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

Abstract

The present invention relates to a positive electrode active material including: a lithium composite transition metal oxide in a single particle or quasi-single particle form, the single particle being composed of one nucleus, the quasi-single particle being a composite of 30 or less nuclei; and a coating layer including a first coating layer formed on a surface of the lithium composite transition metal oxide and a second coating layer formed on a surface of the first coating layer, wherein a circularity of the lithium composite transition metal oxide defined by the following Equation 1 is 0.50 to 0.68, and the second coating layer contains boron at a content of 300 ppm to 2400 ppm based on a total weight of the positive electrode active material.[Equation 1] Circularity = (4 x Area) / (pi x R 2 ). In the above Equation 1, R represents a length of a major axis passing through the center of the lithium composite transition metal oxide, and Area represents an actual area of the lithium composite transition metal oxide.

Description

[0001] Cross-references to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2023-0188929, filed with the Korean Intellectual Property Office on December 21, 2023, and Korean Patent Application No. 10-2024-0191934, filed with the Korean Intellectual Property Office on December 19, 2024, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to a positive electrode active material, a positive electrode comprising the positive electrode active material and a lithium secondary battery, and more specifically, to a positive electrode active material that achieves kinetic balance with a silicon-based negative electrode and improves side reactions with an electrolyte solution, and a positive electrode comprising the positive electrode active material and a lithium secondary battery. Background Technology

[0004] Lithium-ion batteries typically consist of a positive electrode, a negative electrode, a separator, and an electrolyte. The positive and negative electrodes include active materials capable of inserting and deintercalating lithium ions.

[0005] Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMnO2, LiMnO4, etc.), and lithium iron phosphate compounds (LiFePO4) have been used as positive electrode active materials for lithium-ion batteries. Among these, lithium cobalt oxide offers advantages such as high driving voltage and excellent capacity characteristics. However, due to the high price and unstable supply of cobalt as a raw material, its commercial application in high-capacity batteries is difficult. Lithium nickel oxide suffers from poor structural stability, making it difficult to achieve sufficient lifetime characteristics. Meanwhile, lithium manganese oxide exhibits excellent stability but suffers from poor capacity characteristics. Therefore, to compensate for the limitations of lithium transition metal oxides containing only Ni, Co, or Mn, lithium composite transition metal oxides comprising two or more transition metals have been developed, with lithium nickel cobalt manganese oxide (LiNi, Co, and Mn) being widely used.

[0006] Conventional lithium nickel cobalt manganese oxides are typically in the form of spherical secondary particles aggregated from tens to hundreds of primary particles. However, in the case of lithium nickel cobalt manganese oxides in the form of secondary particles with a large number of primary particles aggregated, particle breakage is prone to occur. This includes the primary particles breaking during the rolling process in the manufacturing of the positive electrode, and the problem of internal cracking during charging and discharging. When the particles of the positive electrode active material break or crack, the contact area with the electrolyte solution increases, leading to increased gas generation and active material degradation due to side reactions with the electrolyte solution, thus resulting in a decline in lifetime characteristics.

[0007] In addition, in order to achieve high capacity of lithium secondary batteries, it is necessary to apply silicon-based anode active materials to the anode. However, if silicon-based anode active materials are used, there is a problem of anode deterioration due to the imbalance of kinetic balance between the positive and negative electrodes.

[0008] Meanwhile, to improve the lifetime characteristics of lithium nickel cobalt manganese oxide, a technique has been proposed to suppress contact with the electrolyte solution by forming a coating on the surface of the lithium nickel cobalt manganese oxide. However, the problem with the above method is that the effect of improving lifetime characteristics is insufficient, and the initial resistance characteristics decrease with the increase of coating thickness. Summary of the Invention

[0009] Technical issues

[0010] To address the aforementioned problems, the present invention provides a positive electrode active material, as well as a positive electrode and a lithium secondary battery to which the positive electrode active material is applied, wherein the positive electrode active material exhibits excellent lifetime characteristics by adjusting the shape of lithium composite transition metal oxide particles in the form of single or quasi-single particles and additionally forming a boron coating.

[0011] Technical solution

[0012] [1] The present invention provides a positive electrode active material comprising: a lithium composite transition metal oxide in the form of a single particle or a quasi-single particle, wherein the single particle is composed of a single nodule, and the quasi-single particle is a composite of 30 or fewer nodules; and a coating comprising a first coating formed on the surface of the lithium composite transition metal oxide and a second coating formed on the surface of the first coating, wherein the roundness of the lithium composite transition metal oxide as defined by Equation 1 is 0.50 to 0.68, and the second coating contains boron, wherein the boron content is 300 ppm to 2400 ppm based on the total weight of the positive electrode active material.

[0013] [Equation 1]

[0014] Roundness = (4 × Area) / (π × R) 2 )

[0015] In Equation 1 above, R represents the length of the major axis passing through the center of the lithium complex transition metal oxide, and Area represents the actual area of ​​the lithium complex transition metal oxide.

[0016] [2] In [1] above, the lithium complex transition metal oxide may have a composition represented by the following chemical formula 1.

[0017] [Chemical Formula 1]

[0018] Li 1+x Ni a Co bMn c Al d M 1 e O2

[0019] In the above chemical formula 1, M 1 is one or more selected from the group consisting of W, Zr, Y, Ba, Ca, Ti, Mg, Ta, and Nb, and 0 ≤ x ≤ 0.5, 0.8 ≤ a < 1.0, 0 < b ≤ 0.1, 0 < c < 0.2, 0 < d ≤ 0.05, and 0 ≤ e ≤ 0.05.

[0020] [3] In the above [1] or [2], the nickel content in all metals other than lithium in the lithium composite transition metal compound can be 80 mol% or more.

[0021] [4] In at least one of the above [1] to [3], the average particle size (D 50 ) of the lithium composite transition metal oxide can be 1 μm to 8 μm.

[0022] [5] In at least one of the above [1] to [4], the roundness of the lithium composite transition metal oxide can be 0.58 to 0.65.

[0023] [6] In at least one of the above [1] to [5], the first coating can include one or more selected from the group consisting of Ni, Co, and Al.

[0024] [7] In at least one of the above [1] to [6], based on the total weight of the positive electrode active material, the boron content of the second coating can be 400 ppm to 1400 ppm.

[0025] [8] In at least one of the above [1] to [7], the BET specific surface area of the positive electrode active material can be 0.4 m 2 / g to 1.0 m 2 / g.

[0026] [9] The present invention provides a positive electrode comprising the positive electrode active material described in at least one of the above [1] to [8].

[0027]

[10] The present invention provides a lithium secondary battery comprising the positive electrode described in the above [9], a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0028]

[11] In the above

[10] , the negative electrode can include a negative electrode active material layer comprising a silicon-based negative electrode active material.

[0029]

[12] In the above

[11] , the negative electrode can further include a carbon-based negative electrode active material.

[0030] Beneficial effects

[0031] The positive electrode active material of the present invention can achieve excellent lifetime and achieve kinetic balance with silicon-based negative electrode by adjusting the shape of lithium composite transition metal oxide in single-particle form.

[0032] Furthermore, the positive electrode active material of the present invention is prepared by preparing a lithium composite transition metal oxide in the form of a single particle consisting of a single nodule or a quasi-single particle as a composite of 30 or fewer nodules, followed by high-temperature firing and subsequent formation of a boron coating. If the high-temperature firing and boron coating are performed as in the present invention, the high-temperature firing improves the surface structure of the lithium composite transition metal oxide, thereby suppressing the increase in resistance after coating formation and improving side reactions with the electrolyte solution on the positive electrode surface, thus reducing gas generation. Attached Figure Description

[0033] Figure 1 These are photographs showing the results of scanning electron microscopy-image analysis (SEM-IAM) of the surface and interior of the positive electrode active material prepared in Example 1.

[0034] Figure 2 These are photographs showing the results of scanning electron microscopy-image analysis (SEM-IAM) of the surface and interior of the positive electrode active material prepared in Comparative Example 1. Detailed Implementation

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

[0036] It should be understood that the terms or words used in this specification and claims should not be construed as having the meanings defined in common dictionaries, but should be interpreted as having meanings and concepts consistent with the technical ideas of the invention, based on the inventor's ability to appropriately define the concepts of the terms to best explain the principles of the invention.

[0037] In this invention, "single particle" refers to a particle consisting of a single nodule.

[0038] "Nodule" refers to the sub-particle unit that makes up a single particle or quasi-single particle, and when observed with a scanning electron microscope at a field of view of 5,000 to 20,000 times, it can be a single crystal without grain boundaries or a polycrystalline material without grain boundaries in appearance.

[0039] "Quasi-single particle" refers to a complex formed by the aggregation of no more than 30 nodules, preferably 2 to 30 nodules.

[0040] In this invention, "roundness" is the ratio of the actual area of ​​the lithium complex transition metal oxide to the area of ​​a circle having a diameter with the major axis passing through the center of the lithium complex transition metal oxide.

[0041] "Roundness" can be obtained by extracting an image of the powder to be measured (e.g., lithium complex transition metal oxide) using a scanning electron microscope (SEM), and then analyzing and identifying the particles using image analysis management (IAM) to measure the roundness of the corresponding lithium complex transition metal oxide.

[0042] In this invention, "average particle size D" 50 "" refers to the particle size at the 50% volume cumulative particle size distribution of the powder being measured (e.g., lithium complex transition metal oxide). Average particle size D 50 The particle size can be measured using laser diffraction. For example, the average particle size (D) can be measured by dispersing the powder to be measured in a dispersion medium and then introducing the mixture into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000) and irradiating it with ultrasound at an output of approximately 28 kHz at 60 W. 50 This allows us to obtain a volumetric cumulative particle size distribution map, and then obtain the particle size corresponding to 50% of the cumulative volume.

[0043] In this invention, the "specific surface area" is measured by the BET method, and specifically, it can be calculated from the amount of nitrogen adsorbed at liquid nitrogen temperature (77K) using the Belserp-mino II of BEL Japan Co., Ltd.

[0044] Positive electrode active material

[0045] The positive electrode active material of the present invention will be described below.

[0046] The positive electrode active material of the present invention comprises: (1) a lithium composite transition metal oxide in the form of single particles or quasi-single particles having a specific roundness value, and (2) a boron coating comprising a first coating formed on the surface of the lithium composite transition metal oxide and a second coating formed on the surface of the first coating, wherein the boron content is from 300 ppm to 2400 ppm based on the total weight of the positive electrode active material.

[0047] (1) Lithium-based transition metal oxides

[0048] Lithium complex transition metal oxides are single particles consisting of a single nodule, or quasi-single particles as complexes of 30 or fewer nodules, preferably 2 to 30, more preferably 2 to 20.

[0049] As described above, lithium composite transition metal oxides in single-particle or quasi-single-particle form have higher particle strength than typical lithium composite transition metal oxide particles in secondary-particle form, which consists of tens to hundreds of primary particles aggregated together, and therefore exhibit less particle breakage during rolling.

[0050] Furthermore, the lithium composite transition metal oxide of the present invention in the form of single or quasi-single particles has a small number of nodules constituting the particles, so that the changes caused by the volume expansion and contraction of the nodules during charging and discharging are small, and thus the generation of intraparticle cracks is significantly reduced.

[0051] Lithium complex transition metal oxides can be, for example, lithium nickel oxides, particularly lithium nickel oxides comprising nickel, cobalt, and manganese, wherein the molar percentage of nickel in the metals other than lithium is greater than 50 mol%.

[0052] Preferably, the lithium complex transition metal oxide may have a composition represented by the following [Chemical Formula 1].

[0053] [Chemical Formula 1]

[0054] Li 1+x Ni a Co b Mn c Al d M 1 e O2

[0055] In Equation 1 above, M 1 It is selected from one or more elements in the group consisting of W, Zr, Y, Ba, Ca, Ti, Mg, Ta, and Nb. It does not necessarily include M. 1 Elements, but when M is included in appropriate amounts 1 When elements are present, M 1 Elements can be used to improve the stability of the crystal structure during firing.

[0056] 1+x represents the molar ratio of lithium in the lithium composite transition metal oxide, where x can be 0≤x≤0.5, preferably 0.05≤x≤0.45, and more preferably 0.1≤x≤0.4. If the molar ratio of lithium satisfies the above range, a stable crystal structure of lithium composite transition metal oxide particles can be formed.

[0057] 'a' represents the molar ratio of nickel to all metals other than lithium in a lithium-ion complex transition metal oxide, and 'a' can be 0.8 ≤ a < 1.0, preferably 0.85 ≤ a < 1.0, and more preferably 0.9 ≤ a < 1.0. If the molar ratio of nickel meets the above range, high energy density can be exhibited, enabling high capacity to be achieved.

[0058] b represents the molar ratio of cobalt among all metals other than lithium in the lithium composite transition metal oxide, and b can be 0 < b ≤ 0.1, preferably 0 < b ≤ 0.09, more preferably 0.01 ≤ b ≤ 0.08. If the molar ratio of cobalt satisfies the above range, good resistance characteristics and output characteristics can be achieved.

[0059] c represents the molar ratio of manganese among all metals other than lithium in the lithium composite transition metal oxide, and c can be 0 < c < 0.2, preferably 0 < c ≤ 0.18, more preferably 0.02 ≤ c ≤ 0.17. If the molar ratio of manganese satisfies the above range, the positive electrode active material can exhibit excellent structural stability.

[0060] d represents the molar ratio of aluminum among all metals other than lithium in the lithium composite transition metal oxide, and d can be 0 < d ≤ 0.05, preferably 0 < d ≤ 0.04, more preferably 0 < d ≤ 0.03. If the molar ratio of aluminum satisfies the above range, stability and excellent output characteristics can be achieved.

[0061] e represents the molar ratio of element M among all metals other than lithium in the lithium composite transition metal oxide 1 and e can be 0 ≤ e ≤ 0.05, preferably 0 ≤ e ≤ 0.04, more preferably 0 ≤ e ≤ 0.03. If the molar ratio of element M 1 satisfies the above range, the positive electrode active material can exhibit excellent structural stability.

[0062] The lithium composite transition metal oxide can have the following composition: the content of nickel among all metals other than lithium is 80 mol% or more, preferably 82 mol% or more, and more preferably 85 mol% or more. If the content of nickel in the lithium composite transition metal oxide satisfies the above range, a high energy density can be achieved.

[0063] The average particle size (D 50 ) of the lithium composite transition metal oxide can be 1 μm to 8 μm, preferably 2 μm to 6 μm, more preferably 3 μm to 4 μm. If the average particle size (D 50 ) is too small, the processability during electrode manufacturing is reduced, and the electrolyte solution wettability is reduced, which may increase the electrochemical properties, and if the average particle size (D 50 ) is too large, there are problems of increased resistance and reduced output characteristics.

[0064] The lithium composite transition metal oxide can have a sphericity of 0.50 to 0.68, preferably 0.55 to 0.66, and more preferably 0.58 to 0.65. If the sphericity meets the above range, lithium ions can diffuse more smoothly into the particles, and the contact area between the positive electrode active material and the electrolyte increases, thereby improving battery chemistry. Furthermore, since the distance between particles in the electrode remains relatively constant, current flow in the electrode is promoted, and a uniform particle arrangement is achieved, thereby increasing mechanical stability and improving battery cycle life.

[0065] Having the above average particle size (D) 50 The lithium composite transition metal oxide with the above-mentioned roundness can be prepared by the following method.

[0066] The method for preparing positive electrode active materials according to the present invention includes the following steps: 1) mixing a transition metal precursor and a first lithium raw material, and then subjecting the mixture to a first firing to form a first firing product comprising a single-particle lithium composite transition metal oxide, wherein the single-particle lithium composite transition metal oxide comprises a single particle consisting of 1 to 30 nodules; and 2) subjecting the first firing product to a second firing to form a second firing product.

[0067] 1) Steps for forming a product after one firing

[0068] First, the transition metal precursor and the first lithium raw material are mixed and then calcined once to form a first-calcined product.

[0069] In this case, the transition metal precursor can be a commercially available precursor purchased for use, such as nickel-cobalt-manganese-based hydroxides, or it can be prepared according to methods for preparing precursors known in the art.

[0070] Preferably, the transition metal precursor can be a transition metal hydroxide represented by the following chemical formula 2.

[0071] [Chemical Formula 2]

[0072] Li 1+x1 Ni a1 Co b1 Mn c1 Al d1 M 2 e1 O2

[0073] In Equation 2 above, M 2 Same as defined in Chemical Formula 1. That is, M 2 It is selected from one or more of the group consisting of W, Zr, Y, Ba, Ca, Ti, Mg, Ta and Nb.

[0074] 1 + x1 represents the molar ratio of lithium in the lithium composite transition metal oxide, where x can be 0 ≤ x1 ≤ 0.5, preferably 0.05 ≤ x1 ≤ 0.45, more preferably 0.1 ≤ x1 ≤ 0.4. If the molar ratio of lithium satisfies the above range, the crystal structure of the lithium composite transition metal oxide can be formed stably.

[0075] a1 represents the molar ratio of nickel among all the metals except lithium in the lithium composite transition metal oxide, and a1 can be 0.8 ≤ a1 < 1.0, preferably 0.85 ≤ a1 < 1.0, more preferably 0.9 ≤ a1 < 1.0. If the molar ratio of nickel satisfies the above range, high energy density can be exhibited, and thus high capacity can be achieved.

[0076] b1 represents the molar ratio of cobalt among all the metals except lithium in the lithium composite transition metal oxide, and b1 can be 0 < b1 ≤ 0.1, preferably 0 < b1 ≤ 0.09, more preferably 0.01 ≤ b1 ≤ 0.08. If the molar ratio of cobalt satisfies the above range, good resistance characteristics and output characteristics can be achieved.

[0077] c1 represents the molar ratio of manganese among all the metals except lithium in the lithium composite transition metal oxide, and c1 can be 0 < c1 < 0.2, preferably 0 < c1 ≤ 0.18, more preferably 0.02 ≤ c1 ≤ 0.17. If the molar ratio of manganese satisfies the above range, the positive electrode active material can exhibit excellent structural stability.

[0078] d1 represents the molar ratio of aluminum among all the metals except lithium in the lithium composite transition metal oxide, and d1 can be 0 < d1 ≤ 0.05, preferably 0 < d1 ≤ 0.04, more preferably 0 < d1 ≤ 0.03. If the molar ratio of aluminum satisfies the above range, stability and excellent output characteristics can be achieved.

[0079] e1 represents the molar ratio of element M among all the metals except lithium in the lithium composite transition metal oxide 2 and e1 can be 0 ≤ e1 ≤ 0.05, preferably 0 ≤ e1 ≤ 0.04, more preferably 0 ≤ e1 ≤ 0.03. If the molar ratio of element M 2 satisfies the above range, the positive electrode active material can exhibit excellent structural stability.

[0080] The transition metal precursor can be prepared, for example, by carrying out a coprecipitation reaction while introducing an aqueous solution of transition metals, an ammonium cation complexing agent, and an alkaline compound into a reactor, and then stirring the mixture.

[0081] Transition metal aqueous solutions can be prepared by dissolving transition metal-containing raw materials in a solvent such as water, and for example, by dissolving nickel-containing, cobalt-containing, and aluminum-containing raw materials in water. Furthermore, if desired, the transition metal aqueous solution may further include M-containing raw materials. 2 Raw materials for elements.

[0082] Meanwhile, raw materials containing transition metals can be acetates, carbonates, nitrates, sulfates, halides, sulfides, or oxides of transition metals.

[0083] Contains M 2 The raw material for the element can be M 2 Acetates, carbonates, nitrates, sulfates, halides, sulfides, or oxides of elements.

[0084] The input amount of each raw material containing a transition metal can be determined by taking into account the molar ratio of the transition metal in the final positive electrode active material.

[0085] Simultaneously, the ammonium cation complex forming agent may include at least one compound selected from the group consisting of NH4OH, (NH4)2SO4, NH4NO3, NH4Cl, CH3COONH4, and NH4CO3, and said compound may be introduced into the reactor in the form of a solution of said compound dissolved in a solvent. In this case, water, or a mixture of water and an organic solvent (specifically, alcohols, etc.) that can be uniformly mixed with water, may be used as the solvent.

[0086] The basic compound can be at least one compound selected from the group consisting of NaOH, KOH, and Ca(OH)2, and the compound can be introduced into the reactor in the form of a solution in which the compound is dissolved in a solvent. In this case, water, or a mixture of water and an organic solvent (specifically, alcohols, etc.) that can be homogeneously mixed with water can be used as the solvent.

[0087] If an aqueous solution of a transition metal, an ammonium cation complexing agent, and an alkaline compound are introduced into a reactor and stirred as described above, the transition metal in the aqueous solution will co-precipitate to produce precursor particles in the form of transition metal hydroxides.

[0088] In this case, an aqueous solution of transition metal, an ammonium cation complexing agent, and an alkaline compound are introduced in an amount that brings the pH of the reaction solution within the desired range.

[0089] When precursor particles are formed using the method described above, the particles are separated from the reaction solution to obtain the transition metal precursor. For example, the precursor particles can be separated from the reaction solution by filtering, and then the separated precursor particles can be washed with water and dried to obtain the transition metal precursor.

[0090] Next, the transition metal precursor and the first lithium feedstock are mixed and then subjected to a single calcination to form a single-calcination product comprising a single-particle lithium composite transition metal oxide containing 1 to 30 primary particles. In this case, if desired, the M-containing precursor can be added. 2 The raw materials of the elements are mixed together and then fired.

[0091] As the first lithium raw material, lithium-containing sulfates, nitrates, acetates, carbonates, oxalates, citrates, halides, hydroxides, hydroxyoxides, etc., can be used, and for example, Li2CO3, LiNO3, LiNO2, LiOH, LiOH H2O, LiH, LiF, LiCl, LiBr, LiI, CH3COOL i, Li2O, Li2SO4, Li3C6H5O7 or mixtures thereof can be used.

[0092] Meanwhile, the firing process is carried out under the condition that the particles grow to more than 1 μm in one step, thereby forming a single-particle lithium composite transition metal oxide.

[0093] The appropriate primary firing temperature can vary depending on the metal composition of the precursor, and for example, if the nickel content is 80 mol% or more, the primary firing can be carried out at a temperature of 750°C to 1000°C, preferably 800°C to 950°C, and more preferably 825°C to 950°C.

[0094] In addition, the firing time can be 6 to 30 hours, preferably 8 to 25 hours, and more preferably 8 to 20 hours.

[0095] Furthermore, firing can be carried out in an oxygen atmosphere. In this specification, an oxygen atmosphere refers to an atmosphere containing a sufficient amount of oxygen for firing, including atmospheric atmosphere. In particular, firing is preferably carried out in an atmosphere where the oxygen partial pressure is higher than that of atmospheric atmosphere.

[0096] If a single firing is performed under the above conditions, a single-particle lithium composite transition metal oxide with excellent electrochemical properties can be formed. If the temperature and time of the single firing are too low, the primary particles will not grow sufficiently, resulting in a secondary particle form of the positive electrode active material. If the temperature and time of the single firing are too high, a large amount of electrically inert rock salt phase will be formed during the firing process, resulting in a fired product with unstable structure and low crystallinity, which may reduce the electrochemical properties.

[0097] 2) Second firing step

[0098] The secondary firing can be carried out at a temperature 50°C to 200°C lower than the primary firing temperature. If the secondary firing temperature is higher than the primary firing temperature, an excessive rock salt phase will form on the surface of the lithium composite transition metal oxide, resulting in surface degradation and residual lithium byproducts. This may degrade the electrochemical properties. Furthermore, if the secondary firing is carried out at a temperature more than 200°C lower than the primary firing temperature, the lithium intercalation rate will decrease, leading to electrochemical performance degradation, and the effect of improving the sphericity of the primary particles will be insignificant. For example, the secondary firing temperature can be 700°C to 900°C, preferably 700°C to 875°C, and more preferably 680°C to 850°C.

[0099] In order to increase the crystallinity of the internal crystal structure of the positive electrode active material, the secondary firing time can be, for example, 6 hours to 15 hours, preferably 8 hours to 15 hours, and more preferably 8 hours to 14 hours.

[0100] Furthermore, the secondary firing can be carried out in an oxygen atmosphere.

[0101] If the secondary firing is carried out under the above conditions, a positive electrode active material with excellent electrochemical properties and the desired primary particle surface shape can be easily formed.

[0102] (2) Coating

[0103] The positive electrode active material of the present invention includes a first coating formed on the surface of a lithium composite transition metal oxide and a second coating formed on the surface of the first coating.

[0104] The first coating may include one or more elements selected from the group consisting of Ni, Co, and Al, preferably one or more of Co and Al, and more preferably Co and Al. By including the above elements, the structural stability and particle strength of the active material can be improved, and the effects of improved high-temperature lifetime characteristics and resistivity increase rate can be obtained. Preferably, the first coating may include Al2O3 and / or Co3O4 components.

[0105] Next, the second coating contains boron. If the second coating contains boron, it promotes the movement of lithium on the surface of the lithium complex transition metal oxide, thereby achieving an improved initial capacity compared to the case where no coating is formed.

[0106] In this case, the boron content can be from 300 ppm to 2400 ppm, preferably from 350 ppm to 2000 ppm, and more preferably from 400 ppm to 1400 ppm, based on the total weight of the positive electrode active material. If the boron content is less than 300 ppm based on the total weight of the positive electrode active material, the amount of gas generated increases, thus leading to a deterioration in lifetime characteristics. If it is greater than 2400 ppm, there are problems with increased initial resistance and decreased capacity retention.

[0107] Based on the total weight of the positive electrode active material, the boron content can be from 300 ppm to 2400 ppm, preferably from 400 ppm to 1400 ppm, and more preferably from 1000 ppm to 1400 ppm. If the above range is met, boron reacts with oxygen to form surface-protective oxides such as B2O3, thereby suppressing side reactions with the electrolyte, reducing gas generation, and thus improving high-temperature life characteristics.

[0108] The BET specific surface area of ​​the positive electrode active material can be 0.4 m². 2 / g to 1.0 m 2 / g, preferably 0.45 m 2 / g to 0.95m 2 / g, and more preferably 0.45 m 2 / g to 0.9 m 2 / g. If the BET specific surface area meets the above range, appropriate electrochemical properties can be ensured. If the specific surface area is too small, the reaction sites decrease, which can degrade the electrochemical properties, and if the specific surface area is too large, the reaction area with the electrolyte increases, which can further activate side reactions at high voltages.

[0109] positive electrode

[0110] Next, the positive electrode of the present invention will be described.

[0111] The positive electrode of the present invention includes a positive electrode active material layer containing a positive electrode active material. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material.

[0112] In the positive electrode, there are no particular restrictions on the positive electrode current collector, as long as it is conductive and does not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., can be used. Furthermore, the positive electrode current collector typically has a thickness from 3 μm to 500 μm, and microscopic 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 forms, such as films, sheets, foils, meshes, porous bodies, foams, and nonwovens.

[0113] In addition to the aforementioned positive electrode active material, the positive electrode active material layer may include conductive materials and adhesives.

[0114] Conductive materials are used to impart conductivity to the electrodes, and any conductive material can be used without particular limitation, as long as it has electronic conductivity without causing chemical changes that constitute the battery. Specific examples may include: graphite, such as natural or artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermally cracked carbon black, carbon fibers, and carbon nanotubes; metal powders or fibers of copper, nickel, aluminum, and silver, etc.; 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 mixtures of any one or more of these may be used. Based on the total weight of the positive electrode active material layer, the content of the conductive material is typically from 1% to 30% by weight, preferably from 1% to 20% by weight, and more preferably from 1% to 10% by weight.

[0115] The adhesive is used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples may include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and mixtures of any one or more thereof may be used. Based on the total weight of the positive electrode active material layer, the adhesive content may be from 1% to 30% by weight, preferably from 1% to 20% by weight, and more preferably from 1% to 10% by weight.

[0116] The positive electrode can be manufactured according to typical methods for manufacturing positive electrodes. For example, the positive electrode can be manufactured by mixing positive electrode active materials, binders and / or conductive materials in a solvent to prepare a positive electrode slurry, applying the positive electrode slurry onto a positive electrode current collector, and then drying and rolling it.

[0117] The solvent can be a solvent commonly used in the art, and can be dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), acetone, or water, etc., and any one of them or a mixture of two or more thereof can be used. Considering the applied thickness and preparation yield of the slurry, and the ability of the slurry to have a viscosity that can exhibit excellent thickness uniformity when later used to manufacture the positive electrode, the solvent can be used in an amount sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder.

[0118] In another method, the positive electrode can be manufactured by casting the positive electrode slurry onto a separate carrier and then pressing the membrane layer peeled off from the carrier onto the positive electrode current collector.

[0119] Lithium secondary battery

[0120] Next, the lithium secondary battery of the present invention will be described.

[0121] Specifically, the lithium secondary battery includes the above-described positive electrode, a negative electrode positioned to face the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. Since the positive electrode is the same as the above-described positive electrode, its detailed description will be omitted, and hereinafter, only the remaining components will be described in detail.

[0122] (1) Negative electrode

[0123] The negative electrode of the present invention includes a negative electrode active material layer containing a negative electrode active material, and if necessary, the negative electrode active material layer may contain a conductive material and / or an adhesive.

[0124] As the negative electrode active material, various negative electrode active materials used in the art can be used, such as silicon-based negative electrode active materials, carbon-based negative electrode active materials, or metal alloys, etc.

[0125] The negative electrode active material may include a silicon-based negative electrode active material.

[0126] Since the silicon-based negative electrode active material has a higher theoretical capacity than the carbon-based negative electrode active material and has a higher reaction rate with lithium compared to the carbon-based negative electrode active material, if the negative electrode includes a silicon-based negative electrode active material, the energy density and fast charging performance are improved. However, since the silicon-based negative electrode active material has a large irreversible capacity and has a large volume expansion during charge and discharge, the life characteristics of the silicon-based negative electrode active material are poor. In particular, if used in combination with a lithium composite transition metal oxide having a large gas generation amount, there is a problem of further exacerbating the deterioration of the life characteristics. However, if a single particle or quasi-single particle form of lithium composite transition metal oxide is applied as in the present invention, the gas generation is less than that of the conventional lithium composite transition metal oxide in the form of secondary particles, so that the deterioration of the life characteristics can be minimized, and the excess lithium generated by the rock salt phase during the activation process of the lithium composite transition metal oxide can compensate for the irreversible capacity of the silicon-based negative electrode active material.

[0127] The silicon-based negative electrode active material can be, for example, Si, SiO w (where 0 < w ≤ 2), Si-C composite material, Si-M a alloy (M a is one or more selected from the group consisting of Al, Sn, Mg, Cu, Fe, Pb, Zn, Mn, Cr, Ti, and Ni), or a combination thereof.

[0128] Meanwhile, if necessary, the silicon-based negative electrode active material can be doped with M b metal, and in this case, Mb The metal may be a Group 1 alkali metal element and / or a Group 2 alkaline earth metal element, such as Li, Mg, etc. Specifically, the silicon-based anode active material may be Si, SiO doped with M b metal (where 0 < w ≤ 2), or a Si-C composite material, etc. Due to the doping element, the metal-doped silicon-based anode active material has a reduced active material capacity, but has high efficiency, and thus can achieve a high energy density. w

[0129] In addition, if necessary, the silicon-based anode active material may further include a carbon coating on the particle surface. In this case, based on the total weight of the silicon-based anode active material, the carbon coating amount may be 20 wt% or more, preferably 0.1 wt% to 20 wt%. When the carbon coating is applied, the conductivity of the silicon surface is improved, thereby improving the uniformity of the SEI layer, and there is an effect of improving the initial efficiency and life characteristics.

[0130] The carbon coating can be formed by dry coating, wet coating, chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD), etc.

[0131] The capacity of the silicon-based anode active material is preferably 1000 mAh / g to 4000 mAh / g, preferably 1000 mAh / g to 3800 mAh / g, more preferably 1200 mAh / g to 3800 mAh / g. If a silicon-based anode active material that satisfies the above capacity range is used, high capacity characteristics can be achieved.

[0132] The initial efficiency of the silicon-based anode active material may be 60% to 95%, preferably 70% to 95%, more preferably 75% to 95%. The initial efficiency of the silicon-based anode active material refers to the percentage of the discharge capacity to the charge capacity measured by manufacturing a half-cell using the anode (where the silicon-based anode is used 100% as the anode active material) and a lithium counter electrode, and then charging and discharging the half-cell at a 0.1 C rate between 0.01 V and 1.5 V. If the initial efficiency of the silicon-based anode active material satisfies the above range, the lithium provided from the positive electrode can be used reversibly, and excellent fast charging performance can be achieved.

[0133] The anode may further include a carbon-based anode active material as the anode active material. The carbon-based anode active material may be, for example, artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, soft carbon, or hard carbon, etc., but is not limited thereto.

[0134] Based on the total weight of the anode active material, the content of silicon-based anode active material can be 1% to 100% by weight, 1% to 50% by weight, 1% to 30% by weight, 1% to 15% by weight, 10% to 70% by weight, or 10% to 50% by weight.

[0135] Based on the total weight of the anode active material, the content of carbon-based anode active material can be 0% to 99% by weight, 50% to 99% by weight, 70% to 99% by weight, 85% to 99% by weight, 30% to 90% by weight, or 50% to 90% by weight.

[0136] The negative electrode active material can be a mixture of silicon-based and carbon-based negative electrode active materials, and in this case, the mixing ratio of silicon-based to carbon-based negative electrode active material by weight can be 1:99 to 50:50, preferably 3:97 to 30:70. If the mixing ratio of silicon-based to carbon-based negative electrode active material meets the above range, the volume expansion of the silicon-based negative electrode active material is suppressed, while the capacity characteristics are improved, thus ensuring excellent cycle performance.

[0137] Based on the total weight of the negative electrode active material layer, the content of the negative electrode active material can be from 80% to 99% by weight. If the content of the negative electrode active material meets the above range, excellent capacity and electrochemical properties can be obtained.

[0138] Examples of conductive materials may include, for example, spherical or flake graphite, natural or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fibers, single-walled carbon nanotubes, or multi-walled carbon nanotubes; metal powders or metal fibers such as copper, nickel, aluminum, or silver; conductive whiskers such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides such as titanium oxide; conductive polymers such as polyphenylene derivatives; and any one or mixtures of two or more of these. Based on the total weight of the negative electrode active material layer, the content of the conductive material may be from 0.1% to 30% by weight, 1% to 20% by weight, or 1% to 10% by weight.

[0139] Preferably, single-walled carbon nanotubes can be used as conductive materials. If single-walled carbon nanotubes are used as conductive materials, conductive paths are uniformly formed on the surface of the negative electrode active material, and thus, improved cycle characteristics can be obtained.

[0140] Examples of adhesives may include, for example, polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylic acid, polyacrylamide, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM) rubber, sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one or mixtures of two or more thereof may be used. Based on the total weight of the negative electrode active material layer, the adhesive content may be from 1% to 20% by weight, 2% to 20% by weight, or 2% to 10% by weight.

[0141] In the negative electrode, the negative electrode active material layer can be a single layer or a multilayer structure consisting of two or more layers. If the negative electrode active material layer has a multilayer structure consisting of two or more layers, each layer can have different types and / or contents of negative electrode active materials, binders, and / or conductive materials. For example, in the negative electrode of the present invention, the content of carbon-based negative electrode active material in the lower layer can be formed to be higher than the content of carbon-based negative electrode active material in the upper layer, and the content of silicon-based negative electrode active material can be formed to be higher in the upper layer. In this case, compared with the case where the negative electrode active material layer is formed as a single layer, the effect of improving fast charging performance can be obtained.

[0142] (2) Diaphragm

[0143] 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 a separator commonly used in lithium-ion secondary batteries. Particularly preferred are separators that exhibit excellent moisture retention in the electrolyte solution and low resistance to ion movement within the electrolyte. Specifically, porous polymer membranes can be used, such as those made from polyolefin polymers (e.g., ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers), or stacked structures having two or more of these layers. Furthermore, typical porous nonwoven fabrics can be used, such as nonwoven fabrics made from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Additionally, coated separators including ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength and can be selectively used in single-layer or multi-layer structures.

[0144] (3) Electrolytes

[0145] In lithium secondary batteries, the electrolyte can be any organic liquid electrolyte, inorganic liquid electrolyte, solid polymer electrolyte, gel polymer electrolyte, solid inorganic electrolyte, molten inorganic electrolyte, etc., which can be used to manufacture lithium secondary batteries, but is not limited to these.

[0146] Specifically, electrolytes may include organic solvents and lithium salts.

[0147] As organic solvents, any organic solvent can be used without particular limitation, as long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, as organic solvents, the following can be used: ester solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents, such as dibutyl ether or tetrahydrofuran; ketone solvents, such as cyclohexanone; aromatic solvents, such as benzene and fluorobenzene; 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 to C20 hydrocarbon group, and may include double-bonded aromatic rings or ether bonds); amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane; or sulfolane.

[0148] Any compound can be used as the lithium salt without particular limitation, as long as it is a compound capable of providing lithium ions for lithium secondary batteries. Specifically, the anion of the lithium salt can be 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 -At least one of the following groups can be used as a lithium salt: LiPF6, LiN(FSO2)2, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The lithium salt can be used in concentrations ranging from 0.1 M to 5.0 M. If the concentration of the lithium salt is within the above range, the electrolyte has suitable conductivity and viscosity, thus exhibiting excellent electrolyte performance, and lithium ions can move efficiently.

[0149] To improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity, the electrolyte may also include one or more additives, such as halogenated alkyl carbonate compounds like ethylene difluorocarbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol diether, hexamethylphosphotriamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolides, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride. In this case, based on the total weight of 100 parts by weight of the electrolyte, the content of the additives can be from 0.1 parts by weight to 5 parts by weight.

[0150] The invention will be described in more detail below with reference to embodiments. However, the following embodiments are merely illustrative and are not intended to limit the scope of the invention.

[0151] Examples and Comparative Examples

[0152] Preparation Example 1

[0153] Average particle size (D) 50 Ni, a transition metal precursor with a thickness of 3.7 μm, 0.8 Co 0.09 Mn 0.1 Al 0.01 (OH)2 and LiOH are mixed to make the molar ratio of Ni+Co+Mn+Al:Li 1:1.03, and Zr(SO4) is added based on the total weight of the transition metal precursor. 4H₂O was mixed with it at a concentration of 1000 ppm to prepare a mixture. The mixture was then subjected to a single calcination at 850°C for 18 hours to obtain a single-calcination product.

[0154] Next, it was calcined again at 780°C for 10 hours to prepare lithium composite transition metal oxides with the roundness described in Table 1 below.

[0155] Preparation Example 2

[0156] Lithium composite transition metal oxides having the sphericity described in Table 1 below were prepared in the same manner as in Preparation Example 1, except that an average particle size (D) was used. 50 It is a transition metal precursor with a thickness of 3.8 μm.

[0157] Preparation Example 3

[0158] Lithium composite transition metal oxides having the roundness described in Table 1 below were prepared in the same manner as in Preparation Example 1, except that a first firing at 900°C for 12 hours was performed, and a second firing at 800°C for 10 hours was performed.

[0159] Preparation Example 4

[0160] Lithium composite transition metal oxides having the roundness described in Table 1 below were prepared in the same manner as in Preparation Example 1, except that a first firing at 800°C for 12 hours was performed, and a second firing at 750°C for 10 hours was performed.

[0161] [Table 1]

[0162] Roundness was measured by photographing lithium complex transition metal oxides at 2000x magnification using a scanning electron microscope (SEM) and then obtaining approximately 300 particle images using an image analysis management (IAM) program.

[0163] Example 1

[0164] The lithium composite transition metal oxide of Preparation Example 1 and Co(OH)2 were mixed at a weight ratio of 100:2 and heat-treated at 700°C for 6 hours to prepare an intermediate in which a cobalt coating was formed as the first coating.

[0165] The intermediate and H3BO3 were mixed at a weight ratio of 100:0.2 and heat-treated at 400°C for 6 hours to prepare a positive electrode active material in which a boron coating is formed as a second coating.

[0166] The boron content in the positive electrode active material is shown in Table 2 below.

[0167] Example 2

[0168] The positive electrode active material was prepared in the same manner as in Example 1, except that the lithium composite transition metal oxide of Preparation Example 2 was used, and the intermediate and H3BO3 were mixed in a weight ratio of 100:0.1.

[0169] The boron content in the positive electrode active material is shown in Table 2 below.

[0170] Comparative Example 1

[0171] The positive electrode active material was prepared in the same manner as in Example 1, except that the process of forming a boron coating as a second coating was omitted.

[0172] The boron content in the positive electrode active material is shown in Table 2 below.

[0173] Comparative Example 2

[0174] The positive electrode active material was prepared in the same manner as in Example 1, except that the intermediate and H3BO3 were mixed in a weight ratio of 100:0.2.

[0175] The boron content in the positive electrode active material is shown in Table 2 below.

[0176] Comparative Example 3

[0177] The positive electrode active material was prepared in the same manner as in Example 1, except that the lithium composite transition metal oxide of Preparation Example 3 was used.

[0178] The boron content in the positive electrode active material is shown in Table 2 below.

[0179] Comparative Example 4

[0180] The positive electrode active material was prepared in the same manner as in Example 1, except that the lithium composite transition metal oxide of Preparation Example 4 was used.

[0181] The boron content in the positive electrode active material is shown in Table 2 below.

[0182] [Table 2]

[0183] The boron content in the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 to 4 was measured by inductively coupled plasma (ICP) using an ICP-OES (PerkinElmer, Optima 7300DV) device.

[0184] Experimental Example 1

[0185] The surface and internal structures of the positive electrode active materials prepared in Example 1 and Comparative Example 1 were confirmed by scanning electron microscopy (SEM). The measurement results are as follows: Figure 1 and Figure 2 As shown.

[0186] refer to Figure 1 and Figure 2As can be seen, the positive electrode active materials prepared in Example 1 and Comparative Example 1 are composed of lithium composite transition metal compound particles in the form of single particles or quasi-single particles. The single particle is composed of a single nodule, and the quasi-single particle is a composite of 30 or fewer nodules.

[0187] Experiment Example 2: Measurement of Gas Production

[0188] <Manufacturing of Lithium Secondary Batteries>

[0189] The positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 to 4, namely carbon nanotubes and PVDF, were mixed in N-methylpyrrolidone at a weight ratio of 95:2:3 to prepare a positive electrode slurry. The positive electrode slurry was applied to an aluminum current collector, dried, and then rolled to manufacture the positive electrode.

[0190] A negative electrode slurry was prepared by mixing SiO and graphite in a weight ratio of 5:95, carbon nanotubes, and styrene-butadiene rubber (SBR) in water at a weight ratio of 95.5:1:3.5. The negative electrode slurry was applied to a copper current collector sheet, dried, and then rolled to manufacture the negative electrode.

[0191] A separator is placed between the positive and negative electrodes manufactured as described above to manufacture an electrode assembly, and the electrode assembly is placed inside a housing. Then, an electrolyte solution is injected into the housing to manufacture a lithium secondary battery.

[0192] In this case, an electrolyte solution is prepared by dissolving 1.0 M lithium hexafluorophosphate (LiPF6) in an organic solvent in a volume ratio of ethylene carbonate / ethyl methyl carbonate / diethyl carbonate of 3:4:4.

[0193] <Measurement of Gas Production>

[0194] Each of the manufactured lithium-ion batteries was charged to SOC 100, and then each lithium-ion battery was disassembled to separate the positive electrode. The positive electrode and 400 µl of electrolyte solution were then placed in a pouch-type battery casing, and the casing was sealed to manufacture a cell. The cell was stored at 60°C for 8 weeks to measure the change in cell volume (Δcell volume, in µl) before and after high-temperature storage. The cell volume change was measured by placing the cell in water and then measuring the change in water volume. The measurement results are shown in Table 3 below.

[0195] [Table 3]

[0196] As shown in Table 3 above, the lithium secondary batteries using the positive electrode active materials prepared in Examples 1 and 2 of the present invention produce less gas than the lithium secondary batteries using the positive electrode active materials prepared in Comparative Examples 1 to 4.

[0197] Experimental Example 3: Evaluation of High-Temperature Lifetime Characteristics

[0198] Each of the manufactured lithium secondary batteries was charged to 4.25 V at 0.5 C and then discharged to 2.5 V at 1 C. This entire process was set as one cycle, and 100 charge-discharge cycles were performed to measure capacity retention and rate of increase in resistance, thereby evaluating lifetime characteristics. The measurement results are shown in Table 4 below.

[0199] In this case, the capacitance retention rate and the resistance increase rate are calculated using the following equations 2 and 3.

[0200] Equation 2: Capacity retention rate (%) = (Discharge capacity after 100 cycles / Discharge capacity after 1 cycle) × 100

[0201] Equation 3: Resistance increase rate (%) = {(Resistance after 100 cycles / Resistance after 1 cycle) × 100} - 100

[0202] [Table 4]

[0203] As shown in Table 4 above, compared with the lithium secondary batteries using the positive electrode active materials prepared in Comparative Examples 1 to 4, the lithium secondary batteries using the positive electrode active materials prepared in Examples 1 and 2 of the present invention have higher capacity retention and lower resistance increase rate, thus demonstrating excellent high-temperature life characteristics.

Claims

1. A positive electrode active material, comprising: Lithium composite transition metal oxides in the form of single particles or quasi-single particles, wherein the single particle consists of a single nucleus, and the quasi-single particle is a composite with fewer than 30 nuclei; and The coating comprises a first coating formed on the surface of the lithium composite transition metal oxide and a second coating formed on the surface of the first coating. in, The roundness of the lithium composite transition metal oxide, as defined by Equation 1, is between 0.50 and 0.68, and The second coating contains boron, wherein the boron content, based on the total weight of the positive electrode active material, is between 300 ppm and 2400 ppm by weight. [Equation 1] Roundness = (4 × Area) / (π × R) 2 ) In Equation 1 above, R represents the length of the major axis passing through the center of the lithium composite transition metal oxide, and Area represents the actual area of ​​the lithium composite transition metal oxide.

2. The positive electrode active material according to claim 1, wherein, The lithium composite transition metal oxide has a composition represented by the following chemical formula 1: [Chemical Formula 1] Li 1+x Ni a Co b Mr c Al d M 1 e O2 Among them, in the above Chemical Formula 1, M 1 is one or more selected from the group consisting of W, Zr, Y, Ba, Ca, Ti, Mg, Ta, and Nb, and 0 ≤ x ≤ 0.5, 0.8 ≤ a < 1.0, 0 < b ≤ 0.1, 0 < c < 0.2, 0 < d ≤ 0.05, and 0 ≤ e ≤ 0.

05.

3. The positive electrode active material according to claim 1, wherein, The nickel content in all metals other than lithium in the lithium-ion complex transition metal compound is above 80 mol%.

4. The positive electrode active material according to claim 1, wherein, The average particle size D of the lithium composite transition metal oxide 50 The range is from 1 μm to 8 μm.

5. The positive electrode active material according to claim 1, wherein, The roundness of the lithium composite transition metal oxide is 0.58 to 0.

65.

6. The positive electrode active material according to claim 1, wherein, The first coating comprises one or more selected from the group consisting of Ni, Co, and Al.

7. The positive electrode active material according to claim 1, wherein, Based on the total weight of the positive electrode active material, the boron content is between 400 ppm and 1400 ppm by weight.

8. The positive electrode active material according to claim 1, wherein, The BET specific surface area of ​​the positive electrode active material is 0.4 m². 2 / g to 1.0 m 2 / g.

9. A positive electrode comprising the positive electrode active material of claim 1.

10. A lithium secondary battery, comprising: The positive electrode as described in claim 9; negative electrode; A membrane located between the positive electrode and the negative electrode; and Electrolytes.

11. The lithium secondary battery according to claim 10, wherein, The negative electrode includes a negative electrode active material layer, which includes a silicon-based negative electrode active material.

12. The lithium secondary battery according to claim 11, wherein, The negative electrode also includes carbon-based negative electrode active materials.

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