Positive electrode active material for secondary battery

Abstract

The present invention provides a positive electrode active material for a secondary battery, the positive electrode active material comprising a lithium composite transition metal oxide containing nickel (Ni) and at least one selected from the group consisting of cobalt (Co), manganese (Mn), and aluminum (Al), the lithium composite transition metal oxide including at least one element selected from the group consisting of cobalt (Co), manganese (Mn), and aluminum (Al), and the lithium composite transition metal oxide including at least one element selected from the group consisting of cobalt (Co), manganese (Mn), and aluminum (Al). The content of nickel (Ni) in the metal other than lithium is 60 mol% or more, and the lithium composite transition metal oxide satisfies relational expression 2: [relational expression 2] Delta size (crystallite size IB-crystallite size FWHM) < = 20; wherein, in relational expression 2, the crystallite size FWHM is a crystallite size calculated from X-ray diffraction (XRD) data using a full width at half maximum (FWHM) method, and the crystallite size IB is a crystallite size calculated from XRD data using an integral width (IB) method.

Description

[0001] This application is a divisional application of the original application of International Application No. PCT / KR2020 / 001526, Chinese National Stage Application No. 202080003990.8, filed on January 31, 2020, with the title of "Cathode Active Material for Secondary Battery and Lithium Secondary Battery Comprising the Same".

[0002] Cross Reference to Related Applications

[0003] This application claims priority to Korean Patent Application No. 10-2019-0013809, filed on February 1, 2019, the disclosure of which is incorporated herein by reference. TECHNICAL FIELD

[0004] The present invention relates to a cathode active material for a secondary battery and a lithium secondary battery comprising the same. BACKGROUND

[0005] In recent years, as electronic devices using batteries such as mobile phones, notebook computers, electric vehicles, etc. have rapidly spread, the demand for lightweight secondary batteries having relatively high capacity and small size has rapidly increased. In particular, since lithium secondary batteries are lightweight and have high energy density, lithium secondary batteries have attracted attention as driving power sources for portable devices. Therefore, research and development to improve the performance of lithium secondary batteries has been actively conducted.

[0006] In a lithium secondary battery, in a state in which an organic electrolyte solution or a polymer electrolyte solution is filled between a cathode and an anode respectively formed of active materials capable of intercalating and deintercalating lithium ions, electric energy is generated by oxidation and reduction reactions when lithium ions are intercalated / deintercalated in the cathode and the anode.

[0007] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2or LiMn2O4, etc.), or lithium iron phosphate compound (LiFePO4) has been used as a cathode active material for a lithium secondary battery. In addition, as a method of improving low thermal stability while maintaining the excellent reversible capacity of LiNiO2, lithium composite metal oxides (hereinafter, referred to as "NCM-based lithium transition metal oxide" or "NCA-based lithium transition metal oxide") in which a part of nickel (Ni) is substituted with cobalt (Co) or manganese (Mn) / aluminum (Al) have been developed. However, since the capacity characteristics of the conventionally developed NCM-based / NCA-based lithium transition metal oxide are insufficient, the application of the NCM-based / NCA-based lithium transition metal oxide is limited.

[0008] 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 having a high nickel content, a higher capacity can be obtained compared to layered-structure cathode active materials having different compositions, but the surface thereof is unstable, and structural degradation occurs during charging and discharging, and the lattice constant greatly changes, i.e., a large volume change occurs in the unit cell. Since such a volume change causes cracks in the cathode active material, and these cracks become severe during charging and discharging, the cracks act as voids where the electrolyte solution cannot reach or reduce the electrical conductivity. To prevent this, the performance of high-Ni cathode active materials has been improved by coating or doping. However, coating / doping can incur additional costs, and it can be difficult to uniformly coat / dope the surface or lattice of the cathode active material. SUMMARY

[0009] [TECHNICAL PROBLEM]

[0010] One aspect of the present application provides a cathode active material that can minimize cracks in the cathode active material occurring during charging and discharging, can increase the capacity retention rate as cycling progresses, and can suppress the resistance increase rate by optimizing the crystallite size and reducing the strain in a nickel-based lithium composite transition metal oxide cathode active material.

[0011] [TECHNICAL SOLUTION]

[0012] According to one aspect of the present application, there is provided a cathode active material for a secondary battery, the cathode active material comprising a nickel-based lithium composite transition metal oxide containing nickel (Ni), wherein the lithium composite transition metal oxide satisfies the following relation expression 1 and relation expression 2.

[0013] [RELATION EXPRESSION 1]

[0014] 80 nm ≤ crystallite size FWHM ≤ 150 nm

[0015] [RELATION EXPRESSION 2]

[0016] Δsize (|crystallite size IB – crystallite size FWHM |) ≤ 20

[0017] In the relation expression 1 and the relation expression 2, “crystallite size FWHM ” is a crystallite size calculated from X-ray diffraction (XRD) data using a full width at half maximum (FWHM) method, and “crystallite size IB ” is a crystallite size calculated from XRD data using an integral breadth (IB) method.

[0018] According to another aspect of the present application, there is provided a cathode comprising the cathode active material and a lithium secondary battery.

[0019] [Advantageous Effects]

[0020] According to the present application, in a nickel-based lithium composite transition metal oxide cathode active material, cracks of the cathode active material occurring during charge and discharge can be minimized, capacity retention rate with the progress of cycles can be increased, and resistance increase rate can be suppressed. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is a graph evaluating secondary battery life characteristics of cathode active materials using the present application examples and comparative examples. DETAILED DESCRIPTION

[0022] Hereinafter, the present application will be described in more detail to allow a clearer understanding of the present application. In this case, it should be understood that the words or terms used in the specification and claims should not be interpreted as being defined in commonly used dictionaries. It will be further understood that the words or terms used in the specification and claims should be interpreted as having a meaning that is consistent with the meaning of the terms in the context of relevant art and the technical idea of the present application, based on the principle that an inventor can properly define the meaning of the words or terms to best explain the present application.

[0023] <Positive electrode active material>

[0024] The cathode active material for a secondary battery of the present application is a nickel-based lithium composite transition metal oxide containing nickel (Ni), wherein the lithium composite transition metal oxide satisfies the following relation expression 1 and relation expression 2.

[0025] [Relation Expression 1]

[0026] 80 nm ≤ crystallite size FWHM ≤ 150 nm

[0027] [Relation Expression 2]

[0028] Δ size (| crystallite size IB – crystallite size FWHM |) ≤ 20

[0029] In the relation expression 1 and relation expression 2, the crystallite size FWHM is a crystallite size calculated from X-ray diffraction (XRD) data using a full width at half maximum (FWHM) method, and the crystallite size IB is a crystallite size calculated from XRD data using an integral breadth (IB) method.

[0030] The positive electrode active material of the present application is a nickel-based lithium complex transition metal oxide containing nickel (Ni). Preferably, the lithium complex transition metal oxide contains nickel (Ni) and can further contain at least one of cobalt (Co), manganese (Mn), and aluminum (Al). Furthermore, the lithium complex transition metal oxide can be a high-Ni lithium complex transition metal oxide in which the content of nickel (Ni) is 60 mol% or more, more preferably the content of nickel (Ni) is 70 mol% or more, for example, 80 mol% or more, among the metals other than lithium. As described above, since the content of nickel (Ni) is 60 mol% or more among the metals other than lithium, a high capacity can be ensured.

[0031] Furthermore, the lithium complex transition metal oxide of one embodiment of the present application can be represented by the following formula 1.

[0032] [Formula 1]

[0033] Li a Ni 1-(x+y+z) Co x M’ y M” z O 2+δ

[0034] 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, and 0 ≤ δ ≤ 1.0.

[0035] In the lithium complex transition metal oxide of formula 1, the content of lithium (Li) can correspond to a, i.e., 1.0 ≤ a ≤ 1.5. When a is less than 1.0, the capacity can be reduced, and when a is greater than 1.5, since the strength of the sintered positive electrode active material increases, grinding becomes difficult, and the amount of gas generated can increase due to an increase in Li by-products. In view of the balance between the capacity characteristics improvement effect of the positive electrode active material due to control of the amount of Li and the sinterability during preparation of the active material, the content of Li is more preferably 1.0 ≤ a ≤ 1.1.

[0036] In the lithium complex transition metal oxide of formula 1, the content of Ni can correspond to 1 - (x + y + z), for example, 0.6 ≤ 1 - (x + y + z) ≤ 0.9. If the amount of Ni in the lithium complex transition metal oxide of formula 1 is 0.6 or more, since the amount of nickel sufficient to promote charge and discharge is ensured, a higher capacity can be achieved. The content of Ni is more preferably 0.6 ≤ 1 - (x + y + z) ≤ 0.8.

[0037] In the lithium composite transition metal oxide of Formula 1, the content of Co can correspond to x, and the content of M' can correspond to y. In this case, the contents of Co and M' can satisfy x < y, and since the structural stability of the layered structure increases when y is greater than x, the occurrence of a lattice volume change during charge and discharge processes decreases, and thus, the occurrence of structural degradation and cracks can be effectively inhibited.

[0038] In the lithium composite transition metal oxide of Formula 1, M" can be a doping element included in the crystal structure of the lithium composite transition metal oxide, and the content of M" can correspond to z, i.e., 0 ≤ z ≤ 0.1.

[0039] As described above, for a high-Ni lithium composite transition metal oxide (in which, among the metals other than lithium, the content of nickel (Ni) is 60 mol% or more) as a positive electrode active material, it can achieve a high capacity, but its surface is unstable, structural degradation can occur during charge and discharge processes, and the lattice constant greatly changes, i.e., a large volume change occurs in the unit cell. Since such a volume change can cause cracks in the positive electrode active material, and these cracks become severe during charge and discharge processes, these cracks can act as voids where the electrolyte cannot reach or reduce the electrical conductivity.

[0040] Therefore, in the present application, by optimizing the crystallite size and reducing the strain to satisfy the relationship 1 and the 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 processes, the capacity retention rate as the secondary battery cycles can be increased, and the resistance increase rate can be inhibited.

[0041] The particles of the lithium composite transition metal oxide of the present application are composed of secondary particles in which primary particles are aggregated. In the present application, the expression "primary particle" indicates a primary structure of a single particle, and the expression "secondary particle" indicates an aggregate in which primary particles are aggregated by physical or chemical bonding between the primary particles without a deliberate aggregation or assembly process of the primary particles to constitute a secondary particle, i.e., a secondary structure.

[0042] In the present application, the expression "particle" indicates a particle having a micrometer size, which can be identified as a "grain" having a crystalline form with a size of several tens of nanometers when the particle is observed at a magnification. When the grain is further magnified, a separate region having a form in which atoms form a lattice structure in a predetermined direction can be identified, which is referred to as a "crystallite", and the particle size observed by X-ray diffraction (XRD) is defined as the crystallite size. As for the method of measuring the crystallite size, the crystallite size can be determined using XRD data.

[0043] The positive electrode active material of the present application satisfies the following relationship 1.

[0044] [Equation 1]

[0045] 80nm≤Crystalline size FWHM ≤150nm

[0046] In equation 1, the crystallite size FWHM The crystallite size of the positive electrode active material of one embodiment of the present invention is calculated from the XRD data using the full width at half maximum (FWHM) method. FWHM It may preferably be 90 nm to 150 nm, more preferably 100 nm to 145 nm, most preferably 130 nm to 132 nm.

[0047] Furthermore, the positive electrode active material of the present invention satisfies the following Relationship 2 at the same time.

[0048] [Equation 2]

[0049] Δ size (|crystallite size IB – Crystallite size FWHM |)≤20

[0050] In equation 2, the crystallite size FWHM is the crystallite size calculated from XRD data using the full width at half maximum (FWHM) method. IB is the crystallite size calculated from XRD data using the integrated width (IB) method. Calculate crystallite size FWHM When calculating the crystallite size without considering the strain IB When considering strain, the crystallite size FWHM and crystallite size IB The difference can be called the degree of strain. Generally speaking, when calculating the crystallite size, the peak shape is assumed to be a symmetrical bell shape, but the actual peak shape is not symmetrical 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 that corrects the area by the double Voigt method that combines the Lorentz and Gauss methods, it is a calculation method that takes into account the peak broadening caused by strain. Therefore, the crystallite size FWHM and crystallite size IB The difference can be called the degree of strain.

[0051] The Δ size (|crystal size) of the positive electrode active material of one embodiment of the present invention IB – Crystallite size FWHM |) can be 18 or less, i.e., 0 to 18, preferably 17 or less, i.e., 0 to 17, more preferably 16 or less, i.e., 0 to 16.

[0052] Since the positive electrode active material of the present application satisfies both the relationship 1 and the relationship 2, i.e., the positive electrode active material of the present application satisfies the crystallite size FWHM is 80 nm to 150 nm and Δsize (|crystallite size IB - the crystallite size FWHM |) is 20 or less, it is possible to minimize the cracks of the positive electrode active material occurring during the charge and discharge process, to increase the capacity retention rate as the cycle progresses, and to suppress the resistance increase rate.

[0053] By adjusting the sintering temperature and the sintering time, it is possible to obtain the positive electrode active material of the present application satisfying the relationship 1 and the relationship 2. When sintering after mixing the positive electrode active material precursor with the lithium source to prepare the lithium composite transition metal oxide, it is possible to perform sintering for a longer time at a sintering temperature that is relatively lower than the conventional sintering temperature, to increase the degree of completion of sintering and to reduce the strain that can occur in the crystallite.

[0054] Specifically, after mixing the positive electrode active material precursor containing nickel (Ni) (e.g., a high-Ni positive electrode active material precursor in which the content of nickel (Ni) is 60 mol% or more in the total metal) with a lithium source (e.g., LiOH) and performing first sintering at about 450°C to about 500°C (around the melting point of LiOH), 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, i.e., the total sintering time is about 30 hours or more, to prepare the lithium composite transition metal oxide. This allows the Li ions to sufficiently diffuse into the structure, and thus, it is possible to minimize the strain that can exist in the positive electrode active material.

[0055] However, otherwise, there is no particular limitation as long as it is a production method in which the crystallite size and the strain in the crystallite can satisfy the relationship 1 and the relationship 2 by adjusting the sintering conditions.

[0056] <Positive electrode and secondary battery>

[0057] According to another embodiment of the present application, there are provided a positive electrode for a secondary battery and a lithium secondary battery including the positive electrode active material produced as described above.

[0058] Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer containing the positive electrode active material provided on the positive electrode current collector.

[0059] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity and does not cause an adverse chemical change in the battery, and, for example, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel that has been surface-treated with one of carbon, nickel, titanium, or silver, etc. can be used. Also, the positive electrode current collector can generally have a thickness of 3 μm to 500 μm, and micro-irregularities can be formed on the surface of the current collector to improve adhesion of the positive electrode active material. The positive electrode current collector can be used in various shapes, such as the shape of a film, sheet, foil, mesh, porous body, foam body, and non-woven fabric body, etc.

[0060] Also, the positive electrode active material layer can include a conductive agent and a binder in addition to the above-described positive electrode active material.

[0061] In this case, the conductive agent is used to provide conductivity to the electrode, and any conductive agent can be used without particular limitation as long as it has suitable electronic conductivity and does not cause an adverse chemical change 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 crack 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 these or a mixture of two or more of these can be used. The content of the conductive agent can generally be 1 to 30% by weight, based on the total weight of the positive electrode active material layer.

[0062] Further, the binder improves adhesion between the positive electrode active material particles and 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), fluoro rubber, or various copolymers thereof, and any one of these or a mixture of two or more of these can be used. The content of the binder can be 1 to 30% by weight, based on the total weight of the positive electrode active material layer.

[0063] The positive electrode can be prepared according to a general positive electrode preparation method, in addition to using the above-described positive electrode active material. Specifically, a positive electrode active material layer formation composition, which includes the above-described positive electrode active material and, optionally, a binder and a conductive agent, can be coated on a 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 described previously.

[0064] The solvent can be a solvent generally used in the art, for example, the solvent can include dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl pyrrolidone (NMP), acetone, or water, any one thereof or a mixture of two or more thereof can be used. The amount of the solvent used is sufficient if the solvent can dissolve or disperse the positive electrode active material, the conductive agent, and the binder (taking into account the coating thickness of the slurry and the manufacturing yield), and can allow a viscosity to be obtained which can provide excellent thickness uniformity in a subsequent coating process for preparing the positive electrode.

[0065] Further, as another method, the positive electrode can be prepared by casting a composition for positive electrode active material layer formation on a separate support, and then laminating a film separated from the support on a positive electrode current collector.

[0066] According to another embodiment of the present application, 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.

[0067] 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. Further, the lithium secondary battery can also optionally include a battery container accommodating an electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member sealing the battery container.

[0068] 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.

[0069] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not cause adverse chemical changes of the battery, for example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, or silver, and an aluminum-cadmium alloy can be used. Further, the negative electrode current collector can generally have a thickness of 3 μm to 500 μm, and, similarly to the positive electrode current collector, micro-irregularities can be formed on the surface of the current collector to improve adhesion of the negative electrode active material. The negative electrode current collector can be used in various shapes, for example, the shape of a film, a sheet, a foil, a mesh, a porous body, a foam body, a non-woven fabric body, and the like.

[0070] The negative electrode active material layer optionally includes a binder and a conductive agent in addition to the negative electrode active material. The negative electrode active material layer can be prepared by coating a composition for negative electrode formation in the form of a slurry containing the negative electrode active material and optionally the binder and the conductive agent on a negative electrode current collector, and drying the coated negative electrode current collector, or can be prepared by casting the composition for negative electrode formation on a separate support, and then laminating a film separated from the support on a negative electrode current collector.

[0071] A compound capable of reversibly intercalating and deintercalating 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 alloys, Sn alloys, or Al alloys; metal oxides that can be doped and undoped with lithium such as SnO, SnO2, vanadium oxides, and lithium vanadium oxides; or composites containing a metal compound and a carbonaceous material such as Si-C composites or Sn-C composites, and any one thereof or a mixture of two or more thereof can be used. In addition, a thin metal lithium film can be used as the negative electrode active material. Furthermore, both low-crystalline carbon and high-crystalline carbon can be used as the carbonaceous material. Common examples of the low-crystalline carbon can be soft carbon and hard carbon, and common examples of the high-crystalline carbon can be irregular, planar, flaky, spherical, or fibrous natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbead, mesophase pitch, and high-temperature sintered carbon such as petroleum or coal tar pitch-derived coke. β (0 < β < 2), SnO2, vanadium oxides, and lithium vanadium oxides; or composites containing a metal compound and a carbonaceous material such as Si-C composites or Sn-C composites, and any one thereof or a mixture of two or more thereof can be used. In addition, a thin metal lithium film can be used as the negative electrode active material. Furthermore, both low-crystalline carbon and high-crystalline carbon can be used as the carbonaceous material. Common examples of the low-crystalline carbon can be soft carbon and hard carbon, and common examples of the high-crystalline carbon can be irregular, planar, flaky, spherical, or fibrous natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbead, mesophase pitch, and high-temperature sintered carbon such as petroleum or coal tar pitch-derived coke.

[0072] In addition, the binder and the conductive agent can be the same as those previously described in the positive electrode.

[0073] In the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a path for movement of lithium ions, wherein any separator can be used as the separator without particular limitation as long as it is generally used in lithium secondary batteries, and in particular, a separator having a high moisture retaining ability with respect to the electrolyte solution and a low resistance to the transport of electrolyte ions can be used. 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 / butylene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminate structure having two or more layers of the above-described polymer can be used. Also, a common porous nonwoven fabric such as a nonwoven fabric formed of high-melting point glass fibers or polyethylene terephthalate fibers can be used. In addition, a coated separator including a ceramic component or a polymeric material can be used to secure heat resistance or mechanical strength, and a separator having a single layer or a multi-layer structure can be selectively used.

[0074] In addition, the electrolyte used in the present application 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 application is not limited thereto.

[0075] Specifically, the electrolyte can include an organic solvent and a lithium salt.

[0076] Any organic solvent can be used as the organic solvent without particular limitation, as long as it can be used as a medium that can move ions participating in the electrochemical reaction of the battery. Specifically, as the organic solvent, an ester-based solvent such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; an ether-based solvent such as dibutyl ether or tetrahydrofuran; a ketone-based solvent such as cyclohexanone; an aromatic hydrocarbon-based solvent such as benzene and fluorobenzene; or a carbonate-based solvent such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methylethyl carbonate (MEC), ethylmethyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); an alcohol-based solvent such as ethanol and isopropanol; a nitrile such as R-CN (wherein R is a linear, branched, or cyclic C2 to C20 hydrocarbon group, and can include a double-bonded aromatic ring or an ether bond); an amide such as dimethylformamide; dioxolane such as 1,3-dioxolane; or a sulfolane can be used. Among these solvents, a carbonate-based solvent can be used, for example, a mixture of a cyclic carbonate having high ionic conductivity and high dielectric constant (e.g., ethylene carbonate or propylene carbonate) and a low-viscosity linear carbonate compound (e.g., ethylmethyl 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 in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte solution can be excellent.

[0077] A 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 0.1 M to 2.0 M. In the case where the concentration of the lithium salt is in the above range, since the electrolyte can have appropriate conductivity and viscosity, excellent electrolyte performance can be obtained, and lithium ions can move effectively.

[0078] To improve the life characteristics of the battery, suppress the decrease in the battery capacity, and increase the discharge capacity of the battery, at least one additive, such as a halogenated alkylene carbonate compound (e.g., difluoroethylene carbonate), pyridine, triethyl phosphite, triethanolamine, a cyclic ether, ethylenediamine, n-glyme, triamide hexaphosphate, a nitrobenzene derivative, sulfur, a quinonimine dye, an N-substituted oxazolidinone, an N,N-substituted imidazolidine, an ethylene glycol dialkyl ether, an ammonium salt, a pyrrole, 2-methoxyethanol, or aluminum trichloride, can be further added to the electrolyte in addition to the electrolyte components. In this case, the content of the additive can be 0.1 to 5% by weight based on the total weight of the electrolyte.

[0079] As described above, since the lithium secondary battery including the positive electrode active material of the present application stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, the lithium secondary battery is suitable for portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEV).

[0080] Therefore, according to another embodiment of the present application, there are provided a battery module including the lithium secondary battery as a cell unit and a battery pack including the battery module.

[0081] The battery module or the battery pack can be used as a power source for at least one of medium- and large-sized devices such as power tools, electric vehicles including electric vehicles (EV), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEV), or power storage systems.

[0082] Hereinafter, embodiments of the present application will be described in detail so that those of ordinary skill in the art to which the present application pertains can easily practice the present application. However, the present application can be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein.

[0083] Example 1

[0084] Ni 0.69 Co 0.13 Mn 0.18 The Ni

[0085] Example 2

[0086] 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.

[0087] Comparative Example 1

[0088] The positive electrode active material was prepared in the same manner as in Example 1, except that sintering was performed at 830°C for 20 hours.

[0089] Comparative Example 2

[0090] The positive electrode active material was prepared in the same manner as in Example 1, except that sintering was performed at 810°C for 31 hours.

[0091] [Experimental Example 1: Measurement of crystallite size and strain]

[0092] The crystallite size of the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2 was measured. 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.

[0093] [Table 1]

[0094] Crystallite size FWHM (nm) Delta size (crystallite size IB crystallite size FWHM |)]]> Example 1 130 8 Example 2 132 17 Comparative Example 1 180 34 Comparative Example 2 175 25

[0095] Referring to Table 1, for Examples 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 the relationship 1 and / or the relationship 2, and deviation occurs.

[0096] [Experimental Example 2: Evaluation of life characteristics]

[0097] Each of the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2, a carbon black conductive agent, and a PVdF binder were mixed in 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.

[0098] Lithium metal was used as a negative electrode.

[0099] Each lithium secondary battery was prepared as follows: a porous polyethylene separator was disposed between the positive electrode and the negative electrode prepared as described above to prepare an electrode assembly, the electrode assembly was put into a case, and then an electrolyte solution was injected into the case. In this case, the electrolyte solution was prepared as follows: 1.0 M lithium hexafluorophosphate (LiPF6) was dissolved in an organic solvent consisting of ethylene carbonate / ethyl methyl carbonate / diethyl carbonate (a mixed volume ratio of EC / EMC / DEC = 3 / 4 / 3).

[0100] Each lithium secondary battery half-cell prepared as described above was charged at 45°C in a constant current / constant voltage (CCCV) mode to a voltage of 4.3 V at 0.5 C and discharged to a voltage of 2.5 V at a constant current of 0.5 C, thereby measuring the capacity retention rate and the resistance increase rate when 400 charge / discharge cycles were performed. The results thereof are shown in Table 2 and Figure 1

[0101] [Table 2]

[0102] 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

[0103] Referring to Table 2, for Examples 1 and 2, the crystallite size FWHM satisfied 80 nm to 150 nm and Δsize (|crystallite size IB – crystallite size FWHM |) (i.e., strain) satisfied 20 or less, the capacity retention rate was improved and the resistance increase rate was reduced as compared with Comparative Examples 1 and 2.​

Claims

1. A positive electrode active material for a secondary battery, comprising a lithium composite transition metal oxide, wherein the lithium composite transition metal oxide comprises nickel (Ni) and at least one selected from the group consisting of cobalt (Co), manganese (Mn) and aluminum (Al), in, In the lithium composite transition metal oxide, the content of nickel (Ni) in the metals other than lithium is 60 mol% or more, Wherein, the lithium composite transition metal oxide satisfies the relationship 2: [Equation 2] Δ size (|crystallite size IB – crystallite size FWHM|) ≤ 20 In equation 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 the XRD data using the integrated width (IB) method.

2. The positive electrode active material for a secondary battery according to claim 1, wherein The lithium composite transition metal oxide particles are secondary particles formed by aggregation of primary particles.

3. The positive electrode active material for a secondary battery according to claim 1, wherein The lithium composite transition metal oxide is represented by Formula 1: [Formula 1] Li a Ni 1-(x+y+z) Co x M' y M" z O 2+δ 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。 4. The positive electrode active material for a secondary battery according to claim 1, wherein The Δ size (|crystallite size IB−crystallite size FWHM|) in Relational Expression 2 is 17 or less.

5. The positive electrode active material for a secondary battery according to claim 1, wherein The lithium composite transition metal oxide satisfies the relationship 1: [Equation 1] 80nm≤Crystalline size FWHM≤150nm Here, in Relational Formula 1, the crystallite size FWHM is the crystallite size calculated from X-ray diffraction (XRD) data using the full width at half maximum (FWHM) method.

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

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