Positive electrode active material, positive electrode for lithium secondary battery, and lithium secondary battery

Abstract

A positive electrode active material according to one embodiment of the present invention comprises: a core containing a first active material as a lithium-rich oxide; and a shell provided on the surface of the core and containing a second active material having an olivine structure, in which the weight% of the first active material is greater than or equal to the weight% of the second active material based on the total weight of the positive electrode active material. According to one embodiment of the present invention, a positive electrode for a lithium secondary battery having excellent life performance can be provided.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material, a positive electrode for a lithium secondary battery, and a lithium secondary battery. Background Art

[0002] In recent years, extensive research has been conducted on electric vehicles (EVs) that can replace vehicles using fossil fuels such as gasoline vehicles and diesel vehicles, which are one of the main causes of air pollution. As the power source for these electric vehicles (EVs), lithium secondary batteries with high discharge voltage and power stability are mainly used.

[0003] In order to improve the performance of the lithium secondary battery, a technology that can improve the energy density, life performance, etc. of the positive electrode for a lithium secondary battery needs to be developed. Summary of the Invention

[0004] (I) Technical Problems to be Solved

[0005] According to one aspect of the present invention, the life performance of the positive electrode for a lithium secondary battery can be improved.

[0006] According to another aspect of the present invention, a positive electrode for a secondary battery having excellent thermal stability can be provided.

[0007] According to another aspect of the present invention, the energy density of the positive electrode for a lithium secondary battery can be improved.

[0008] (II) Technical Solutions

[0009] The positive electrode active material according to a specific embodiment of the present invention includes: a core containing a first active material as a lithium-rich oxide; and a shell provided on the surface of the core, and the shell contains a second active material having an olivine structure, wherein, based on the total weight of the positive electrode active material, the weight % of the first active material is greater than or equal to the weight % of the second active material.

[0010] In some specific embodiments, the first active material may be represented by the following Chemical Formula 1.

[0011] [Chemical Formula 1]

[0012] Li a [M x Ni y Mn z O b

[0013] In the Chemical Formula 1, M is at least one element selected from Co, Mg, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, and 0 ≤ x ≤ 0.9, 0 ≤ y ≤ 0.9, x + y > 0, 0.1 ≤ z ≤ 0.9, 1.8 ≤ a + x + y + z ≤ 2.2, 1.05 ≤ a / (x + y + z) ≤ 1.95, 1.8 ≤ b ≤ 2.2.

[0014] In some specific embodiments, the second active material may be represented by the following Chemical Formula 2.

[0015] [Chemical Formula 2]

[0016] LiMePO4

[0017] In the Chemical Formula 2, Me is at least one element selected from Co, Ni, Fe, and Mn.

[0018] In some specific embodiments, based on the total weight of the positive electrode active material, the weight ratio of the first active material to the second active material may be from 60:40 to 95:5.

[0019] In some specific embodiments, the BET specific surface area of the first active material may be from 0.9 m² / g to 2.0 m² / g.

[0020] In some specific embodiments, the average particle size (D50) of the first active material may be greater than or equal to the average particle size (D50) of the second active material.

[0021] In some specific embodiments, the average particle size (D50) of the first active material may be from 5 μm to 12 μm.

[0022] In some specific embodiments, the average particle size (D50) of the second active material may be from 0.3 μm to 5 μm.

[0023] The positive electrode for a lithium secondary battery according to a specific embodiment of the present invention includes the positive electrode active material described in any one of the above specific embodiments.

[0024] The method for manufacturing a positive electrode for a lithium secondary battery according to a specific embodiment of the present invention includes the following steps: preparing a positive electrode slurry, the positive electrode slurry including a first active material as a lithium-rich oxide and a second active material having an olivine structure; and forming a positive electrode mixture layer on at least one surface of a positive electrode current collector with the positive electrode slurry, wherein the positive electrode mixture layer includes the positive electrode active material described in any one of the above specific embodiments.

[0025] In some specific embodiments, the first active material may be represented by the chemical formula 1.

[0026] In some specific embodiments, the second active material may be represented by the chemical formula 2.

[0027] In some specific embodiments, the positive electrode paste may be prepared by adding the first active material to a pre-dispersed solution of the second active material.

[0028] In some specific embodiments, the solid content of the pre-dispersed solution may be 50 wt% to 70 wt%.

[0029] A lithium secondary battery according to a specific embodiment of the present invention includes a positive electrode for a lithium secondary battery according to any one of the above specific embodiments.

[0030] (III) Beneficial Effects

[0031] According to a specific embodiment of the present invention, a positive electrode for a lithium secondary battery having excellent life performance can be provided.

[0032] According to another specific embodiment of the present invention, the thermal stability of the positive electrode for a secondary battery can be improved.

[0033] According to another specific embodiment of the present invention, a positive electrode for a lithium secondary battery having excellent energy density can be provided. Description of the Drawings

[0034] Figure 1 is a cross-sectional view schematically showing the morphology of a positive electrode active material according to a specific embodiment.

[0035] Figure 2a and Figure 2b are diagrams showing scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis images of a cross-section of a positive electrode active material according to Example 1.

[0036] Figure 3a and Figure 3b are diagrams showing scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis images of a cross-section of a positive electrode active material according to Example 5.

[0037] Description of the Reference Numerals:

[0038] 100: Positive electrode active material

[0039] 10: Core

[0040] 11: Shell Detailed implementation mode

[0041] Hereinafter, with reference to the drawings, the technologies disclosed in this specification and their specific implementation modes will be described in detail. However, the implementation forms of the above technologies can be modified into various other forms, and their scope is not limited to the specific implementation modes described below. In addition, the technologies disclosed in this specification can not only be applied limited to the configurations of the specific implementation modes to be described below, but also be configured by optionally combining all or part of each specific implementation mode to achieve various modifications.

[0042] In this specification, "lithium-rich oxide" can refer to oxides with a lithium-rich composition such as lithium-rich layered oxide (LLO), over-lithiated oxide (OLO), and lithium-rich manganese (LMR)-based oxide. Specifically, the "lithium-rich oxide" can refer to an oxide with a lithium-rich composition in which the content of lithium is higher than that of the transition metal and exhibits a high capacity under high voltage conditions.

[0043] With the increasing demand for lithium secondary batteries, a technology for manufacturing a high-capacity lithium secondary battery with excellent life characteristics is required. According to a specific implementation mode, by applying lithium-rich oxide particles with a high discharge capacity as the positive electrode active material, the capacity of the positive electrode can be improved. The lithium-rich oxide particles can improve the capacity of the positive electrode through the oxidation / reduction reaction of oxygen during the charging process of the battery, and can be driven at a higher upper limit voltage compared with the existing nickel-cobalt-manganese (NCM)-based active materials, thereby improving the energy density of the positive electrode.

[0044] However, when the lithium-rich oxide particles are applied as the positive electrode active material, during repeated continuous charge / discharge cycles of the battery, oxygen in the lattice on the surface of the active material may be released, and irreversible phase changes may occur.

[0045] Therefore, when the lithium-rich oxide particles are applied as the positive electrode active material, the amount of gas generated inside the secondary battery may increase, and problems such as a decrease in the life characteristics of the battery may occur, and these problems may be aggravated as the driving temperature and upper limit voltage of the battery increase.

[0046] According to another specific embodiment, by applying lithium metal oxide particles having an olivine structure with excellent structural stability as a positive electrode active material, the life characteristics of the positive electrode can be improved. The lithium metal oxide particles having the olivine structure have excellent thermal stability. Therefore, when they are applied as a positive electrode active material, even if the battery undergoes continuous charge / discharge cycles in a high-temperature environment, the degree of material deterioration can be relatively low. In addition, when the lithium metal oxide particles having the olivine structure are applied as a positive electrode active material, the electrochemical reaction of the active material usually terminates at a driving voltage of 3.2 - 3.4V. Therefore, at a high voltage above 4.5V, side reactions of the active material can occur less frequently.

[0047] However, the lithium metal oxide particles having the olivine structure have a low capacity. Therefore, when they are applied as a positive electrode active material, it may be difficult to improve the energy density of the battery.

[0048] According to a specific embodiment of the present invention, by alleviating the above problems, a positive electrode for a high-capacity lithium secondary battery having excellent thermal stability and life characteristics, etc., can be provided. Hereinafter, with reference to Figures 1 to 3b , specific embodiments of the present invention will be specifically described.

[0049] Figure 1 is a cross-sectional view schematically showing the morphology of a positive electrode active material according to a specific embodiment.

[0050] Figure 2a and Figure 2b are diagrams showing scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis images of the cross-section of the positive electrode active material according to Example 1.

[0051] Figure 3a and Figure 3b are diagrams showing scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis images of the cross-section of the positive electrode active material according to Example 5.

[0052] Positive electrode active material

[0053] The positive electrode active material 100 according to a specific embodiment includes: a core 10 containing a first active material as a lithium-rich oxide; and a shell 11 disposed on the surface of the core, and the shell 11 contains a second active material having an olivine structure, wherein, based on the total weight of the positive electrode active material 100, the weight percentage of the first active material is greater than or equal to the weight percentage of the second active material.

[0054] In some specific embodiments, the first active material may be represented by the following Chemical Formula 1.

[0055] [Chemical Formula 1]

[0056] Li a [M x Ni y Mn z O b

[0057] In the above Chemical Formula 1, M is at least one element selected from Co, Mg, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, and 0 ≤ x ≤ 0.9, 0 ≤ y ≤ 0.9, x + y > 0, 0.1 ≤ z ≤ 0.9, 1.8 ≤ a + x + y + z ≤ 2.2, 1.05 ≤ a / (x + y + z) ≤ 1.95, 1.8 ≤ b ≤ 2.2.

[0058] In some specific embodiments, the second active material can be represented by the following Chemical Formula 2.

[0059] [Chemical Formula 2]

[0060] LiMePO4

[0061] In the above Chemical Formula 2, Me is at least one element selected from Co, Ni, Fe, and Mn.

[0062] The first active material represented by the above Chemical Formula 1 is a lithium-rich oxide particle with a high discharge capacity, and can be a composite of 1) a lithium-rich phase oxide such as Li2MnO3 and 2) a layered structure oxide such as an NCM-based cathode active material. Therefore, different from active materials that only contain lithium-rich phase oxides such as Li2M1O2 (where M1 can be Ni, Co, Fe, Mn, Zn, Mg, Ca, Cu, etc.) and Li2M2O3 (where M2 can be Mn, Sn, Mo, Ru, Ir, etc.), the first active material can also contain a layered structure oxide. Therefore, compared with the above active materials that only contain lithium-rich phase oxides, the first active material can have more excellent crystal structure stability and long-term life performance.

[0063] In some specific embodiments, in the above Chemical Formula 1, it can be 0 < x ≤ 0.9, 0.05 ≤ x ≤ 0.9, 0.1 ≤ x ≤ 0.9, 0 < x ≤ 0.8, 0.05 ≤ x ≤ 0.8, or 0.1 ≤ x ≤ 0.8.

[0064] In some specific embodiments, in the above Chemical Formula 1, it can be 0 < y ≤ 0.9, 0.05 ≤ y ≤ 0.9, 0.1 ≤ y ≤ 0.9, 0 < y ≤ 0.8, 0.05 ≤ y ≤ 0.8, or 0.1 ≤ y ≤ 0.8.

[0065] In some specific embodiments, in Formula 1, it may be 1.1 ≤ a / (x + y + z) ≤ 1.95, 1.15 ≤ a / (x + y + z) ≤ 1.95, 1.2 ≤ a / (x + y + z) ≤ 1.95, or 1.3 ≤ a / (x + y + z) ≤ 1.95.

[0066] In some specific embodiments, in the lithium-rich oxide particles, the molar fraction of manganese relative to all elements other than lithium and oxygen may be from 0.5 to 0.75. Exemplarily, in Formula 1, it may be 0.5 ≤ z / (x + y + z) ≤ 0.75.

[0067] In some specific embodiments, it may be 0.25 ≤ (x + y) / (x + y + z) ≤ 0.5.

[0068] In some specific embodiments, in the lithium-rich oxide particles, the molar fraction of cobalt relative to all elements other than lithium and oxygen may be from 0 to 0.02. Exemplarily, the lithium-rich oxide particles may not contain cobalt.

[0069] In some specific embodiments, in Formula 1, it may be 1.9 ≤ b ≤ 2.1 or 1.95 ≤ b ≤ 2.05.

[0070] The chemical structure represented by Formula 1 represents the binding relationship included in the structure of the first active material, and does not exclude other additional elements. Exemplarily, M may include Co, and Co may be provided together with Ni and Mn as the main active elements of the first active material. Formula 1 is provided to represent the binding relationship of the main active elements, and it should be understood that Formula 1 is a formula including the introduction and substitution of additional elements.

[0071] In some specific embodiments, in addition to including the main active elements, auxiliary elements for enhancing the chemical stability of the structure of the first active material may be further included. The auxiliary elements may be mixed together into the first active material to form a binding, and it should be understood that this situation is also included within the scope of the chemical structure represented by Formula 1.

[0072] Exemplarily, the auxiliary elements may include at least one of Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, or Zr. The auxiliary elements may act as auxiliary active elements that contribute to the capacity / power activity of the first active material together with Co or Mn, such as Al.

[0073] In some specific embodiments, the first active material may further include a coating material or a doping material containing a coating element or a doping element. Exemplarily, an element substantially the same as or similar to the above auxiliary element may be used as the coating element or the doping element. Exemplarily, one or a combination of two or more of the above elements may be used as the coating element or the doping element. In this case, the upper operating voltage of the lithium secondary battery can be adjusted, thereby suppressing the voltage drop (voltage decay) of the lithium secondary battery.

[0074] The coating element or the doping element may be present on the surface of the particles of the first active material, or may penetrate through the surface of the particles of the first active material and be included in the bonding structure represented by Chemical Formula 1.

[0075] In some specific embodiments, the coating material may form a sea-type coating or an island-type coating.

[0076] In some specific embodiments, in the particles of the first active material, based on the total weight of all elements other than lithium and oxygen, the content of the coating element may be 500 - 8000 ppm, 1000 - 8000 ppm, or 1500 - 8000 ppm. When the content of the coating element is as described above, a decrease in the initial capacity of the lithium secondary battery and an increase in resistance can be prevented, and the voltage drop of the lithium secondary battery can be further suppressed.

[0077] In some specific embodiments, the coating material may be formed by a dry coating method or a wet coating method. Exemplarily, the particles of the first active material and the coating source may be dry-mixed or wet-mixed, and then heat-treated (e.g., calcined or dried) to form a coating material on the surface of the particles of the first active material. The coating source may be a coating source well-known in the art. Exemplarily, the coating source may contain B, Al, W, Zr, Ti, Mg, Co, etc.

[0078] The particle composition of the first active material represented by Chemical Formula 1 can be confirmed by inductively coupled plasma (ICP) analysis. Exemplarily, the particles of the first active material are analyzed using ICP, and the total number of metal atoms such as lithium (i.e., the value of a + x + y + z in Chemical Formula 1) is normalized to 1.8 to 2.2 (exemplarily, 2), thereby obtaining the chemical formula of the first active material.

[0079] The second active material represented by Formula 2 may be lithium metal oxide particles with an olivine structure having excellent structural stability. In some specific embodiments, the second active material may be a lithium iron phosphate (LFP)-based active material containing iron (Fe) or a lithium manganese iron phosphate (LMFP)-based active material in a form in which part of the iron (Fe) in the lithium iron phosphate (LFP)-based active material is replaced by manganese (Mn).

[0080] The particle composition of the second active material represented by Formula 2 can also be confirmed by inductively coupled plasma (ICP) analysis. Exemplarily, the particles of the second active material are analyzed using ICP, and the number of phosphorus atoms is normalized to 1, whereby the chemical formula of the second active material can be obtained.

[0081] In some specific embodiments, the second active material may include a carbon coating on its surface. When the second active material includes a carbon coating on its surface, the ionic conductivity and electronic conductivity of the second active material can be further improved.

[0082] The positive electrode active material 100 may have a core-shell structure in which a shell containing a second active material is provided on the surface of a core 10 containing a first active material, wherein the first active material is lithium-rich oxide particles having a high discharge capacity, and the second active material is lithium metal oxide particles with an olivine structure having excellent structural stability. The positive electrode active material having such a core-shell structure may have a form in which the second active material uniformly distributed on the surface of the first active material coats the first active material, and may have further improved thermal stability compared to the case of simply mixing the first active material and the second active material.

[0083] In addition, the first active material can be driven at a higher upper limit voltage compared to the NCM-based active material, thereby significantly increasing the energy density of the positive electrode. Therefore, when the first active material and the second active material are mixed in a core-shell structure, a high energy density can be ensured to an extent that cannot be achieved by mixing the NCM-based active material and the LFP-based active material.

[0084] Therefore, even when the battery is driven under high voltage / high temperature conditions, oxygen detachment from the surface of the first active material, which is lithium-rich oxide particles, or irreversible phase change can be suppressed, and a positive electrode for a high-capacity lithium secondary battery with excellent life characteristics can be manufactured.

[0085] Based on the total weight of the positive electrode active material 100, the weight percentage of the first active material can be greater than or equal to the weight percentage of the second active material. In some specific embodiments, based on the total weight of the positive electrode active material 100, the weight ratio of the first active material to the second active material can be from 60:40 to 95:5. Exemplarily, based on the total weight of the positive electrode active material 100, the weight ratio of the first active material to the second active material can be from 70:30 to 90:10.

[0086] When the content relationship between the first active material and the second active material is as described above, the positive electrode active material 100 can contain a second active material with a content above a certain level, so that the life characteristics of the active material can be ensured to reach an excellent level. At the same time, the positive electrode active material 100 can contain a relatively large amount of the first active material, so that the energy density can be maximized.

[0087] In some specific embodiments, the BET specific surface area of the first active material can be from 0.9 m² / g to 2.0 m² / g. Exemplarily, the BET specific surface area of the first active material can be 1.0 m² / g or more and can be 1.7 m² / g or less. When the BET specific surface area value of the first active material is lower than 0.9 m² / g, the capacity of the positive electrode containing the positive electrode active material may be reduced. When the BET specific surface area value of the first active material exceeds 2.0 m² / g, the life performance of the positive electrode containing the positive electrode active material may be reduced. The measurement method of the BET specific surface area is not particularly limited. Exemplarily, the specific surface area of the first active material can be calculated by measuring the nitrogen adsorption amount of the first active material at the liquid nitrogen temperature (77K) using a specific surface area analysis device (Micromeritics, ASAP™ 2420).

[0088] In some specific embodiments, the average particle size (D50) of the first active material can be greater than or equal to the average particle size (D50) of the second active material. When the first active material contained in the positive electrode active material 100 is applied as large particles and the second active material is applied as small particles, due to the bi-modal effect, the energy density of the positive electrode active material 100 can be further improved.

[0089] In some specific embodiments, the average particle size (D50) of the first active material can be from 5 μm to 12 μm. Exemplarily, the average particle size (D50) of the first active material can be 6 μm or more and can be 11 μm or less.

[0090] In some specific embodiments, the average particle size (D50) of the second active material may be from 0.3 μm to 5 μm. Exemplarily, the average particle size (D50) of the second active material may be 0.5 μm or more and may be 2 μm or less.

[0091] Positive electrode for lithium secondary battery

[0092] The positive electrode for a lithium secondary battery according to a specific embodiment includes 100 of the positive electrode active material described in any one of the above specific embodiments. Exemplarily, the positive electrode for a lithium secondary battery may include a positive electrode current collector and a positive electrode mixture layer on at least one surface of the positive electrode current collector.

[0093] The composition of the positive electrode current collector is not particularly limited. Exemplarily, the positive electrode current collector may be a plate or foil formed of one or more of indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), and their alloys. In some specific embodiments, the positive electrode current collector may be an aluminum foil (Al-foil).

[0094] The thickness of the positive electrode current collector is not particularly limited. Exemplarily, the thickness of the positive electrode current collector may be 0.1 - 50 μm.

[0095] The positive electrode mixture layer may further contain a binder. The binder is not particularly limited. Exemplarily, the binder may include one or more of styrene-butadiene rubber (SBR), polytetrafluoroethylene, polyvinylidene fluoride, vinylidene fluoride / hexafluoropropylene copolymer, polyacrylonitrile, and polymethyl methacrylate.

[0096] The positive electrode mixture layer may further contain a conductive material. The conductive material is not particularly limited. Exemplarily, the conductive material may include one or more of the following substances: graphite such as natural graphite or artificial graphite; carbon-based substances such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, carbon fiber, carbon nanotube (CNT); metal powders or metal fibers such as copper, nickel, aluminum, silver; conductive whiskers such as zinc oxide, potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives.

[0097] Method for manufacturing a positive electrode for a lithium secondary battery

[0098] A method for manufacturing a positive electrode for a lithium secondary battery according to a specific embodiment includes the following steps: preparing a positive electrode paste, the positive electrode paste containing a first active material that is a lithium-rich oxide and a second active material having an olivine structure; and forming a positive electrode mixture layer on at least one surface of a positive electrode current collector with the positive electrode paste, wherein the positive electrode mixture layer contains 100 of the positive electrode active material according to any one of the above specific embodiments.

[0099] In some specific embodiments, the first active material may be represented by the following Chemical Formula 1.

[0100] [Chemical Formula 1]

[0101] Li a [M x Ni y Mn z O b

[0102] In Chemical Formula 1, M is at least one element selected from Co, Mg, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, and 0 ≤ x ≤ 0.9, 0 ≤ y ≤ 0.9, x + y > 0, 0.1 ≤ z ≤ 0.9, 1.8 ≤ a + x + y + z ≤ 2.2, 1.05 ≤ a / (x + y + z) ≤ 1.95, 1.8 ≤ b ≤ 2.2.

[0103] In some specific embodiments, the second active material may be represented by the following Chemical Formula 2.

[0104] [Chemical Formula 2]

[0105] LiMePO4

[0106] In Chemical Formula 2, Me is at least one element selected from Co, Ni, Fe, and Mn.

[0107] Detailed descriptions of the first active material, the second active material, etc. are repeated with the above content, and thus the descriptions are omitted.

[0108] <Preparation of Positive Electrode Paste>

[0109] In some specific embodiments, the positive electrode paste may be prepared by adding the first active material to a pre-dispersion solution of the second active material. The pre-dispersion solution of the second active material may be a solution in the form of a paste in which the second active material is pre-dispersed in a solvent. The solvent is not particularly limited. Exemplarily, the solvent may be N-methyl-2-pyrrolidone (NMP).

[0110] When adding the first active material to the pre-dispersion solution prepared separately by pre-dispersing the second active material, agglomeration between the second active material particles can be prevented, and the second active material can be evenly distributed on the surface of the first active material, so that the core-shell structure of the positive electrode active material 100 can be well formed.

[0111] In some specific embodiments, the solid content of the pre-dispersion solution can be 50 wt% to 70 wt%. Exemplarily, the solid content of the pre-dispersion solution can be 60 wt% to 65 wt%. When the solid content of the pre-dispersion solution is within the above range, the finally prepared positive electrode paste can have excellent dispersibility and fluidity.

[0112] In some specific embodiments, the content of the first active material added to the pre-dispersion solution of the second active material can be adjusted by considering the contents of the first active material and the second active material in the finally prepared positive electrode active material 100.

[0113] In some specific embodiments, the positive electrode paste may further contain components such as a binder and a conductive material. The binder and / or the conductive material can be added in two separate times according to the weight ratio of the first active material to the second active material when preparing the pre-dispersion solution and the paste. In some specific embodiments, the binder and / or the conductive material can be added in the form of a separate pre-dispersion solution. The detailed description of the binder and the conductive material is repeated with the above content, so the description is omitted.

[0114] <Formation of the positive electrode mixture layer>

[0115] In some specific embodiments, the positive electrode mixture layer can be formed by drying the positive electrode paste coated on at least one surface of the positive electrode current collector. The coating method of the positive electrode paste is not particularly limited. Exemplarily, the positive electrode paste can be coated by methods such as bar coating, casting, or spraying.

[0116] In some specific embodiments, the drying of the positive electrode paste can be carried out at 100°C to 200°C. Exemplarily, the drying of the positive electrode paste can be carried out at 130°C to 170°C.

[0117] Lithium secondary battery

[0118] The lithium secondary battery according to a specific embodiment includes the positive electrode for a lithium secondary battery according to any one of the above specific embodiments. Exemplarily, the lithium secondary battery may include a single cell, and the single cell includes the above positive electrode for a lithium secondary battery, a negative electrode, and a separator. The separator can be disposed between the positive electrode and the negative electrode in the single cell.

[0119] The negative electrode is not particularly limited. Exemplarily, the negative electrode may include a negative electrode current collector and a negative electrode mixture layer on at least one surface of the negative electrode current collector.

[0120] The composition of the negative electrode current collector is not particularly limited. Exemplarily, the negative electrode current collector may be a plate or foil formed of one or more of indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), and their alloys. In some specific embodiments, the negative electrode current collector may be a copper foil (Cu-foil).

[0121] The thickness of the negative electrode current collector is not particularly limited. Exemplarily, the thickness of the negative electrode current collector may be 0.1 - 50 μm.

[0122] The negative electrode mixture layer may contain a negative electrode active material. The negative electrode active material is not particularly limited. Exemplarily, the negative electrode active material may be one or more selected from the following: carbon-based materials such as crystalline carbon, amorphous carbon, carbon composites, and carbon fibers; lithium metal; lithium alloys; silicon-containing substances and tin-containing substances.

[0123] Exemplarily, the crystalline carbon may be graphite-based carbon such as natural graphite, artificial graphite, graphitized coke, graphitized mesocarbon microbead (MCMB), and graphitized mesophase pitch-based carbon fiber (MPCF).

[0124] Exemplarily, the amorphous carbon may be hard carbon, soft carbon, coke, mesocarbon microbead (MCMB), or mesophase pitch-based carbon fiber (MPCF).

[0125] Exemplarily, the elements contained in the lithium alloy may be aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium.

[0126] The silicon-containing substance is not particularly limited as long as it contains silicon, and the silicon-containing substance may be an active material that can be alloyed with lithium (Li). Exemplarily, the silicon-containing substance may be one or more selected from silicon (Si), silicon oxide (SiO x , 0 < x < 2), metal-doped silicon oxide (SiO x , 0 < x < 2), carbon-coated silicon oxide (SiO x , 0 < x < 2), silicon-carbon composite (Si-C), and silicon alloy.

[0127] The negative electrode mixture layer may further contain a binder. The binder is not particularly limited. Exemplarily, the binder may be a rubber-based binder such as styrene-butadiene rubber (SBR), fluoro-based rubber, ethylene-propylene rubber, butadiene rubber, isoprene rubber, silalkyl rubber, etc.; a cellulose-based binder such as carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose, methyl cellulose or alkali metal salts thereof; and any combination thereof.

[0128] The negative electrode mixture layer may further contain a conductive material. The conductive material is not particularly limited. Exemplarily, the conductive material may be one or more selected from particulate carbon materials and fibrous carbon materials. The particulate carbon material may be carbon black such as Super-P, Super-C, acetylene black, Ketjen black, etc., and the fibrous carbon material may be carbon fiber, carbon nanotube (CNT), vapor grown carbon fiber (VGCF), etc.

[0129] The separator is not particularly limited. Exemplarily, the separator may include a porous polymer film made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer, etc. In addition, the separator may further include a non-woven fabric formed of glass fibers with a high melting point, polyethylene terephthalate fibers, etc.

[0130] In some specific embodiments, the lithium secondary battery can be manufactured by accommodating the above-mentioned single cell in a soft package as a battery case and then injecting an electrolyte.

[0131] The electrolyte may contain an organic solvent and a lithium salt. The organic solvent serves as a medium for enabling the migration of ions participating in the electrochemical reaction of the battery. Exemplarily, the organic solvent may be used alone or a mixture of two or more of the following solvents: carbonate-based solvents, ester-based solvents, ether-based solvents, ketone-based solvents, alcohol-based solvents or aprotic solvents. When using a mixture of two or more solvents, the mixing ratio can be appropriately adjusted according to the target battery performance.

[0132] The lithium salt is a substance that is dissolved in an organic solvent and serves as a source of lithium ions in the battery, enables the basic operation of the lithium secondary battery, and promotes the migration of lithium ions between the positive electrode and the negative electrode. As the lithium salt, a known substance can be used at a concentration suitable for the purpose. The electrolyte may further contain known solvents as needed to improve charge-discharge characteristics, flame retardant characteristics, etc., and may contain known additives.

[0133] In some specific embodiments, the single cell can include a solid electrolyte between the positive electrode and the negative electrode and does not include a separator. The solid electrolyte is not particularly limited. Exemplarily, the solid electrolyte can be an oxide-based solid electrolyte, a sulfide-based solid electrolyte, or a polymer-based solid electrolyte.

[0134] Example

[0135] 1. Manufacture of positive electrode for lithium secondary battery

[0136] 1) Example 1 and Example 2

[0137] (1) Preparation of first active material

[0138] Add deoxygenated distilled water to a closed reactor, and further add NiSO4·6H2O and MnSO4·H2O with a molar ratio of 36.5:63.5. Then, further add a precipitating agent (NaOH) and a chelating agent (NH4OH) to the reactor, and perform a coprecipitation reaction for 72 hours to prepare metal hydroxide particles.

[0139] Dry the metal hydroxide particles at 100 °C for 12 hours, and then dry them again at 120 °C for 12 hours. Add the dried metal hydroxide particles and lithium carbonate to a dry mixer to prepare a mixture. Adjust the mixing ratio of the metal hydroxide particles and the lithium carbonate so that the finally prepared first active material in the form of lithium metal oxide particles satisfies the composition according to the following ICP analysis.

[0140] Load the mixture into a calcination furnace, raise the temperature of the calcination furnace to 250 °C at a rate of 2 °C / minute, and hold it at 250 °C for 4 hours for primary calcination. After the primary calcination, raise the temperature of the calcination furnace to 900 °C at a rate of 2 °C / minute, and hold it at 900 °C for 9 hours for secondary calcination.

[0141] After completion of the calcination, allow the calcined product to cool naturally to room temperature, and perform pulverization and classification to prepare lithium-rich oxide particles as the first active material. As a result of analyzing the lithium-rich oxide particles using inductively coupled plasma (ICP) (normalizing the number of oxygen atoms to 2), it was confirmed that the chemical formula of the lithium-rich oxide particles is Li 1.13 Ni 0.32 Mn 0.55 O2, which corresponds to a lithium-rich manganese (LMR) active material as a compound represented by the following Chemical Formula 1.

[0142] [Chemical Formula 1]

[0143] Lia [M x Ni y Mn z O b

[0144] In the Chemical Formula 1, M is at least one element selected from Co, Mg, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, and 0 ≤ x ≤ 0.9, 0 ≤ y ≤ 0.9, x + y > 0, 0.1 ≤ z ≤ 0.9, 1.8 ≤ a + x + y + z ≤ 2.2, 1.05 ≤ a / (x + y + z) ≤ 1.95, 1.8 ≤ b ≤ 2.2.

[0145] In addition, the BET analysis (Brunauer - Emmett - Teller Analysis) was performed on the first active material to measure the specific surface area. Specifically, the specific surface area was calculated by measuring the nitrogen adsorption amount of the first active material at the liquid nitrogen temperature (77K) using a specific surface area analyzer (Micromeritics, ASAP™ 2420). As a result, the BET specific surface area of the first active material was 1.63 m² / g (Example 1) and 0.91 m² / g (Example 2).

[0146] Furthermore, as a result of analyzing the average particle size (D50) of the first active material using the laser diffraction method (Microtrac MT3000), it was confirmed that the average particle size (D50) of the first active material was 10.1 μm (Example 1) and 6.8 μm (Example 2).

[0147] (2) Preparation of the second active material

[0148] Lithium carbonate (Li2CO3) as a lithium source, iron phosphate (FePO4) as a metal phosphate, polyethylene as a first carbon source, and glucose as a second carbon source were added to distilled water, and the particles were mixed and pulverized by a ball mill to form a mixed solution containing LiFePO4. The formed mixed solution containing LiFePO4 was dried using a spray dryer in the form of a micro nozzle.

[0149] Under a nitrogen atmosphere, at approximately 600 °C to 750 °C, the dried powder is calcined for 5 hours to 12 hours, and then subjected to a classification and iron removal process to prepare a lithium iron phosphate (LFP)-based active material as the second active material. The LFP-based active material is in the form of composite particles having a carbon coating formed on lithium metal phosphate particles represented by the chemical formula LiFePO4. As a result of analyzing the average particle diameter (D50) of the second active material using the same method as for the first active material, it was confirmed that the average particle diameter (D50) of the second active material was 0.7 μm.

[0150] (3) Fabrication of the positive electrode

[0151] The second active material is mixed with N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a pre-dispersion solution having a solids content of 62 wt%, and the first active material is added to the pre-dispersion solution to prepare a slurry having a solids content of 66 wt%. At this time, the addition of the first active material makes the weight ratio of the first active material to the second active material 80:20. In addition, when preparing the pre-dispersion solution and the slurry, a predetermined content of a conductive material (carbon black) and a binder (polyvinylidene fluoride, PVDF) are added in two portions according to the weight ratio of the first active material to the second active material and mixed to prepare a positive electrode slurry.

[0152] The positive electrode slurry is coated on one side of a positive electrode current collector (aluminum foil) having a thickness of 15 μm at 12 mg / cm² and dried at 150 °C to form a positive electrode mixture layer. Thereafter, the positive electrode current collector and the positive electrode mixture layer are rolled to a density of 2.7 g / cubic centimeter (cc) to fabricate a positive electrode containing a core-shell structured positive electrode active material, the core-shell structure being a structure in which a shell containing the second active material is coated on the surface of a core containing the first active material. At this time, based on the solids content, the contents of the positive electrode active material, the binder, and the conductive material contained in the positive electrode mixture layer are 92 wt%, 3 wt%, and 5 wt%, respectively.

[0153] In addition, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis images of the cross-section of the positive electrode active material according to Example 1 are as Figure 2a and Figure 2b shown. At this time, elemental mapping of nickel (Ni) and iron (Fe) is performed using EDS, Figure 2a the bright part of Figure 2b corresponds to the first active material (LMR), Figure 2a and Figure 2b, it can be confirmed that a core-shell structure in which the second active material uniformly covers the first active material on the surface of the first active material is well formed.

[0154] 2) Example 3

[0155] The positive electrode of Example 3 was manufactured by the same method as in Example 1, except that the positive electrode paste was prepared without pre-dispersing the second active material.

[0156] 3) Example 4

[0157] The positive electrode of Example 4 was manufactured by the same method as in Example 1, except that the average particle size (D50) of the first active material was 3.8 μm and the average particle size (D50) of the second active material was 7.7 μm.

[0158] 4) Example 5

[0159] The positive electrode of Example 5 was manufactured by the same method as in Example 1, except that the weight ratio of the first active material to the second active material was 60:40.

[0160] In addition, the scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis images of the cross-section of the positive electrode active material of Example 5 are as Figure 3a and Figure 3b shown. At this time, element mapping of nickel (Ni) and iron (Fe) was performed using EDS, Figure 3a the bright part of Figure 3b corresponds to the first active material (LMR), Figure 3a and Figure 3b the bright part of

[0161] 5) Comparative Example 1

[0162] The first active material was used as the positive electrode active material of Comparative Example 1.

[0163] 6) Comparative Example 2

[0164] The positive electrode of Comparative Example 2 was manufactured by the same method as in Example 1, except that the positive electrode paste was prepared such that the weight ratio of the first active material to the second active material was 20:80.

[0165] 2. Evaluation of Performance (DSC Analysis)

[0166] For the positive electrode for a lithium secondary battery manufactured as described above, the thermal stability was evaluated by performing differential scanning calorimetry (DSC) analysis. Specifically, a differential scanning calorimeter (Mettler Toledo, model: DSC 1 STAR System) was used to evaluate the heat generation amount of the charged positive electrode, and the results are shown in Table 1 below together with the discharge capacity of the positive electrode active material. At this time, the charged positive electrode was prepared by the following method.

[0167] First, a coin-type half-cell including positive electrodes of examples and comparative examples and using lithium metal as the counter electrode was manufactured. The coin-type half-cell was charged to 4.6 V at a current of 0.1 C and then discharged to 2.0 V, and then charged again to 4.6 V, and then the battery was disassembled, and the obtained charged positive electrode was washed and dried to prepare a sample for DSC analysis.

[0168] [Table 1]

[0169]

[0170] In Table 1, "O" indicates that the second active material was pre-dispersed, and "X" indicates that the second active material was not pre-dispersed.

[0171] Referring to Table 1, the positive electrode of Comparative Example 1 contains only the first active material (LMR) and does not contain the second active material (LFP) with excellent thermal stability, thus showing the highest DSC heat generation amount. Therefore, it was judged that the thermal stability of the positive electrode of Comparative Example 1 was relatively reduced.

[0172] In addition, the positive electrode of Comparative Example 2 contains a relatively large amount of the second active material (LFP) with excellent thermal stability, thus showing the lowest DSC heat generation amount, but the content of the first active material (LMR) is relatively low, so the discharge capacity is more than 20% lower than that of Examples 1 to 4.

[0173] On the other hand, in the positive electrodes of Examples 1 to 5 that contain both the first active material and the second active material and the content of the first active material is adjusted to be higher than the content of the second active material, compared with the positive electrode of Comparative Example 1, it shows a relatively low heat generation amount, and compared with the positive electrode of Comparative Example 2, it shows a higher discharge capacity.

[0174] Among them, the positive electrode of Example 3 in which the positive electrode paste is prepared without pre-dispersing the second active material (LFP) shows a relatively high DSC calorific value. It is judged that this is because when the second active material is not pre-dispersed, agglomeration occurs between the second active material particles, resulting in a weakened effect of the second active material (LFP) coating the first active material (LMR). Therefore, compared with Example 1 and Example 2, the core-shell structure of the positive electrode active material cannot be formed relatively well.

[0175] In addition, the positive electrode of Example 4 in which the particle size of the second active material (LFP) is larger than the particle size of the first active material (LMR) also shows a relatively high DSC calorific value. It is judged that this is because when the particle size of the first active material (LMR) corresponding to the core is smaller than the particle size of the second active material (LFP), the core-shell structure in which the second active material (LFP) coats the first active material (LMR) cannot be formed relatively well.

[0176] In addition, in the positive electrode of Example 5 where the weight ratio of the first active material (LMR) to the second active material (LFP) is 60:40, since it contains a relatively larger amount of the second active material with excellent thermal stability, the DSC calorific value is about 18% lower than that of Example 1, but the discharge capacity is reduced by about 7% compared with Example 1.

[0177] Therefore, it is judged that when the first active material is added to the pre-dispersed solution of the second active material as in Example 1 and Example 2 and the particle size of the first active material is adjusted to be larger than the particle size of the second active material, a positive electrode active material with a well-formed core-shell structure can be included, and thus a high-capacity positive electrode with further improved thermal stability can be manufactured.

Claims

1. A positive electrode active material, the positive electrode active material comprising: a core containing a first active material which is a lithium-rich oxide; and a shell provided on the surface of the core, and the shell containing a second active material having an olivine structure, Among them, Based on the total weight of the positive electrode active material, the weight percentage of the first active material is greater than or equal to the weight percentage of the second active material.

2. The positive electrode active material according to claim 1, wherein, The first active material is represented by the following Chemical Formula 1, [Chemical Formula 1] Li a [M x Ni y Mn z O b In Chemical Formula 1, M is at least one element selected from Co, Mg, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, and 0 ≤ x ≤ 0.9, 0 ≤ y ≤ 0.9, x + y > 0, 0.1 ≤ z ≤ 0.9, 1.8 ≤ a + x + y + z ≤ 2.2, 1.05 ≤ a / (x + y + z) ≤ 1.95, 1.8 ≤ b ≤ 2.

2.

3. The positive electrode active material according to claim 1, wherein, The second active material is represented by the following Chemical Formula 2, [Chemical Formula 2] LiMePO4 In Chemical Formula 2, Me is at least one element selected from Co, Ni, Fe, and Mn.

4. The positive electrode active material according to claim 1, wherein, Based on the total weight of the positive electrode active material, the weight ratio of the first active material to the second active material is 60:40 to 95:

5.

5. The positive electrode active material according to claim 1, wherein, The BET specific surface area of the first active material is 0.9 m² / g to 2.0 m² / g.

6. The positive electrode active material according to claim 1, wherein, The average particle size D50 of the first active material is greater than or equal to the average particle size D50 of the second active material.

7. The positive electrode active material according to claim 6, wherein, The average particle size D50 of the first active material is 5 μm to 12 μm.

8. The positive electrode active material according to claim 6, wherein, The average particle size D50 of the second active material is 0.3 μm to 5 μm.

9. A positive electrode for a lithium secondary battery, comprising the positive electrode active material according to any one of claims 1 to 8.

10. A method for manufacturing a positive electrode for a lithium secondary battery, comprising the following steps: Preparing a positive electrode paste, the positive electrode paste comprising a first active material which is a lithium-rich oxide and a second active material having an olivine structure; And Forming a positive electrode mixture layer on at least one surface of a positive electrode current collector with the positive electrode paste, wherein the positive electrode mixture layer comprises the positive electrode active material according to any one of claims 1 to 8.

11. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 10, wherein, The first active material is represented by the following Chemical Formula 1, [Chemical Formula 1] Li a [M x Ni y Mn z O b In Chemical Formula 1, M is at least one element selected from Co, Mg, Ti, Al, Fe, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, and 0 ≤ x ≤ 0.9, 0 ≤ y ≤ 0.9, x + y > 0, 0.1 ≤ z ≤ 0.9, 1.8 ≤ a + x + y + z ≤ 2.2, 1.05 ≤ a / (x + y + z) ≤ 1.95, 1.8 ≤ b ≤ 2.

2.

12. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 10, wherein, The second active material is represented by the following Chemical Formula 2, [Chemical Formula 2] LiMePO4 In Chemical Formula 2, Me is at least one element selected from Co, Ni, Fe, and Mn.

13. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 10, wherein, The positive electrode paste is prepared by adding the first active material to a pre-dispersed solution of the second active material.

14. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 13, wherein, The solid content of the pre-dispersion solution is 50% to 70% by weight.

15. A lithium secondary battery comprising the positive electrode for a lithium secondary battery according to claim 9.

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