 ≤ z ≤ 0.1, 0.1 ≤ x + y + z ≤ 0.4, 0 ≤ δ ≤ 1.0.

[0034] In the lithium composite transition metal oxide of Formula 1, the content of lithium (Li) may correspond to a, that is, 1.0 ≤ a ≤ 1.5. When a is less than 1.0, the capacity may decrease, and when a is greater than 1.5, due to the increased strength of the sintered positive electrode active material, grinding becomes difficult, and due to the increase in Li by-products, the amount of gas generated may increase. Considering the balance between the improvement effect of the capacity characteristics of the positive electrode active material due to controlling the amount of Li and the sinterability during the preparation of the active material, the content of Li more preferably satisfies 1.0 ≤ a ≤ 1.1.

[0035] In the lithium composite transition metal oxide of Formula 1, the content of Ni may correspond to 1-(x + y + z), for example, 0.6 ≤ 1-(x + y + z) ≤ 0.9. If the amount of Ni in the lithium composite transition metal oxide of Formula 1 is 0.6 or more, higher capacity can be achieved since a sufficient amount of nickel to promote charge and discharge is ensured. The content of Ni more preferably satisfies 0.6 ≤ 1-(x + y + z) ≤ 0.8.

[0036] In the lithium composite transition metal oxide of Formula 1, the content of Co may correspond to x, and the content of M' may correspond to y. In this case, the contents of Co and M' may satisfy x < y. Since the structural stability of the layered structure increases when y is greater than x, a decrease in lattice volume change occurs during charge and discharge. Therefore, the occurrence of structural deterioration and cracks can be effectively suppressed.

[0037] In the lithium composite transition metal oxide of Formula 1, M'' may be a doping element contained in the crystal structure of the lithium composite transition metal oxide, and the content of M'' may correspond to z, that is, 0 ≤ z ≤ 0.1.

[0038] As described above, for the positive electrode active material of a high-Ni lithium composite transition metal oxide (where the content of nickel (Ni) is 60 mol% or more among the metals other than lithium), it can achieve a high capacity, but its surface is unstable, structural deterioration occurs during charge and discharge, and a large change in lattice constant occurs, that is, a large volume change occurs in the unit cell. Since this volume change causes cracks to appear in the positive electrode active material, and these cracks become severe during charge and discharge, these cracks can act as voids that the electrolyte cannot reach or reduce the conductivity.

[0039] Therefore, in the present invention, by optimizing the crystallite size and reducing the strain to satisfy Relationship 1 and Relationship 2, the positive electrode active material of the high-Ni lithium composite transition metal oxide (having 60 mol% or more of nickel (Ni)) can minimize cracks during charge and discharge, increase the capacity retention rate as the secondary battery cycles, and suppress the resistance increase rate.

[0040] The particles of the lithium composite transition metal oxide of the present invention are composed of secondary particles aggregated from primary particles. In the present invention, the term "primary particle" refers to the primary structure of a single particle, and the term "secondary particle" refers to an aggregate in which primary particles are aggregated through physical or chemical bonding between the primary particles without an intentional aggregation or assembly process of the primary particles to form secondary particles, that is, a secondary structure.

[0041] In the present invention, the term "particle" refers to a particle having a micron size. When the particle is observed under magnification, the particle can be recognized as a "crystal grain" having a crystal form with a size of several tens of nanometers. When the crystal grain is further magnified, a separated region can be recognized, and this region has a form in which atoms form a lattice structure in a predetermined direction, and this region is called a "crystallite". The particle size observed by X-ray diffraction (XRD) is defined as the crystallite size. Regarding the method for measuring the crystallite size, XRD data can be used to determine the crystallite size.

[0042] The positive electrode active material of the present invention satisfies the following Relationship 1.

[0043] [Relationship 1]

[0044] 80 nm ≤ Crystallite size FWHM ≤ 150 nm

[0045] In Relationship 1, the crystallite size FWHM is the crystallite size calculated from XRD data using the full width at half maximum (FWHM) method. The crystallite size of the positive electrode active material according to one embodiment of the present invention FWHM may preferably be from 90 nm to 150 nm, more preferably from 100 nm to 145 nm, and most preferably from 130 nm to 132 nm.

[0046] In addition, the positive electrode active material of the present invention simultaneously satisfies the following Relationship 2.

[0047] [Relationship 2]

[0048] Δ size (|Crystallite size IB – Crystallite size FWHM |) ≤ 20

[0049] In Relationship 2, the crystallite size FWHM is the crystallite size calculated from XRD data using the full width at half maximum (FWHM) method, and the crystallite size IB is the crystallite size calculated from XRD data using the integral breadth (IB) method. When calculating the crystallite size FWHM strain is not considered, and when calculating the crystallite size IB strain is considered, where the difference between the crystallite size FWHM and the crystallite size IB can be referred to as the degree of strain. Generally, when calculating the crystallite size, the calculation is performed by assuming that the peak shape is a symmetric bell shape, but the actual peak shape is not symmetric, and peak broadening occurs due to the strain effect. The FWHM method ignores the peak broadening and uses the Lorentz method for calculation; since the IB method is a calculation method for correcting the area by the double Voigt method that combines the Lorentz and Gaussian methods, it is a calculation method that takes into account the peak broadening caused by strain. Therefore, the difference between the crystallite size FWHM and the crystallite size IB can be referred to as the degree of strain.

[0050] The Δ size (|Crystallite size IB – Crystallite size FWHM |) of the positive electrode active material according to one embodiment of the present invention can be 18 or less, that is, from 0 to 18, preferably 17 or less, that is, from 0 to 17, and more preferably 16 or less, that is, from 0 to 16.

[0051] Since the positive electrode active material of the present invention satisfies both Relationship 1 and Relationship 2, that is, the positive electrode active material of the present invention satisfies a crystallite size FWHM of 80 nm to 150 nm and a Δ size (|crystallite size IB – crystallite size FWHM |) of 20 or less, it is possible to minimize the cracks in the positive electrode active material that occur during charge and discharge, increase the capacity retention rate as the cycle progresses, and suppress the resistance increase rate.

[0052] By adjusting the sintering temperature and sintering time, the positive electrode active material of the present invention that satisfies Relationship 1 and Relationship 2 can be obtained. When sintering is performed after mixing a positive electrode active material precursor and a lithium source to prepare a lithium composite transition metal oxide, sintering can be performed at a sintering temperature relatively lower than the conventional sintering temperature for a longer time to increase the degree of sintering and reduce the strain that may occur in the crystallites.

[0053] Specifically, after mixing a nickel (Ni)-containing positive electrode active material precursor (for example, a high-Ni positive electrode active material precursor in which the content of nickel (Ni) among all metals is 60 mol% or more) and a lithium source (for example, LiOH) and performing a first sintering at about 450 °C to about 500 °C (near the melting point of LiOH), a second sintering is performed at about 730 °C to about 780 °C (lower than the conventional sintering temperature of 790 °C to 830 °C), and the sintering can be performed for a longer time, that is, the total sintering time is about 30 hours or more, to prepare a lithium composite transition metal oxide. This allows Li ions to diffuse sufficiently into the structure, and thus, the strain that may be present in the positive electrode active material can be minimized.

[0054] However, in addition, as long as it is a preparation method in which the crystallite size and the strain in the crystallites can satisfy Relationship 1 and Relationship 2 by adjusting the sintering conditions, there is no particular limitation.

[0055] <Positive electrode and secondary battery>

[0056] According to another embodiment of the present invention, a positive electrode for a secondary battery and a lithium secondary battery including the positive electrode active material prepared as described above are provided.

[0057] Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector and containing the positive electrode active material.

[0058] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used. Moreover, the positive electrode current collector generally can have a thickness of 3 μm to 500 μm, and microscopic irregularities can be formed on the surface of the current collector to improve the adhesion of the positive electrode active material. The positive electrode current collector can be used in various shapes, such as in the shapes of a film, sheet, foil, net, porous body, foam body, and non-woven fabric body, etc.

[0059] Moreover, in addition to the above positive electrode active material, the positive electrode active material layer can include a conductive agent and a binder.

[0060] In this case, the conductive agent is used to provide conductivity to the electrode. Any conductive agent can be used without particular limitation as long as it has suitable electron conductivity and does not cause adverse chemical changes in the battery. Specific examples of the conductive agent can be: 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 cracking carbon black, and carbon fiber; powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one of them or a mixture of two or more of them can be used. Based on the total weight of the positive electrode active material layer, the content of the conductive agent can generally be 1 wt% to 30 wt%.

[0061] In addition, the binder improves the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples of the binder can be polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one of them or a mixture of two or more of them can be used. Based on the total weight of the positive electrode active material layer, the content of the binder can be 1 wt% to 30 wt%.

[0062] In addition to using the above positive electrode active material, the positive electrode can be prepared according to the usual positive electrode preparation method. Specifically, a composition for forming a positive electrode active material layer (which contains the above positive electrode active material and optionally a binder and a conductive agent) can be coated on the positive electrode current collector, and then the coated positive electrode current collector can be dried and roll-pressed to prepare the positive electrode. In this case, the types and amounts of the positive electrode active material, binder, and conductive agent are the same as those described above.

[0063] The solvent can be a solvent commonly used in the art. For example, the solvent can include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water, and any one of them or a mixture of two or more of them can be used. If the solvent can dissolve or disperse the positive electrode active material, the conductive agent, and the binder (considering the coating thickness of the slurry and the manufacturing yield), and can allow a viscosity that can provide excellent thickness uniformity in the subsequent coating process for preparing the positive electrode, the amount of the solvent used is sufficient.

[0064] In addition, as another method, a positive electrode can be prepared by the following process: casting a composition for forming a positive electrode active material layer on a separate support, and then laminating a film separated from the support on the positive electrode current collector.

[0065] According to another embodiment of the present invention, an electrochemical device including the positive electrode is provided. The electrochemical device can specifically be a battery or a capacitor, and for example, can be a lithium secondary battery.

[0066] The lithium secondary battery specifically includes a positive electrode, a negative electrode disposed opposite to the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, wherein the positive electrode is as described above. In addition, the lithium secondary battery can optionally further include a battery container for accommodating the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member for sealing the battery container.

[0067] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector.

[0068] There is no particular limitation on the negative electrode current collector as long as it has high conductivity and does not cause adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloy can be used. In addition, the negative electrode current collector usually can have a thickness of 3 μm to 500 μm, and, similar to the positive electrode current collector, microscopic irregularities can be formed on the surface of the current collector to improve the adhesion of the negative electrode active material. The negative electrode current collector can be used in various shapes, such as the shapes of a film, a sheet, a foil, a mesh, a porous body, a foam body, and a non-woven fabric body, etc.

[0069] In addition to the negative electrode active material, the negative electrode active material layer optionally includes a binder and a conductive agent. The negative electrode active material layer can be prepared by the following process: coating a negative electrode forming composition in the form of a slurry containing the negative electrode active material and optionally the binder and the conductive agent on the negative electrode current collector, and drying the coated negative electrode current collector; or can be prepared by the following process: casting a negative electrode forming composition on a separate support, and then laminating a film separated from the support on the negative electrode current collector.

[0070] Compounds capable of reversibly embedding and de-embedding lithium can be used as the negative electrode active material. Specific examples of the negative electrode active material can be: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metal compounds capable of alloying with lithium such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium (Cd), Si alloy, Sn alloy, or Al alloy; metal oxides that can be doped and de-doped with lithium such as SiO β (0 < β < 2), SnO2, vanadium oxides, and lithium vanadium oxides; or composites containing metal compounds and carbonaceous materials such as Si-C composites or Sn-C composites, and any one of them or a mixture of two or more thereof can be used. In addition, a thin film of metallic lithium can be used as the negative electrode active material. Furthermore, both low-crystalline carbon and high-crystalline carbon can be used as carbonaceous materials. Common examples of low-crystalline carbon can be soft carbon and hard carbon, and common examples of high-crystalline carbon can be irregular, planar, flaky, spherical, or fibrous natural graphite or artificial graphite, floating (Kish) graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microspheres, mesophase pitch, and high-temperature sintered carbon such as coke derived from petroleum or coal tar pitch.

[0071] In addition, the binder and the conductive agent can be the same as those previously described for the positive electrode.

[0072] In a lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a path for the movement of lithium ions. Any separator can be used as the separator without particular limitation as long as it is commonly used in lithium secondary batteries. In particular, a separator having a high moisture retention capacity for the electrolyte and a low resistance to the transport of electrolyte ions can be used. Specifically, a porous polymer film can be used, for example, a porous polymer film made of polyolefin polymers (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer), or a laminated structure having two or more layers of the above polymers can be used. Moreover, common porous non-woven fabrics can be used, for example, non-woven fabrics formed of high melting point glass fibers or polyethylene terephthalate fibers. In addition, a coated separator including a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and a separator having a single-layer or multi-layer structure can be selectively used.

[0073] In addition, the electrolyte used in the present invention can include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten-type inorganic electrolyte that can be used to prepare a lithium secondary battery, but the present invention is not limited thereto.

[0074] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0075] Any organic solvent can be used as the organic solvent without particular limitation as long as it can be used as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as the organic solvent, the following can be used: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; or carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropyl alcohol; nitriles such as R-CN (where R is a linear, branched, or cyclic C2 to C20 hydrocarbon group and may include a double bond aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes. Among these solvents, carbonate solvents can be used. For example, a mixture of a cyclic carbonate (such as ethylene carbonate or propylene carbonate) having a high ion conductivity and a high dielectric constant and a low viscosity linear carbonate compound (such as ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) that can improve the charge / discharge performance of the battery can be used. In this case, when the cyclic carbonate and the chain carbonate are mixed at a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte solution can be excellent.

[0076] The lithium salt can be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, as the lithium salt, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2 can be used. The lithium salt can be used in a concentration range of 0.1M to 2.0M. When the concentration of the lithium salt is within the above range, since the electrolyte can have an appropriate conductivity and viscosity, excellent electrolyte performance can be obtained, and lithium ions can move effectively.

[0077] In order to improve the life characteristics of the battery, suppress the reduction of the battery capacity, and increase the discharge capacity of the battery, in addition to the electrolyte components, at least one additive can be further added to the electrolyte, for example, a halogenated alkylene carbonate compound (such as fluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, diethylene glycol dimethyl ether, hexaphosphoric triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, based on the total weight of the electrolyte, the content of the additive can be from 0.1% by weight to 5% by weight.

[0078] As described above, since the lithium secondary battery including the positive electrode active material of the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, the lithium secondary battery is suitable for portable devices such as mobile phones, laptop computers, and digital cameras, as well as electric vehicles such as hybrid electric vehicles (HEVs).

[0079] Therefore, according to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a cell unit and a battery pack including the battery module.

[0080] The battery module or battery pack can be used as a power source for at least one of the following medium and large-sized devices: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0081] Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement the present invention. However, the present invention can be implemented in various different forms and should not be construed as being limited to the embodiments described herein.

[0082] Example 1

[0083] Add Ni 0.69 Co 0.13 Mn 0.18 (OH)2 positive electrode active material precursor and a lithium raw material (LiOH) to a Henschel mixer (700L) so that the final molar ratio of Li / M (Ni, Co, Mn) is 1.05, and then mix at a central speed of 300 rpm for 20 minutes. Put the mixed powder into an alumina crucible with a size of 330 mm × 330 mm, hold it at 500 °C for 10 hours in an oxygen (O2) atmosphere, and then sinter at 730 °C for 30 hours to prepare the positive electrode active material.

[0084] Example 2

[0085] The positive electrode active material was prepared in the same manner as in Example 1, except that the mixed powder was held at 500 °C for 10 hours and then sintered at 750 °C for 20 hours.

[0086] Comparative Example 1

[0087] The positive electrode active material was prepared in the same manner as in Example 1, except that it was sintered at 830 °C for 20 hours.

[0088] Comparative Example 2

[0089] The positive electrode active material was prepared in the same manner as in Example 1, except that it was sintered at 810 °C for 31 hours.

[0090] [Experimental Example 1: Measurement of crystallite size and strain]

[0091] The crystallite sizes of the positive electrode active materials prepared in Example 1 and 2 and Comparative Examples 1 and 2 were measured. The crystallite size FWHM is the crystallite size calculated from the XRD data using the full width at half maximum (FWHM) method, and the crystallite size IB is the crystallite size calculated from the XRD data using the integral breadth (IB) method.

[0092] [Table 1]

[0093] <![CDATA[Microcrystalline size FWHM (nm)]]> <![CDATA[Δ size (| microcrystalline size IB – microcrystalline size FWHM |)]]> Example 1 130 8 Example 2 132 17 Comparative Example 1 180 34 Comparative Example 2 175 25

[0094] Referring to Table 1, for Example 1 and 2, the crystallite size FWHM satisfies 80 nm to 150 nm, and the Δ size (|crystallite size IB – crystallite size FWHM |) (i.e., strain) satisfies 20 or less, while Comparative Examples 1 and 2 do not satisfy Relation 1 and / or Relation 2, showing a deviation.

[0095] [Experimental Example 2: Evaluation of life characteristics]

[0096] Each of the positive electrode active materials, carbon black conductive agent, and PVdF binder prepared in Example 1 and 2 and Comparative Examples 1 and 2 was mixed at a weight ratio of 96:2:2 in an N-methylpyrrolidone solvent to prepare a positive electrode material mixture. The positive electrode material mixture was coated on one surface of an aluminum current collector, dried at 100 °C, and then roll-pressed to prepare a positive electrode.

[0097] Lithium metal was used as the negative electrode.

[0098] Each lithium secondary battery is prepared as follows: A porous polyethylene separator is disposed between the positive electrode and the negative electrode prepared as described above to prepare a battery assembly, the electrode assembly is placed in a case, and then an electrolyte solution is injected into the case. In this case, the electrolyte solution is prepared as follows: 1.0 M lithium hexafluorophosphate (LiPF6) is dissolved in an organic solvent composed of ethylene carbonate / ethyl methyl carbonate / diethyl carbonate (mixing volume ratio of EC / EMC / DEC = 3 / 4 / 3).

[0099] Each lithium secondary battery half-cell prepared as described above is charged at 45 °C in a constant current / constant voltage (CCCV) mode at 0.5C to a voltage of 4.3 V and discharged at a constant current of 0.5C to a voltage of 2.5 V, thereby measuring the capacity retention rate and the resistance increase rate when performing 400 charge and discharge cycles. The results are shown in Table 2 and Figure 1 in.

[0100] [Table 2]

[0101] Capacity retention rate (%) (@400 cycles) Resistance increase rate (%) (@400 cycles) Example 1 90.4 124 Example 2 86.2 135 Comparative Example 1 78.8 161 Comparative Example 2 82.5 146

[0102] Referring to Table 2, for Example 1 and Example 2, the microcrystalline size FWHM satisfies 80 nm to 150 nm and the Δ size (|microcrystalline size IB – microcrystalline size FWHM |) (i.e., strain) satisfies 20 or less. Compared with Comparative Examples 1 and 2, the capacity retention rate is increased and the resistance increase rate is decreased.

Claims

1. A positive electrode active material for a secondary battery, the positive electrode active material comprising a nickel (Ni)-containing nickel-based lithium composite transition metal oxide represented by Formula 1, [Formula 1] Li a Ni 1-(x+y+z) Co x M’ y M” z O 2+δ Among them, In Formula 1, M ’ is at least one of Mn and Al, M” is at least one of barium (Ba), calcium (Ca), zirconium (Zr), titanium (Ti), magnesium (Mg), tantalum (Ta) and niobium (Nb), 1.0 ≤ a ≤ 1.5, x < y, 0 ≤ z ≤ 0.1, 0.1 ≤ x + y + z ≤ 0.4, 0 ≤ δ ≤ 1.0, wherein the lithium composite transition metal oxide satisfies Relationship 1 and Relationship 2: [Relationship 1] 80nm ≤ crystallite size FWHM ≤ 150nm [Relationship 2] Δ Size (|Microcrystalline Size IB – Microcrystalline Size FWHM |) ≤ 20 Among them, in Relationship 1 and Relationship 2, the crystallite size FWHM is the crystallite size calculated from X-ray diffraction (XRD) data using the full width at half maximum (FWHM) method, and the crystallite size IB is the crystallite size calculated from XRD data using the integral breadth (IB) method; wherein, among the metals other than lithium in the lithium composite transition metal oxide, the content of nickel (Ni) is 60 mol% or more; wherein the particles of the lithium composite transition metal oxide are secondary particles aggregated from primary particles.

2. The positive electrode active material for a secondary battery according to claim 1, wherein, The lithium composite transition metal oxide contains nickel (Ni) and further contains at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

3. The positive electrode active material for a secondary battery according to claim 1, wherein, Among the metals other than lithium in the lithium composite transition metal oxide, the content of nickel (Ni) is 70 mol% or more.

4. The positive electrode active material for a secondary battery according to claim 1, wherein In Formula 1, z = 0.

5. The positive electrode active material for a secondary battery according to claim 1, wherein, The microcrystalline size in Relationship 1 FWHM is in the range of 130 nm to 132 nm.

6. The positive electrode active material for a secondary battery according to claim 1, wherein, The Δ dimension (|microcrystalline size IB – microcrystalline size FWHM |) in Formula 2 is 17 or less.

7. A method for preparing the positive electrode active material for a secondary battery according to claim 1, the method comprising: mixing a lithium source and a positive electrode active material precursor containing nickel (Ni), and performing a first sintering at 450 °C to 500 °C; and after the first sintering, performing a second sintering at 730 °C to 780 °C, wherein the total sintering time of the first sintering and the second sintering is 30 hours or more; wherein, among the metals other than lithium in the lithium composite transition metal oxide, the content of nickel (Ni) is 60 mol% or more; wherein the particles of the lithium composite transition metal oxide are secondary particles aggregated from primary particles.

8. The method according to claim 7, wherein, The lithium source is LiOH.

9. The method according to claim 7, wherein Among all the metals of the positive electrode active material precursor, the content of nickel (Ni) is 70 mol% or more.

10. A positive electrode for a secondary battery, the positive electrode comprising the positive electrode active material according to any one of claims 1 to 6.

11. A lithium secondary battery, the lithium secondary battery comprising the positive electrode according to claim 10.

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