Positive active material, method for preparing the same, and positive electrode for secondary battery and lithium secondary battery comprising the same

Abstract

The present invention relates to a positive electrode active material, a method for preparing the same, and a positive electrode for a secondary battery and a lithium secondary battery comprising the same. The positive electrode active material comprises a lithium transition metal oxide represented by the following Formula 1, wherein the lithium transition metal oxide comprises a central portion having a layered structure and a surface portion having a second phase different in structure from the central portion. The positive electrode active material has improved structural stability, thereby enabling the preparation of a battery having high capacity and long life. [Formula 1] Li 1+a (Ni x Co y M 1 z M 2 w ) 1‑a O2in Formula 1, 0≤a≤0.2, 0.6 1 is at least one selected from the group consisting of Mn and Al, and M 2 is at least one selected from the group consisting of Zr, B, W, Mo, Cr, Ta, Nb, Mg, Ce, Hf, Ta, La, Ti, Sr, Ba, Ce, F, P, S, and Y.

Description

[0001] This invention patent application is a divisional application of Chinese patent application filed on December 5, 2018, with application number 201880058902.7 and invention title "Positive electrode active material for lithium secondary batteries, preparation method thereof, positive electrode for lithium secondary batteries containing said positive electrode active material and lithium secondary batteries".

[0002] Cross-reference of related applications

[0003] This application claims the benefit of Korean Patent Application No. 2017-0169449, filed with the Korean Intellectual Property Office on December 11, 2017, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0004] This invention relates to a positive electrode active material for lithium secondary batteries, a method for preparing the positive electrode active material, a positive electrode for lithium secondary batteries comprising the positive electrode active material, and a lithium secondary battery. Background Technology

[0005] With the development of mobile device technology and increasing demand, the demand for secondary batteries as an energy source has increased significantly. Among these secondary batteries, lithium secondary batteries, which have high energy density, high voltage, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0006] Lithium transition metal composite oxides have been used as positive electrode active materials for lithium secondary batteries, and among these oxides, lithium-cobalt composite metal oxides such as LiCoO2, which have high operating voltage and excellent capacity characteristics, are mainly used. However, LiCoO2 exhibits very poor thermal properties due to its unstable crystal structure caused by delithiation. Furthermore, the high cost of LiCoO2 limits its use in large quantities as a power source for applications such as electric vehicles.

[0007] Lithium manganese composite metal oxides (LiMnO2 or LiMn2O4), lithium iron phosphate compounds (LiFePO4, etc.), or lithium nickel composite metal oxides (LiNiO2, etc.) have been developed as alternatives to LiCoO2. Among these materials, lithium nickel composite metal oxides have been studied and developed more actively, as they can easily achieve large-capacity batteries due to their high reversible capacity of approximately 200 mAh / g. However, a limitation of LiNiO2 is that it has worse thermal stability than LiCoO2. When an internal short circuit occurs during charging due to external pressure, the positive electrode active material decomposes, causing the battery to rupture and catch fire. Therefore, as a method to improve the low thermal stability while maintaining the excellent reversible capacity of LiNiO2, lithium nickel cobalt manganese oxides, in which a portion of the nickel (Ni) is replaced by cobalt (Co), manganese (Mn), or aluminum (Al), have been developed.

[0008] However, lithium nickel cobalt manganese oxides have low structural stability and capacity, and their limitations lie in the fact that stability further decreases, especially when the amount of nickel is increased to improve capacity characteristics.

[0009] Therefore, among cathode active materials containing high nickel content that exhibit high capacity characteristics, there is a need to develop a cathode active material that can be used to prepare high-capacity and long-life batteries due to its excellent stability. Summary of the Invention

[0010] Technical issues

[0011] One aspect of the present invention provides a positive electrode active material with improved structural stability.

[0012] Another aspect of the present invention provides a method for preparing a positive electrode active material.

[0013] Another aspect of the present invention provides a positive electrode for a lithium secondary battery, the positive electrode comprising the positive electrode active material.

[0014] Another aspect of the present invention provides a lithium secondary battery comprising a positive electrode for the lithium secondary battery.

[0015] Technical solution

[0016] According to one aspect of the present invention, a positive electrode active material is provided, the positive electrode active material comprising a lithium transition metal oxide represented by Formula 1, wherein the lithium transition metal oxide comprises a central portion having a layered structure and a surface portion having a second phase having a structure different from the central portion.

[0017] [Formula 1]

[0018] Li1+a (Ni x Co y M 1 z M 2 w ) 1-a O2

[0019] In Equation 1,

[0020] 0≤a≤0.2, 0.6 <x≤1,0<y≤0.4,0<z≤0.4,M 1 It is selected from at least one of the following: manganese (Mn) and aluminum (Al), and M 2 It is selected from at least one of the following: zirconium (Zr), boron (B), tungsten (W), molybdenum (Mo), chromium (Cr), tantalum (Ta), niobium (Nb), magnesium (Mg), cerium (Ce), hafnium (Hf), lanthanum (La), titanium (Ti), strontium (Sr), barium (Ba), fluorine (F), phosphorus (P), sulfur (S), and yttrium (Y).

[0021] According to another aspect of the present invention, a method for preparing a positive electrode active material is provided, the method comprising the steps of: mixing a positive electrode active material precursor with a lithium raw material and subjecting it to a primary heat treatment; and performing a secondary heat treatment at a temperature lower than that of the primary heat treatment to prepare the positive electrode active material, wherein the primary heat treatment and the secondary heat treatment are respectively performed in an oxygen atmosphere, and the secondary heat treatment is performed in an oxygen atmosphere with an oxygen concentration of 50% or higher.

[0022] According to another aspect of the present invention, a positive electrode for a lithium secondary battery is provided, the positive electrode comprising: 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 comprises a positive electrode active material according to the present invention.

[0023] According to another aspect of the present invention, a lithium secondary battery is provided, the lithium secondary battery comprising: a positive electrode according to the present invention; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte.

[0024] Beneficial effects

[0025] According to the present invention, a positive electrode active material can be prepared by controlling the heat treatment conditions during the preparation of positive electrode active material particles. The positive electrode active material comprises a central portion having a layered structure and a surface portion having a second phase with a structure different from that of the central portion. Specifically, a positive electrode active material with improved structural stability can be prepared by having a layered structure in the central portion of the positive electrode active material particles and having a second phase (spinel structure and / or rock salt structure) with a structure different from that of the central portion only in the surface portion. The surface portion is specifically located within 30 nm from the surface of the particle toward the center.

[0026] Furthermore, since improving structural stability as described above improves lifetime characteristics, it is possible to fabricate lithium secondary batteries with long lifespans. Attached Figure Description

[0027] Figure 1 This is a schematic diagram showing the positive electrode active material particles according to the present invention;

[0028] Figure 2 It contains data on small-angle diffraction patterns (SADP) that show the layered structure of the positive electrode active material particles;

[0029] Figure 3 This is SADP data showing the rock salt structure of the positive electrode active material particles; and

[0030] Figure 4 This is SADP data showing the spinel structure of the positive electrode active material particles.

[0031] [Figure Labels]

[0032] 100: Positive electrode active material particles

[0033] 10: Central Section

[0034] 20: Surface part Detailed Implementation

[0035] The invention will be described in more detail below.

[0036] It should be understood that the words or terms used in the specification and claims should not be interpreted as having the meaning defined in a common dictionary, and it will be further understood that, based on the principle that the inventor can appropriately define the meaning of the words or terms to best interpret the invention, the words or terms should be interpreted as having a meaning consistent with their meaning in the relevant field context and the technical concept of the invention.

[0037] For lithium nickel cobalt manganese oxide used as a conventional positive electrode active material for a lithium secondary battery, the structural stability of the positive electrode active material is low, and there is a limitation in that, especially when a large amount of nickel is included to prepare a high-capacity battery, the structural stability of the positive electrode active material is further reduced.

[0038] To compensate for this limitation, research has been actively conducted to improve the structural stability by doping the positive electrode active material with a metal element or a metal oxide. However, in the case of doping the positive electrode active material using a metal element as a doping raw material, since the improvement of the structural stability is limited, a coating must be applied to the positive electrode active material, and thus there are limitations such as a rise in unit price or a reduction in energy density.

[0039] Thus, the present inventors have found that by controlling the heat treatment conditions during the preparation of lithium nickel cobalt manganese oxide, a second phase can be formed on the surface of the layered-structured lithium transition metal oxide, and a positive electrode active material having improved structural stability can be prepared, leading to the completion of the present invention.

[0040] (Positive electrode active material)

[0041] First, as Figure 1 shown, the positive electrode active material particles 100 according to the present invention include a lithium transition metal oxide, wherein the lithium transition metal oxide includes a central portion 10 having a layered structure and a surface portion 20 having a structure different from that of the central portion.

[0042] Specifically, the average composition of the lithium transition metal oxide can preferably be represented by the following formula 1.

[0043] [Formula 1]

[0044] Li 1+a (Ni x Co y M 1 [[ID=3l]] z M 2 w ) 1-a O2

[0045] In Formula 1,

[0046] 0 ≤ a ≤ 0.2, 0.6 < x ≤ 1, 0 < y ≤ 0.4, 0 < z ≤ 0.4 and 0 ≤ w ≤ 0.1, for example, 0 ≤ a ≤ 0.1, 0.7 ≤ x ≤ 1, 0 ≤ y ≤ 0.3, 0 ≤ z ≤ 0.3 and 0 ≤ w ≤ 0.05.

[0047] M 1 is at least one selected from the following: manganese (Mn) and aluminum (Al), and M 2It is selected from at least one of the following: zirconium (Zr), boron (B), tungsten (W), molybdenum (Mo), chromium (Cr), tantalum (Ta), niobium (Nb), magnesium (Mg), cerium (Ce), hafnium (Hf), lanthanum (La), titanium (Ti), strontium (Sr), barium (Ba), fluorine (F), phosphorus (P), sulfur (S), and yttrium (Y).

[0048] When a battery is fabricated using a lithium transition metal oxide as described above, a high battery capacity can be achieved, wherein the amount of nickel in the lithium transition metal oxide is greater than 60 moles based on the total molar number of transition metals other than lithium.

[0049] The positive electrode active material comprises a central portion having a layered structure and a surface portion having a second phase with a structure different from that of the central portion.

[0050] The term "layered structure" refers to a structure in which planes of atoms, tightly bonded by covalent bonds or other strong bonds, overlap in parallel through weak binding forces such as van der Waals forces. For lithium transition metal oxides with a layered structure, the dense arrangement of lithium ions, transition metal ions, and oxygen ions—specifically, alternating layers of metal oxides composed of transition metals and oxygen and octahedral layers of oxygen surrounding lithium—and the Coulomb repulsion acting between the metal oxide layers, allows for the insertion and extraction of lithium ions. Furthermore, because lithium ions diffuse along a two-dimensional plane, ionic conductivity is high.

[0051] Therefore, for positive electrode active materials with layered structures, lithium ions can move quickly and smoothly within the particles to promote lithium ion insertion and extraction, thus reducing the initial internal resistance of the battery. This further improves discharge capacity and lifespan characteristics without worrying about a decrease in rate performance and initial capacity characteristics.

[0052] The surface portion having a second phase with a structure different from the central portion refers to the region located within 30 nm from the surface of the positive electrode active material particle toward the center of the particle, wherein a second phase with a layered structure different from that of the central portion exists.

[0053] The surface portion may include at least one of spinel structure and rock salt structure.

[0054] The term "spinel structure" refers to a structure consisting of a metal oxide layer composed of transition metals and oxygen, and an oxygen octahedral layer surrounding lithium, exhibiting a shape resembling... Figure 4 The three-dimensional arrangement is shown. Specifically, lithium transition metal oxides with a spinel structure can be formed from LiMe structures. x1 Mn 2-x1 O4 (where Me comprises at least one selected from Ni, Co, and Al), wherein it is represented by a transition metal ion with an oxidation state of 3+ or less (selected from at least one selected from Ni).2+ Co 2+ And Al 3+ ) Replacement of Mn 3+ Therefore, replacing Mn sites with metals having oxidation states of 2+ or 3+ can increase the average valence of Mn, thereby improving the stability of lithium transition metal oxides.

[0055] The term "rock salt structure" refers to a face-centered cubic structure, such as... Figure 3 The metal atom shown is coordinated with six oxygen atoms arranged in an octahedral shape. Compounds with a rock salt structure exhibit high structural stability, especially at high temperatures.

[0056] In the case of forming a lithium transition metal oxide having a second phase as described above, the structural stability and thermal stability of the cathode active material can be improved due to the formation of the second phase, wherein the second phase comprises at least one of a spinel structure and a rock salt structure on the surface of the lithium transition metal oxide having a layered structure.

[0057] In particular, when the surface portion exists only in the region located within 30 nm from the particle surface toward the center, the effect of improving structural stability and thermal stability can be more pronounced, and when the positive electrode active material is used in a battery, the life characteristics of the secondary battery can be improved.

[0058] In contrast, when positive electrode active material particles are used in batteries, the lifetime characteristics may deteriorate if a single phase exists throughout the entire positive electrode active material particle, or if the proportion of the second phase in the entire particle increases due to the presence of a second phase extending from the particle surface toward the center or even beyond 30 nm.

[0059] Considering the convenience during the preparation process and electrode application, the average particle size (D) of the positive electrode active material particles is... 50 The size can be in the range of 4μm to 20μm, and more preferably in the range of 8μm to 14μm.

[0060] The average particle size D of the positive electrode active material particles 50 This can be defined as the particle size at 50% of the cumulative particle size distribution. In this invention, for example, the particle size distribution of the positive electrode active material particles can be measured using laser diffraction. Specifically, regarding the particle distribution of the positive electrode active material, after dispersing the particles of the positive electrode active material in a dispersion medium, the dispersion medium is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000) and irradiated with ultrasound at a frequency of approximately 28 kHz and an output of 60 W. The average particle size at 50% of the cumulative particle size distribution in the analyzer can then be calculated.

[0061] (Method for preparing a positive electrode active material)

[0062] The method for preparing a positive electrode active material according to the present invention includes: mixing a positive electrode active material precursor with a lithium raw material and performing a first heat treatment; and performing a second heat treatment at a temperature lower than the first heat treatment to prepare the positive electrode active material, wherein the first heat treatment and the second heat treatment are respectively performed in an oxygen atmosphere, and the second heat treatment is performed in an oxygen atmosphere with an oxygen concentration of 50% or more.

[0063] Hereinafter, the method for preparing a positive electrode active material according to the present invention will be described in more detail.

[0064] First, a positive electrode active material precursor is mixed with a lithium raw material, and a first heat treatment is performed.

[0065] The positive electrode active material precursor may contain nickel in an amount greater than 60 mol% based on the total molar amount of transition metals, and may preferably be represented by Ni x1 Co y1 M 1 z1 M 2 w1 (OH)2 (where 0.6 < x1 ≤ 1, 0 < y1 ≤ 0.4, 0 < z1 ≤ 0.4 and 0 ≤ w1 ≤ 0.1, M 1 includes at least one selected from the following: Mn and Al, and M 2 includes at least one selected from the following: Zr, B, W, Mo, Cr, Ta, Nb, Mg, Ce, Hf, La, Ti, Sr, Ba, F, P, S and Y).

[0066] As described above, when the amount of nickel is greater than 60 mol% based on the total molar amount of transition metals in the positive electrode active material precursor, a high-capacity battery can be achieved when using the precursor to prepare a battery.

[0067] In addition, the lithium raw material can be used without particular limitation as long as it is a compound containing a lithium source, but preferably, at least one selected from the following can be used: lithium carbonate (Li2CO3), lithium hydroxide (LiOH), LiNO3, CH3COOLi, and Li2(COO)2.

[0068] In addition, the positive electrode active material precursor and the lithium raw material can be mixed such that the molar ratio of lithium to transition metals (Li / transition metals) is in the range of 1 to 1.2, preferably 1 to 1.1, more preferably 1 to 1.05. When the positive electrode active material precursor and the lithium raw material are mixed within the above range, a positive electrode active material exhibiting excellent capacity characteristics can be prepared.

[0069] A single heat treatment can be carried out at 800℃ or above, preferably 800℃~900℃, more preferably 800℃~850℃ for 10 hours to 20 hours, such as 12 hours to 16 hours.

[0070] Furthermore, the primary heat treatment can be performed in an oxygen atmosphere with an oxygen concentration of 50% or higher. Performing the primary heat treatment in an oxygen atmosphere with an oxygen concentration of 50% or higher promotes the reaction between the cathode active material precursor and lithium. For example, if the primary heat treatment is performed in an air atmosphere or an inert atmosphere, the reaction between the cathode active material precursor and lithium may not proceed smoothly, and unreacted lithium may remain on the surface of the cathode active material. Due to the presence of unreacted lithium, when this cathode active material is used in a battery, the amount of gas generated due to the reaction of the electrolyte with the unreacted lithium present on the surface of the cathode active material may increase, and therefore, the battery may swell.

[0071] Next, after the first heat treatment, a second heat treatment can be performed at a temperature lower than that of the first heat treatment.

[0072] Regarding performing a second heat treatment after the first heat treatment, it can be done by cooling to room temperature after the first heat treatment and then performing the second heat treatment again; or it can be done by performing the second heat treatment immediately after the first heat treatment.

[0073] In this case, the secondary heat treatment can be carried out for 2 to 12 hours, or 3 to 7 hours, in an oxygen atmosphere with an oxygen concentration of more than 50% at a temperature greater than 600°C and less than 800°C, such as 650°C to 750°C.

[0074] As in this invention, when a secondary heat treatment is performed in an oxygen atmosphere with an oxygen concentration of 50% or higher at a temperature greater than 600°C and less than 800°C, a second phase with a structure different from the layered structure can be formed on the surface of a lithium transition metal oxide having a layered structure. In this case, the surface of the lithium transition metal oxide refers to the region located within 30 nm from the surface of the lithium transition metal oxide toward the center.

[0075] In contrast, if either the oxygen concentration or the heat treatment temperature during the secondary heat treatment does not meet the above range, the second phase formed on the surface of the lithium transition metal oxide as described above not only exists in the region within 30 nm from the surface of the lithium transition metal oxide toward the center, but the second phase can also exist in the entire positive electrode active material, or the second phase having a layered structure and having a structure different from the layered structure can exist in a mixed state in the entire positive electrode active material particles.

[0076] (positive electrode)

[0077] In addition, a positive electrode for a lithium secondary battery is provided, the positive electrode comprising the positive electrode active material according to the present invention. Specifically, a positive electrode for a lithium secondary battery is provided, the positive electrode comprising 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 comprises the positive electrode active material according to the present invention.

[0078] In this case, since the positive electrode active material is the same as described above, its detailed description will be omitted, and only the remaining structure will be described in detail below.

[0079] The positive electrode current collector may contain a highly conductive metal. There are no particular limitations on the positive electrode current collector, as long as it is easily bonded to the positive electrode active material layer and is non-reactive within the battery's voltage range. For example, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, or silver can be used as the positive electrode current collector. Furthermore, the positive electrode current collector typically has a thickness of 3 μm to 500 μm, and fine irregularities can be formed on its surface to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various shapes such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0080] In addition to the aforementioned positive electrode active material, the positive electrode active material layer may optionally contain conductive agents, binders, and dispersants, if desired.

[0081] In this case, based on the total weight of the positive electrode active material layer, the positive electrode active material can be included in an amount of 80% to 99% by weight, such as 85% to 98.5% by weight. When the content of the positive electrode active material is within the above range, excellent capacity characteristics can be obtained.

[0082] 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 electronic conductivity and does not cause adverse chemical changes in the battery. Specific examples of conductive agents may be: graphite such as natural or artificial graphite; carbon materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, and carbon fibers; 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 or a mixture of two or more thereof may be used. The conductive agent is typically included in an amount of 0.1% to 15% by weight, based on the total weight of the positive electrode active material layer.

[0083] The adhesive improves the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the current collector. Specific examples of the adhesive may 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, polyacrylic acid and polymers in which hydrogen is replaced by lithium (Li), sodium (Na), or calcium (Ca), or various copolymers thereof, and any one or a mixture of two or more thereof may be used. Based on the total weight of the positive electrode active material layer, the adhesive may be included in an amount of 0.1% to 15% by weight.

[0084] The dispersant may comprise an aqueous dispersant or an organic dispersant such as N-methyl-2-pyrrolidone.

[0085] In addition to using the aforementioned positive electrode active materials, positive electrodes can be prepared according to typical methods for preparing positive electrodes. Specifically, a composition for forming the positive electrode active material layer is coated onto a positive electrode current collector, and then the positive electrode is prepared by drying and calendering the coated positive electrode current collector. The composition is prepared by dissolving or dispersing the positive electrode active material, along with a binder, conductive agent, and dispersant selected as needed, in a solvent.

[0086] The solvent can be any solvent commonly used in the art. The solvent may include dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), acetone, or water, and any one or a mixture of two or more thereof may be used. Considering the coating thickness and manufacturing yield of the slurry, the amount of solvent used may be sufficient if it can dissolve or disperse the positive electrode active material, conductive agent, binder, and dispersant, and can result in a viscosity that provides excellent thickness uniformity during subsequent coating for the preparation of the positive electrode.

[0087] Alternatively, as another method, the positive electrode can be prepared by casting the composition used to form the positive electrode active material layer onto a separate support, and then pressing the film layer separated from the support onto the positive electrode current collector.

[0088] (Secondary battery)

[0089] Furthermore, in this invention, an electrochemical device comprising the positive electrode can be prepared. Specifically, the electrochemical device can be a battery or a capacitor, such as a lithium secondary battery.

[0090] The lithium secondary battery specifically includes a positive electrode, a negative electrode arranged facing the positive electrode, a separator disposed between the positive and negative electrodes, and an electrolyte. Since the positive electrode is the same as described above, its detailed description will be omitted. Only the remaining structures will be described in detail below.

[0091] In addition, the lithium secondary battery may optionally include a battery container that houses an electrode assembly of a positive electrode, a negative electrode, and a separator, and a sealing member that seals the battery container.

[0092] In addition, the lithium secondary battery may also include a current interruption device that stops charging the battery by detecting volume changes in the battery.

[0093] A current interruption device (CID) detects pressure changes in the battery, and can be activated to stop charging the battery when the internal pressure of the battery rises above a predetermined pressure. The current interruption device is preferably connected to a sealed component and can operate to block current from the outside when the internal pressure of the battery rises.

[0094] In a lithium secondary battery, the negative electrode includes a negative electrode current collector and a layer of negative electrode active material disposed on the negative electrode current collector.

[0095] There are no particular limitations on the negative electrode current collector, as long as it has high conductivity and does not cause adverse chemical changes in the battery. It can be made of materials such as copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, or silver, and aluminum-cadmium alloys. Furthermore, the negative electrode current collector typically has a thickness of 3 μm to 500 μm, and similar to the positive electrode current collector, fine irregularities can be formed on its surface to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various shapes such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0096] In addition to the negative electrode active material, the negative electrode active material layer also selectively contains binders and conductive agents.

[0097] Compounds capable of reversibly inserting and de-intercalating lithium can be used as anode active materials. Specific examples of anode active materials include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; (semi-)metallic materials that can be alloyed 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; and metal oxides that can be doped and de-doped with lithium, such as SiO₂. β(0<β<2), SnO2, vanadium oxide and lithium vanadium oxide; or composites containing (semi-)metallic materials and carbonaceous materials such as Si-C composites or Sn-C composites, and any one or a mixture of two or more thereof can be used. Additionally, lithium metal films can be used as the negative electrode active material. Furthermore, both low-crystallinity carbon and high-crystallinity carbon can be used as carbon materials. Typical examples of low-crystallinity carbon can be soft carbon and hard carbon, and typical examples of high-crystallinity carbon can be amorphous, plate-like, sheet-like, spherical or fibrous natural or artificial graphite, condensed graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microspheres, mesophase pitch and high-temperature sintered carbon such as coke derived from petroleum or coal tar pitch.

[0098] Based on the total weight of the negative electrode active material layer, the negative electrode active material can be included in an amount of 80% to 99% by weight.

[0099] The adhesive is a component that facilitates the bonding between the conductive agent, the active material, and the current collector, and is typically added in an amount of 0.1% to 10% by weight based on the total weight of the negative electrode active material layer. Examples of adhesives include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile rubber, fluororubber, and various copolymers thereof.

[0100] The conductive agent is a component used to further improve the conductivity of the negative electrode active material, wherein the conductive agent can be added in an amount of less than 10% by weight, such as less than 5% by weight, based on the total weight of the negative electrode active material layer. There are no particular limitations on the conductive agent, as long as it is conductive and will not cause adverse chemical changes in the battery. For example, conductive materials such as: graphite (e.g., natural or artificial graphite); carbon black (e.g., acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked black); conductive fibers (e.g., carbon fiber or metal fiber); fluorocarbons; metal powders (e.g., aluminum and nickel powder); conductive whiskers (e.g., zinc oxide whiskers and potassium titanate whiskers); conductive metal oxides (e.g., titanium oxide); or polyphenylene derivatives.

[0101] For example, the negative electrode active material layer can be prepared by coating a composition for forming the negative electrode onto a negative electrode current collector and drying the coated negative electrode current collector, wherein the composition is prepared by dissolving or dispersing the negative electrode active material, along with a selective binder and conductive agent, in a solvent; or the negative electrode active material layer can be prepared by streaming the composition for forming the negative electrode onto a separate support and then pressing the film layer separated from the support onto the negative electrode current collector.

[0102] In lithium-ion secondary batteries, the separator separates the negative and positive electrodes and provides a path for lithium ions to move. Any separator can be used without particular limitation, as long as it is commonly used in lithium-ion secondary batteries. Specifically, separators with high electrolyte retention capacity and low resistance to electrolyte ion transfer can be used. Specifically, porous polymer membranes can be used, such as porous polymer membranes prepared from polyolefin polymers like ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers; or laminated structures having two or more layers. Furthermore, typical porous nonwoven fabrics can be used, such as nonwoven fabrics formed from high-melting-point glass fibers or polyethylene terephthalate fibers. Additionally, coated separators containing ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and separators with single-layer or multi-layer structures can be selectively used.

[0103] Furthermore, the electrolyte used in this invention may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, or molten inorganic electrolytes that can be used to prepare lithium secondary batteries, but this invention is not limited thereto.

[0104] Specifically, the electrolyte may contain an organic solvent and a lithium salt.

[0105] Any organic solvent can be used without particular limitation, as long as it serves as a medium through which ions participating in the battery electrochemical reaction can move. Specifically, the following substances can be used as the organic solvent: 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 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 isopropanol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic C2-C20 hydrocarbon group and may contain double bonds, aromatic rings, or ether bonds); amides such as dimethylformamide; dioxolane such as 1,3-dioxolane; or sulfolane. Among these solvents, carbonate solvents can be used. For example, a mixture of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant, which can improve the charge / discharge performance of the battery, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) can be used. In this case, the electrolyte performance may be excellent when the cyclic carbonate and linear carbonate are mixed in a volume ratio of about 1:1 to about 1:9.

[0106] Lithium salts can be used without particular restrictions, as long as they are compounds capable of providing lithium ions used in lithium secondary batteries. Specifically, the anion of the lithium salt may include at least one selected from: F - Cl - ,Br - I - NO3 - N(CN)2 - BF4 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - Furthermore, 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 as lithium salts. Lithium salts can be used in concentration ranges from 0.1 M to 2.0 M. When the lithium salt concentration is within the above range, excellent electrolyte performance can be obtained because the electrolyte can have suitable conductivity and viscosity, and lithium ions can move efficiently.

[0107] To improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity, in addition to the electrolyte components, at least one additive may be added to the electrolyte, such as alkyl halogenated carbonate compounds like ethylene difluorocarbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, (condensed) glycol dimethyl ethers, hexamethylphosphoric triamine, nitrobenzene derivatives, sulfur, quinone imine dyes, and N-substituted compounds. The additives may be acetonides, N,N-substituted imidazolidinyl ethers, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, or aluminum trichloride. In this case, the additives may be included in an amount of 0.1% to 5% by weight, based on the total weight of the electrolyte.

[0108] As described above, because lithium secondary batteries containing the positive electrode active material according to the present invention stably exhibit excellent discharge capacity, output characteristics and lifespan characteristics, the lithium secondary batteries are suitable for use in: portable devices such as mobile phones, laptop computers and digital cameras; and electric vehicles such as hybrid electric vehicles (HEVs).

[0109] Therefore, according to another embodiment of the present invention, a battery module comprising the lithium secondary battery as a unit cell and a battery pack comprising the battery module are provided.

[0110] The battery module or the battery pack can be used as a power source for at least one medium to large-sized device: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles and plug-in hybrid electric vehicles (PHEVs); or energy storage systems.

[0111] The shape of the lithium secondary battery of the present invention is not particularly limited, but it can be cylindrical, prismatic, pouch or coin-shaped.

[0112] The lithium secondary battery according to the present invention can be used not only as a battery cell for use as a power source for small devices, but also as a unit battery in medium and large battery modules containing multiple battery cells.

[0113] Examples of the medium and large-sized devices may be electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems, but the present invention is not limited thereto.

[0114] Preferred implementation scheme

[0115] The invention will now be described in detail with reference to specific embodiments. However, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these exemplary embodiments are provided so that the description will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0116] Example

[0117] Example 1

[0118] Ni 0.8 Co 0.1 Mn 0.1 (OH)₂ and LiOH were mixed in a molar ratio of 1:1.02 and subjected to a first heat treatment at 800°C for 14 hours in an oxygen atmosphere. Subsequently, a second heat treatment was performed at 700°C for 5 hours in a 100% oxygen atmosphere to prepare the positive electrode active material.

[0119] The above-prepared positive electrode active material, carbon black conductive agent, and polyvinylidene fluoride binder were mixed in N-methylpyrrolidone (NMP) solvent at a weight ratio of 95:3:2 to prepare a composition for forming the positive electrode. A 20 μm thick aluminum film was coated using the composition for forming the positive electrode, dried at 130°C for 2 hours, and then calendered to prepare the positive electrode.

[0120] Lithium metal foil is used as the negative electrode.

[0121] After stacking the prepared positive and negative electrodes together with a polyethylene separator (Tonen Chemical Corporation, F20BHE, thickness: 20 μm) to prepare a polymer battery by conventional methods, the polymer battery is placed in a battery case and an electrolyte is injected therein to prepare a coin cell type lithium secondary battery. The electrolyte is obtained by dissolving 1M LiPF6 in a mixed solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 1:2.

[0122] Example 2

[0123] Except for undergoing a secondary heat treatment at 700°C for 5 hours in an 80% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0124] Example 3

[0125] Except for undergoing a secondary heat treatment at 700°C for 5 hours in a 50% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0126] Example 4

[0127] Except for undergoing a secondary heat treatment at 750°C for 4 hours in a 100% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0128] Example 5

[0129] Except for undergoing a secondary heat treatment at 750°C for 5 hours in an 80% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0130] Example 6

[0131] Except for undergoing a secondary heat treatment at 750°C for 7 hours in a 50% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0132] Example 7

[0133] Except for undergoing a secondary heat treatment at 650°C for 7 hours in a 100% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0134] Example 8

[0135] Except for a secondary heat treatment at 650°C for 7 hours in an 80% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0136] Example 9

[0137] Except for undergoing a secondary heat treatment at 650°C for 5 hours in a 50% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0138] Comparative Example 1

[0139] In addition to mixing Ni at a molar ratio of 1:1.02 0.8 Co 0.1 Mn 0.1 (OH)2 and LiOH were subjected to a heat treatment at 800°C for 14 hours in an oxygen atmosphere to prepare a positive electrode active material, and a lithium secondary battery was prepared in the same manner as in Example 1, except for the positive electrode active material.

[0140] Comparative Example 2

[0141] Except for undergoing a secondary heat treatment at 600°C for 5 hours in a 100% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0142] Comparative Example 3

[0143] Except for undergoing a secondary heat treatment at 700°C for 5 hours in a 20% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0144] Comparative Example 4

[0145] Except for undergoing a secondary heat treatment at 700°C for 5 hours in a 40% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0146] Comparative Example 5

[0147] Except for undergoing a secondary heat treatment at 800°C for 5 hours in a 100% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0148] Comparative Example 6

[0149] Except for undergoing a secondary heat treatment at 800°C for 7 hours in an 80% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0150] Comparative Example 7

[0151] Except for the secondary heat treatment at 800°C for 7 hours in a 50% oxygen atmosphere during the secondary heat treatment, the positive electrode active material and the lithium secondary battery containing it were prepared in the same manner as in Example 1.

[0152] Experimental Example 1: Analysis of the surface phase of positive electrode active materials

[0153] The cross-sections of each positive electrode active material were cut into 50 nm thick sections, and the surface of the positive electrode active material was observed using a transmission electron microscope (TEM) (FE-STEM, TITAN G2 80-100 ChemiSTEM). The phase of the positive electrode active material was measured by small angle diffraction pattern (SADP).

[0154] The presence of a second phase in the region within 30 nm from the particle surface towards the center (surface portion) and in the inner portion (center portion) extending even beyond 30 nm from the particle surface were confirmed, and the results are shown in Table 1 below. The presence of a second phase in the surface portion (the region within 30 nm from the particle surface towards the center) is indicated by 'O', while the absence of a second phase is indicated by '×'. Furthermore, the presence of a second phase in the inner portion extending even beyond 30 nm from the particle surface is indicated by '○', and the absence of a second phase in the inner portion extending beyond 30 nm from the particle surface is indicated by '×'.

[0155] [Table 1]

[0156] Does a second phase exist in the surface portion? Does a second phase exist in the central part? Example 1 O × Example 2 O × Example 3 O × Example 4 O × Example 5 O × Example 6 O × Example 7 O × Example 8 O × Example 9 O × Comparative Example 1 × × Comparative Example 2 × × Comparative Example 3 O O Comparative Example 4 O O Comparative Example 5 O O Comparative Example 6 O O Comparative Example 7 O O

[0157] As shown in Table 1, for the positive electrode active material particles prepared in Examples 1 and 2, it can be confirmed that a second phase exists in the surface portion, which is a region located within 30 nm from the particle surface toward the center, but no second phase exists in the center portion, which is the inner side beyond 30 nm from the surface toward the center.

[0158] In contrast, in Comparative Example 1, which did not undergo secondary heat treatment, there was no second phase in either the surface or the central portion.

[0159] Furthermore, for the positive electrode active material particles prepared in Comparative Examples 3 to 7, a second phase exists within 30 nm from the particle surface toward the center, and a second phase also exists in regions exceeding 30 nm from the particle surface toward the center.

[0160] For the positive electrode active material particles prepared in Comparative Example 2, because the heat treatment temperature was low, there was no second phase in the particles.

[0161] Experimental Example 2: Evaluation of charging and discharging capacity and efficiency characteristics

[0162] The coin-type lithium secondary batteries prepared in Examples 1-9 and Comparative Examples 1-7 were charged to 4.25V at a constant current of 0.2C and discharged to 2.5V at a constant current of 0.2C at 25°C. The charge and discharge characteristics of the first cycle were observed, and the results are shown in Table 2 below.

[0163] [Table 2]

[0164] Charging capacity (mAh / g) Discharge capacity (mAh / g) Example 1 225 200 Example 2 225 199 Example 3 225 198 Example 4 226 202 Example 5 226 201 Example 6 226 200 Example 7 224 199 Example 8 224 198 Example 9 224 197 Comparative Example 1 225 203 Comparative Example 2 225 202 Comparative Example 3 225 194 Comparative Example 4 225 195 Comparative Example 5 224 190 Comparative Example 6 224 188 Comparative Example 7 224 186

[0165] As shown in Table 2, for the coin-shaped lithium secondary batteries prepared in Examples 1 to 7, it can be confirmed that they can achieve better charging and discharging efficiency compared with the lithium secondary batteries prepared in Comparative Examples 3 to 7.

[0166] Experiment Example 3: Hot Box Test

[0167] Hot box tests were conducted using coin-type lithium secondary batteries prepared in Examples 1-9 and Comparative Examples 1-7, respectively.

[0168] Specifically, the coin-shaped lithium secondary batteries prepared in Examples 1-9 and Comparative Examples 1-7 were placed in an oven, and the temperature was increased at a rate of 10°C / min and maintained at 150°C for 30 minutes. During the hot-oven test, it was confirmed whether the batteries exploded, and the results are shown in Table 3 below.

[0169] In this case, O represents the case where the secondary battery did not explode, and × represents the case where it exploded.

[0170] Experiment Example 4: Overcharge Test

[0171] Cylindrical batteries were prepared using the positive electrode active materials prepared in Examples 1-9 and Comparative Examples 1-7, respectively, and then overcharge tests were performed.

[0172] Specifically, after activation, each cylindrical battery was charged to 4.25V at a constant current of 0.2C and charging was stopped at 0.01C. Then, each cylindrical battery was discharged to 2.5V at a constant current of 0.2C. Subsequently, each cylindrical battery was charged at a constant current of 0.5C until the current interruption device (CID) of the cylindrical battery was activated, and the temperature of the battery at this point was measured.

[0173] The overcharge test results are shown in Table 3 below. A battery temperature exceeding 150°C after the current interruption device (CID) is activated is considered an overcharge test failure and is indicated by ×. A battery temperature below 150°C after the current interruption device (CID) is activated is considered a stable overcharge test result and is indicated by O.

[0174] [Table 3]

[0175] Has it passed the hot box test? Has it passed the overcharge test? Example 1 O O Example 2 O O Example 3 O O Example 4 O O Example 5 O O Example 6 O O Example 7 O O Example 8 O O Example 9 O O Comparative Example 1 × × Comparative Example 2 × × Comparative Example 3 O O Comparative Example 4 O O Comparative Example 5 O O Comparative Example 6 O O Comparative Example 7 O O

[0176] Referring to Table 3, it was confirmed that the lithium secondary batteries prepared in Examples 1-9 and Comparative Examples 3-7 all passed the hot box test and overcharge test.

[0177] In contrast, it can be confirmed that Comparative Examples 1 and 2 failed the hot box test and overcharge test.

[0178] Therefore, compared with the lithium secondary batteries of Examples 1 to 9, the positive electrode active materials prepared in Comparative Examples 1 and 2 and the lithium secondary batteries containing the positive electrode active materials have lower stability. Therefore, it is predicted that even if the charging and discharging efficiency is excellent, the battery explosion problem will still exist when the positive electrode active materials are used in secondary batteries due to stability issues.

[0179] Experimental Example 5: Lifetime Characteristics Evaluation

[0180] The lifetime characteristics of the coin-type lithium secondary batteries prepared in Examples 1-9 and Comparative Examples 1-7 were measured.

[0181] Specifically, the coin-shaped batteries prepared in Examples 1-9 and Comparative Examples 1-7 were charged at 45°C with a constant current of 0.2C to 4.25V, and charging was stopped at 0.01C. Subsequently, they were initially discharged at a constant current of 0.2C to a voltage of 2.5V. Then, each coin-shaped battery was charged at a constant current of 0.5C to 4.25V, and charging was stopped at 0.01C, followed by discharging at a constant current of 0.5C to a voltage of 2.5V. This charging and discharging behavior was set as one cycle, and after repeating this cycle 50 times, the lifespan characteristics of the lithium secondary batteries according to Examples 1-9 and Comparative Examples 1-7 were measured. The results are shown in Table 4 below.

[0182] [Table 4]

[0183] Capacity retention rate (%) Example 1 96 Example 2 95 Example 3 96 Example 4 95 Example 5 96 Example 6 96 Example 7 96 Example 8 95 Example 9 96 Comparative Example 1 85 Comparative Example 2 86 Comparative Example 3 85 Comparative Example 4 84 Comparative Example 5 86 Comparative Example 6 85 Comparative Example 7 84

[0184] As shown in Table 4, it can be confirmed that the lithium secondary batteries of Examples 1 to 9, in which the second phase exists only in the surface portion of the positive electrode active material particles, have better lifespan characteristics compared to the lithium secondary batteries of Comparative Examples 1 and 2, in which the second phase does not exist, and the lithium secondary batteries of Comparative Examples 3 to 7, in which the second phase exists not only in the surface portion of the positive electrode active material particles but also in the inner portion extending more than 30 nm from the surface.

Claims

1. A positive electrode active material, the positive electrode active material comprising a lithium transition metal oxide represented by Formula 1, wherein the lithium transition metal oxide comprises a central portion having a layered structure and a surface portion having a second phase with a structure different from that of the central portion, wherein the central portion and the surface portion are made of the same material, wherein the surface portion comprises a rock salt structure, wherein the positive electrode active material is prepared by a method comprising the following steps: mixing a positive electrode active material precursor with a lithium raw material and performing a first heat treatment; and performing a second heat treatment at a temperature lower than that of the first heat treatment to prepare the positive electrode active material, wherein the first heat treatment and the second heat treatment are respectively performed in an oxygen atmosphere, and the second heat treatment is performed in an oxygen atmosphere with an oxygen concentration of 50% or more, [Formula 1] Li 1+a (Ni x Co y M 1 z M 2 w ) 1-a O2 wherein in Formula 1, 0≤a≤0.2, 0.6<x≤1, 0<y≤0.4, 0<z≤0.4 and 0≤w≤0.1, M 1 It is selected from at least one of the following: Mn and Al, and M 2 It is selected from at least one of the following: Zr, B, W, Mo, Cr, Ta, Nb, Mg, Ce, Hf, La, Ti, Sr, Ba, F, P, S and Y.

2. The positive electrode active material according to claim 1, wherein the surface portion is a region within 30 nm from the surface of the particle toward the center of the particle.

3. A method for preparing the positive electrode active material according to claim 1, the method comprising the following steps: mixing a positive electrode active material precursor with a lithium raw material and performing a first heat treatment; and performing a second heat treatment at a temperature lower than that of the first heat treatment to prepare the positive electrode active material, wherein the first heat treatment and the second heat treatment are respectively performed in an oxygen atmosphere, and the second heat treatment is performed in an oxygen atmosphere with an oxygen concentration of 50% or more.

4. The method according to claim 3, wherein the first heat treatment is performed at a temperature of 800°C or higher.

5. The method according to claim 3, wherein the first heat treatment is performed in an oxygen atmosphere with an oxygen concentration of 50% or more.

6. The method according to claim 3, wherein the first heat treatment is performed for 10 hours to 20 hours.

7. The method according to claim 3, wherein the second heat treatment is performed at a temperature higher than 600°C and lower than 800°C.

8. The method according to claim 3, wherein the second heat treatment is performed for 2 hours to 12 hours.

9. A positive electrode for a secondary battery, the positive electrode comprising: 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 comprises the positive electrode active material according to any one of claims 1 to 2.

10. A lithium secondary battery, the lithium secondary battery comprising: the positive electrode according to claim 9; a negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte.

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