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

Abstract

Provided are a positive electrode active material for lithium secondary batteries that has improved packing density, improved energy density, and a reduced specific surface area of ​​particles compared to conventional polycrystalline OLO, a method for producing the same, and a lithium secondary battery including the same. [Solution] The positive electrode active material for a lithium secondary battery of the present invention contains an overlithiated layered oxide (OLO), and the overlithiated layered oxide contains primary particles with an average particle size of 300 nm to 10 μm in an amount of 50 to 100% by volume relative to the entire overlithiated layered oxide.

Description

Technical Field

[0001] The present invention relates to a positive electrode active material containing an overlithiated layered oxide (OLO), and more particularly, to a positive electrode active material for a lithium secondary battery in which the size of primary particles is adjusted by a dopant acting as a flux for growing primary particles, a method for manufacturing the same, and a lithium secondary battery including the same.

Background Art

[0002] With the development of mobile electronic devices such as smartphones, MP3 players, and tablet PCs, the demand for secondary batteries capable of storing electrical energy has been rapidly increasing. In particular, electric vehicles, medium- and large-sized energy storage systems, and portable devices requiring high energy density have emerged, and the demand for lithium secondary batteries is increasing.

[0003] In recent years, as a positive electrode active material that has been attracting the most attention, there is lithium nickel manganese cobalt oxide Li(Ni x Co y Mn2)O2 (where x, y, and z are the atomic fractions of the oxide composition elements, respectively, and satisfy <0 < x ≦ 1, 0 < y ≦ 1, 0 < z ≦ 1, and 0 < x + y + z ≦ 1>). Since this material is used at a higher voltage than LiCoO2, which has been actively studied and used as a positive electrode active material, it has the advantage of exhibiting a high capacity, and also has the advantage of being relatively inexpensive because the Co content is relatively low. However, it has the disadvantage that the rate capability and the life characteristics at high temperatures are not sufficient.

[0004] Therefore, research has been conducted to apply an overlithiated layered oxide (OLO) that exhibits a high reversible capacity and exceeds existing Li(Ni x Co y Mn2)O2 to a lithium secondary battery.

[0005] However, the voltage decay phenomenon that occurs during the life cycle is a problem. This is due to the phase transition from a spinel-like structure to cubic due to the movement of transition metals during the life cycle. Such a voltage decay phenomenon is an issue that must be resolved for the commercialization of lithium secondary batteries. Also, the problem of low packing density needs to be improved. Summary of the Invention Problems to be Solved by the Invention

[0006] The positive electrode active material for a secondary battery containing an overlithiated layered oxide according to an embodiment of the present invention aims to increase the energy density and decrease the specific surface area of particles by adjusting the growth of primary particles compared to conventional polycrystalline OLO.

[0007] Furthermore, the present invention aims to provide a dopant material for improving the stability of the internal structure of the positive electrode active material particles. Means for Solving the Problems

[0008] The overlithiated layered oxide (OLO) according to an embodiment of the present invention is represented by the following [Chemical Formula 1], where primary particles aggregate to form secondary particles, and the primary particles having a size of 300 nm to 10 μm are 50 to 100% by volume of all the primary particles constituting the secondary particles.

[0009] [Chemical Formula 1] JPEG2 (where 0 < r ≤ 0.6, 0 < a ≤ 1, 0 ≤ x ≤ 1, 0 ≤ y < 1, 0 ≤ z < 1, and 0 < x + y + z < 1, and M1 is at least one selected from Na, K, Mg, Al, Fe, Cr, Y, Sn, Ti, B, P, Zr, Ru, Nb, W, Ba, Sr, La, Ga, Mg, Gd, Sm, Ca, Ce, Fe, Al, Ta, Mo, Sc, V, Zn, Nb, Cu, In, S, B, and Bi).)

[0010] In the positive electrode active material according to the embodiment of the present invention, the primary particles have an irregular shape, and the size of the primary particles refers to the longest length.

[0011] In the positive electrode active material according to the embodiment of the present invention, the size of the primary particles in the positive electrode active material state is larger than the size of the primary particles in the precursor state, and the ratio of (the size of the primary particles of the positive electrode active material with added dopant) / (the size of the primary particles of the positive electrode active material without added dopant) is 1 or more, preferably 50 or more.

[0012] The positive electrode active material according to the embodiment of the present invention may contain primary particles having a size of 1 μm to 2 μm in a content of 50 to 100% by volume based on the entire lithium-excess layered oxide.

[0013] Furthermore, the secondary particles of the positive electrode active material according to the embodiment of the present invention may have an average particle size of 2 μm to 20 μm.

[0014] In addition, in the positive electrode active material according to an embodiment of the present invention, M1 in [Chemical Formula 1] may be a dopant that acts as a flux that induces the growth of the primary particles in the lithium-excess layered oxide.

[0015] In addition, in the positive electrode active material according to the embodiment of the present invention, M1 in [Chemical Formula 1] is at least one selected from Nb, Ta, Mo, and W, and M1 may be Nb or Ta.

[0016] In the positive electrode active material according to the embodiment of the present invention, M1 may be contained in an amount of 0.001 to 10 mol % based on the total amount of the lithium-excess layered oxide.

[0017] In addition, in the positive electrode active material according to the embodiment of the present invention, M1 is Nb, and the Nb can be contained in an amount of 0.1 to 1 mol% based on the entire lithium-excess layered oxide.

[0018] In addition, the positive electrode active material according to the embodiment of the present invention is [Chemical Formula 2] Li a M1'O b (where 0 < a ≤ 7, 0 < b ≤ 15, and M1' is at least one selected from Ba, Sr, B, P, Y, Zr, Nb, Mo, Ta, and W.) can be further included. The Li of [Chemical Formula 2] a M1'O b can be a substance generated by the reaction of a dopant that induces growth between primary particles with lithium.

[0019] In addition, with the growth of primary particles, in the XRD analysis of the positive electrode active material according to the embodiment of the present invention, the full width at half maximum (FWHM (deg.)) at I(104) can be reduced by 5 to 50% when M1 is included compared to a substance not containing M1 when fired under the same conditions.

[0020] In addition, the energy density (Wh / L) per volume of the positive electrode active material according to the embodiment of the present invention can be 2.7 to 4.0 (Wh / L).

[0021] In addition, the energy density (Wh / L) per volume of the positive electrode active material according to the embodiment of the present invention can be increased by 5 to 30% compared to a substance not containing M1.

[0022] In addition, the packing density (g / cc) of the positive electrode active material according to the embodiment of the present invention can be 2.0 to 4.0 (g / cc). <00001​​​​​​​​​In addition, the specific surface area (BET, m 2 / g) can decrease by 25 to 80% compared to a material that does not contain M1 as the primary particles grow.

[0025] In addition, in the positive electrode active material according to the embodiment of the present invention, the ratio of the number of moles of lithium to the total number of moles of a metal selected from Ni, Co, or Mn (Li / (Ni+Co+Mn)) may be 1.1 to 1.6.

[0026] In the positive electrode active material according to the embodiment of the present invention, the ratio of the number of moles of Mn to the total number of moles of Ni (Mn / Ni) can be 1 to 4.5.

[0027] In addition, the positive electrode active material according to the embodiment of the present invention is a solid solution phase in which Li2MnO3 having a monoclinic structure and LiMO2 having a rhombohedral structure are mixed, and M may be at least one selected from Ni, Co, Mn, and M1.

[0028] In addition, the positive electrode active material according to the embodiment of the present invention can exhibit a plateau due to Li2MnO3 in the 4.4 V region of the initial charge-discharge profile.

[0029] The method for producing a cathode active material according to an embodiment of the present invention includes the steps of: producing a cathode active material precursor containing at least one element selected from Ni, Co, and Mn; and mixing the cathode active material precursor with a lithium compound and a compound containing M1 in Chemical Formula 1, followed by firing the mixture.

[0030] In the method for producing a positive electrode active material according to the embodiment of the present invention, the size of the primary particles in the positive electrode active material state is larger than the size of the primary particles in the precursor state, and the ratio of (the size of the primary particles of the positive electrode active material with added dopant) / (the size of the primary particles of the positive electrode active material without added dopant) is 1 or more, preferably 50 or more.

[0031] In the method for manufacturing a positive electrode active material according to the embodiment of the present invention, the temperature of the firing step may be 750 to 950°C.

[0032] In addition, the method for preparing a cathode active material according to an embodiment of the present invention may further include a step of roasting the obtained precursor after the step of preparing the precursor and before the step of calcining, and the temperature of the roasting step may be 300 to 600°C.

[0033] In the method for producing a positive electrode active material according to the embodiment of the present invention, M1 is Nb, and the compound containing Nb is Nb2O5.

[0034] In addition, the method for manufacturing a positive electrode active material according to an embodiment of the present invention may further include, after the calcination step, washing and drying the calcined positive electrode active material.

[0035] In addition, the method for manufacturing a positive electrode active material according to an embodiment of the present invention may further include, after the firing step, a step of heat-treating the fired positive electrode active material.

[0036] A secondary battery according to an embodiment of the present invention includes the positive electrode active material. [Effects of the Invention]

[0037] The lithium-excess layered oxide-containing cathode active material for secondary batteries according to an embodiment of the present invention contains a dopant material for improving the stability of the internal structure of the cathode active material particles. This allows the primary particles to be single-crystallized, thereby improving the packing density, energy density, and specific surface area of ​​the particles, compared to conventionally known polycrystalline lithium-excess layered oxide (OLO).

[0038] In addition, the secondary battery including the cathode active material has a reduced specific surface area compared to a battery using a conventionally known polycrystalline lithium-excess cathode active material (OLO), thereby significantly reducing problems of lifespan and voltage drop due to a reduced surface area of ​​the cathode active material. [Brief explanation of the drawings]

[0039] [Figure 1] 1 is a SEM analysis image of positive electrode active materials according to comparative examples and examples of the present invention. [Figure 2A] 2a and 2b are SEM analysis images of cross sections of positive electrode active materials according to comparative examples and examples of the present invention. [Figure 2B] 2a and 2b are SEM analysis images of cross sections of positive electrode active materials according to comparative examples and examples of the present invention. [Figure 3A] 3a and 3b are diagrams showing the results of EDX analysis of the positive electrode active materials according to the comparative example and the example of the present invention. [Figure 3B] 3a and 3b are diagrams showing the results of EDX analysis of the positive electrode active materials according to the comparative example and the example of the present invention. [Figure 4] FIG. 2 is a diagram showing the results of XRD analysis of positive electrode active materials according to comparative examples and examples of the present invention. [Figure 5] FIG. 2 is a diagram showing the results of XRD analysis of positive electrode active materials according to comparative examples and examples of the present invention. [Figure 6] 1 is a graph comparing the results of X-ray diffraction (XRD) analysis and full width at half maximum (FWHM (deg.)) at I(104) of positive electrode active materials according to comparative examples and examples of the present invention. [Figure 7]1 is a graph comparing packing densities (g / cc) of positive electrode active materials according to comparative examples and examples of the present invention. [Figure 8] 1 is a graph comparing the specific surface area (BET, m 2 / g) of positive electrode active materials according to comparative examples and examples of the present invention. [Figure 9] 1 is a graph comparing initial voltage profiles of positive electrode active materials according to comparative examples and examples of the present invention. [Figure 10] 1 is a graph comparing the energy density per volume of positive electrode active materials according to comparative examples and examples of the present invention. [Figure 11] 1 is a graph comparing the capacity retention rate according to the cycle number of positive electrode active materials according to comparative examples and examples of the present invention. [Figure 12] 1 is a graph comparing the capacity of positive electrode active materials according to comparative examples and examples of the present invention as a function of the number of cycles. [Figure 13] 1 is a graph comparing voltage retention rates according to cycle numbers of positive electrode active materials according to comparative examples and examples of the present invention. [Figure 14] 1 is a graph comparing the nominal voltage according to the cycle number of positive electrode active materials according to comparative examples and examples of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0040] The term "comprises" as used herein should be understood as an open-ended term that encompasses the possibility of including other embodiments.

[0041] As used herein, the terms "preferred" and "preferably" refer to embodiments of the invention that may provide certain advantages, under certain circumstances, and do not exclude other embodiments from the scope of the invention.

[0042] Hereinafter, the positive electrode active material according to an embodiment of the present invention will be described in detail.

[0043] The positive electrode active material according to an embodiment of the present invention includes an overlithiated layered oxide (OLO).

[0044] The overlithiated layered oxide can be a solid solution phase in which Li2MnO3 having a monoclinic structure and LiMO2 having a rhombohedral structure are mixed, and M can be at least one or more selected from Ni, Co, Mn, and M1.

[0045] Also, the overlithiated layered oxide according to an embodiment of the present invention can have a flat plateau due to Li2MnO3 in the region of 4.4V in the initial charge-discharge profile. In the process of initial charging, the Li2MnO3 phase is electrochemically inactive with respect to lithium up to the region of 4.4V, and at 4.4V or more, a reaction in which lithium desorbs from the Li2MnO3 phase and oxygen evolution may occur.

[0046] The overlithiated layered oxide according to an embodiment of the present invention is represented by the following [Chemical Formula 1]. [Chemical Formula 1] JPEG2026041917000003.jpg7159 (where 0 < r ≤ 0.6, 0 < a ≤ 1, 0 ≤ x ≤ 1, 0 ≤ y < 1, 0 ≤ z < 1, and 0 < x + y + z < 1, and M1 is at least one or more selected from Na, K, Mg, Al, Fe, Cr, Y, Sn, Ti, B, P, Zr, Ru, Nb, W, Ba, Sr, La, Ga, Mg, Gd, Sm, Ca, Ce, Fe, Al, Ta, Mo, Sc, V, Zn, Nb, Cu, In, S, B, and Bi).

[0047] The molar percentage of lithium relative to the total number of moles of the metal selected from Ni, Co, or Mn (Li / (Ni+Co+Mn)) can be 1.1 to 1.6.

[0048] In the lithium-excess layered oxide represented by Chemical Formula 1, the ratio of the number of moles of lithium to the total number of moles of metals selected from Ni, Co, and Mn (Li / (Ni+Co+Mn)) can be 1.1 to 1.6, 1.2 to 1.6, 1.2 to 1.5, 1.2 to 1.4, or 1.2 to 1.3.

[0049] In the above [Chemical Formula 1], the value of x can be greater than 0 to 0.5, greater than 0 to 0.4, greater than 0 to 0.3, greater than 0 to 0.2, or greater than 0 to 0.1.

[0050] In the above [Chemical Formula 1], the value of y can be greater than 0 to 0.5, greater than 0 to 0.4, greater than 0 to 0.3, greater than 0 to 0.2, or greater than 0 to 0.1.

[0051] The ratio of the number of moles of Mn to the total number of moles of Ni (Mn / Ni) can be 1-4.5, 1-4, 2-4.5, 2-4, 3-4.5, or 3-4.

[0052] The cathode active material according to the present invention is an oxide rich in lithium and manganese, and by adjusting the ratio of the Mn and Li contents to the grain boundary density within a predetermined range, it is possible to efficiently improve problems such as density and voltage drop.

[0053] The oxide of the present invention can have a layered structure in which lithium atomic layers and metal atomic layers of Ni, Co, Mn, or M1 are alternately stacked with oxygen atomic layers interposed therebetween.

[0054] The planes constituting the layers of the layered structure of the positive electrode active material may have a crystal orientation in a direction perpendicular to the C-axis. In this case, the mobility of lithium ions contained in the positive electrode active material is improved, and the structural stability of the positive electrode active material is increased. Therefore, when the positive electrode active material is applied to a battery, the initial capacity characteristics, output characteristics, resistance characteristics, and long-life characteristics can be improved.

[0055] Also, for example, the positive electrode active material including the lithium-excess layered oxide according to the present invention may have a single-crystal structure.

[0056] In the positive electrode active material according to the embodiment of the present invention, primary particles may be aggregated to form secondary particles, and the size of the primary particles may be 0.01 to 10 μm.

[0057] In addition, in the positive electrode active material according to the embodiment of the present invention, the primary particles having a size of 300 nm to 5 μm may be adjusted to 50 to 100 volume %, 70 to 100 volume %, or 100 volume % of all the primary particles constituting the secondary particles.

[0058] In addition, in the positive electrode active material according to the embodiment of the present invention, the primary particles having a size of 300 nm to 10 μm may be adjusted to 50 to 100 vol%, 70 to 100 vol%, or 100 vol% of the total primary particles constituting the secondary particles.

[0059] As another example, the positive electrode active material may be adjusted so that primary particles having a size of more than 500 nm and up to 10 μm account for 50 to 100 volume %, 70 to 100 volume %, or 100 volume % of all primary particles constituting the secondary particles.

[0060] As an example, the positive electrode active material may contain primary particles having a size of 1 μm to 2 μm in a content of 50 to 100% of the total primary particles constituting the secondary particles.

[0061] As another example, the positive electrode active material may have primary particles having a size of 1 μm to 10 μm adjusted to 50 to 100 vol%, 70 to 100 vol%, or 100 vol% of the lithium-excess layered oxide as a whole.

[0062] For example, the positive electrode active material may have primary particles having a size greater than 1 μm in a volume ratio of 50 to 100%, 70 to 100%, or 100% by volume of the lithium-excess layered oxide.

[0063] For example, the positive electrode active material may have primary particles having a size of 2 μm or more in an amount of 50 to 100% by volume, or less than 50 to 70% by volume, based on the total volume of the lithium-excess layered oxide.

[0064] The size of the primary particle means the longest length of the particle.

[0065] For example, the average particle size of the primary particles of the positive electrode active material may be adjusted to be greater than 500 nm and 10 μm, or 1 μm to 10 μm.

[0066] In the present invention, the average particle size is defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. The average particle size can be measured, for example, by the laser diffraction method.

[0067] In the present invention, by increasing the size of the primary particles in the lithium-excess layered oxide and adjusting it to have a single crystal structure, when calcined under the same conditions, the full width at half maximum (FWHM (deg.)) at I(104) in XRD analysis can be adjusted so that when M1 is contained, the full width at half maximum (FWHM (deg.)) at I(104) is reduced by 5 to 25%, 5 to 20%, 10 to 25%, or 10 to 20% compared to a comparative example not containing M1.

[0068] In addition, the positive electrode active material according to the present invention can be adjusted to have a single crystal structure by increasing the size of the primary particles in the lithium-excess layered oxide, and thereby the energy density per volume (Wh / L) can be adjusted to increase by 5 to 25%, 5 to 20%, 10 to 25%, or 10 to 20% when M1 is contained compared to a comparative example not containing M1.

[0069] In addition, the positive electrode active material according to the present invention has a specific surface area (BET, m 2 / g) can be adjusted to decrease by 20 to 80% when M1 is contained compared to a comparative example not containing M1.

[0070] Previously, lithium-rich layered oxides have had the problem of voltage drop during cycling. The voltage drop is due to a phase transition from a spinel-like structure to a cubic structure caused by the migration of transition metals during cycling, and this phenomenon occurs primarily at the surface of the positive electrode active material. The present invention induces the growth of the primary particles and controls the positive electrode active material to have a single crystal structure, thereby increasing the energy density per volume and reducing the specific surface area, thereby reducing the surface area of ​​the positive electrode active material and resolving the problems of lifespan and voltage drop. In the present invention, the induction of primary particle growth encompasses all of the concepts of nucleation, ostwald ripening, and particle aggregation.

[0071] Furthermore, the reduction in the specific surface area makes it possible to solve the problem of side reactions occurring with the electrolyte.

[0072] In the positive electrode active material according to the present invention, the larger the portion corresponding to the single crystal structure, the more the problem of voltage drop occurring in polycrystals can be improved.

[0073] The cathode active material according to the present invention includes M1 in Chemical Formula 1 as a dopant that induces interparticle growth. More preferably, M1 is a dopant that acts as a flux that induces interparticle growth in the lithium-excess layered oxide and can be doped into the lattice structure. As an example, the size of the primary particles can be increased by adding and mixing the flux dopant during the calcination step with the lithium compound and then performing a heat treatment together. "Acting as a flux" means that the dopant can act as a dopant that increases the size of the primary particles by interparticle growth.

[0074] The M1 is at least one selected from Na, K, Mg, Al, Fe, Cr, Y, Sn, Ti, B, P, Zr, Ru, Nb, W, Ba, Sr, La, Ga, Mg, Gd, Sm, Ca, Ce, Fe, Al, Ta, Mo, Sc, V, Zn, Nb, Cu, In, S, B, and Bi, more preferably at least one selected from Ba, Sr, B, P, Y, Zr, Nb, Mo, Ta, and W, which can further grow the size of the primary particles and more appropriately adjust the size to a specific range, and most preferably at least one selected from Nb and Ta.

[0075] In the positive electrode active material according to the present invention, when a dopant element that induces inter-particle growth is mixed with a lithium compound in a calcination step and then heat-treated together, the surface area of ​​the positive electrode active material is reduced, thereby improving the problems of lifespan and voltage drop.

[0076] In the positive electrode active material according to the present invention, the dopant element is uniformly contained on the surface and inside of the positive electrode active material, thereby improving the structural stability of the positive electrode active material, and improving the life characteristics and thermal stability.

[0077] In the positive electrode active material according to the present invention, M1 can be contained in an amount of 0.01 to 3 mol%, based on the entire lithium-excess layered oxide. Further, it can be contained in an amount of 0.1 to 2 mol%, more preferably 0.1 to 1 mol%. When the dopant M1 contained as a flux for inducing the growth of primary particles exceeds the above range, the lithium composite oxide becomes excessive and causes a decrease in capacity and efficiency. When it is less than the above range, the effect of growing primary particles cannot be sufficiently obtained.

[0078] Further, the positive electrode active material for a secondary battery according to an embodiment of the present invention may further contain a lithium-excess layered oxide (overlithiated layered oxide: OLO) represented by the following [Chemical Formula 2]. [Chemical Formula 2] JPEG2026041917000004.jpg12159(In the formula, 0 < a ≤ 7 and 0 < b ≤ 15, and M1' is at least one selected from Ba, Sr, B, P, Y, Zr, Nb, Mo, Ta, and W.)

[0079] The Li in the above [Chemical Formula 2] a M1'O b can be a substance generated by the reaction of a dopant that induces the growth between primary particles with lithium.

[0080] When performing XRD analysis on the positive electrode active material according to an embodiment of the present invention, the full width at half maximum (FWHM (deg.)) at I(104) can be 0.1 to 2.45 (deg.), but the value can vary depending on the manganese content. Therefore, by adjusting the addition and content of the dopant M1 to adjust the reduction rate of the full width at half maximum, problems such as lifespan and voltage drop can be solved.

[0081] Further, the energy density (Wh / L) per volume of the positive electrode active material according to an embodiment obtained by adjusting the addition and content of the dopant M1 can be 2.7 to 4.0 (Wh / L).

[0082] In addition, the specific surface area (BET, m 2 / g) is 0.01 to 2 (BET, m 2 / g).

[0083] The positive electrode active material particles according to the embodiment of the present invention may have an average particle size of 0.1 to 30 μm, or 0.1 to 25 μm, or 0.1 to 20 μm, or 0.1 to 15 μm, or 0.1 to 10 μm.

[0084] The positive electrode active material including the lithium-excess layered oxide according to the embodiment of the present invention may have a structure in which primary particles grow to form secondary particles.

[0085] The particle form of the positive electrode active material may be fibrous, film-like, or spherical, and is preferably spherical.

[0086] In the present invention, the average particle size is defined as the particle size corresponding to 0% of the cumulative volume in the particle size distribution curve. The average particle size can be measured, for example, by the laser diffraction method.

[0087] The shape of the primary particles may be rod-like, plate-like, spherical, elliptical, disk-like, or irregular, and preferably the shape of the primary particles is at least one of plate-like and irregular.

[0088] In the positive electrode active material according to the embodiment of the present invention, the number of primary particles in the secondary particles can be adjusted to 1 to 10,000, 1 to 1,000, 1 to 100, or 1 to 10 by adjusting the size of the primary particles.

[0089] The positive electrode active material according to the embodiment of the present invention may further include a coating layer.

[0090] The coating layer may include, but is not limited to, one or more coating materials selected from P, Nb, Si, Sn, Al, Pr, Al, Ti, Zr, Fe, Al, Fe, Co, Ca, Mn, Ti, Sm, Zr, Fe, La, Ce, Pr, Mg, Bi, Li, W, Co, Zr, B, Ba, F, K, Na, V, Ge, Ga, As, Sr, Y, Ta, Cr, Mo, W, Mn, Ir, Ni, Zn, In, Na, K, Rb, Cs, Fr, Sc, Cu, Ru, Rh, Pd, Ag, Cd, Sb, Hf, Ta, Re, Os, Pt, Au, Pb, Bi, and Po.

[0091] The coating layer blocks contact between the positive electrode active material and the electrolyte contained in the lithium secondary battery, thereby suppressing the occurrence of side reactions, thereby improving the life characteristics and increasing the packing density, and the coating layer enables the positive electrode active material to function as a lithium ion conductor.

[0092] In addition, the coating layer may be formed between the grain boundaries of the primary particles.

[0093] The coating layer may have a thickness of 0.1 to 500 nm. The coating layer may be formed on the entire surface of the positive electrode active material or on part of the surface.

[0094] The coating layer may be in the form of a single layer coating, a double layer coating, a grain boundary coating, a uniform coating, or an island coating.

[0095] In the cathode active material including the lithium-excess layered oxide according to the embodiment of the present invention, the cathode active material may include a concentration gradient portion in which at least one of Ni, Co, Mn, and M1 exhibits a concentration gradient in the inside or outside of the cathode active material particle, i.e., at least a portion of the inside or outside of the secondary particle or the inside or outside of the primary particle.

[0096] In the positive electrode active material according to the embodiment of the present invention, lithium ion diffusion paths may be provided within the primary particles.

[0097] In the positive electrode active material according to the embodiment of the present invention, the surfaces forming the layers of the layered structure have a crystal orientation in a direction perpendicular to the C-axis within the primary particle, and a lithium ion migration path can be provided inside or outside the primary particle toward the center of the positive electrode active material particle.

[0098] Hereinafter, a method for manufacturing a positive electrode active material according to an embodiment of the present invention will be described in detail.

[0099] The method for manufacturing a positive electrode active material for a secondary battery according to an embodiment of the present invention includes a step of manufacturing a precursor of a positive electrode active material containing at least one element selected from the group consisting of Ni, Co, and Mn.

[0100] To produce the precursor, raw materials for nickel, cobalt, manganese, and dopants may be sulfates, nitrates, acetates, halides, hydroxides, or oxyhydroxides containing the respective metal elements, and any raw materials that can be dissolved in a solvent such as water may be used without any particular limitation.

[0101] The precursor can be prepared by co-precipitation, spray-drying, solid phase method, wet grinding, fluidized bed drying, vibration drying, etc.

[0102] The method for manufacturing the cathode active material for a secondary battery according to the embodiment of the present invention includes the steps of mixing the obtained precursor of the cathode active material with a lithium compound and a compound containing M1 in [Chemical Formula 1], which is a dopant contained as a flux for inducing growth between primary particles, and firing the mixture.

[0103] In the present invention, the dopant M1, a flux dopant, is added as a flux that induces growth between primary particles during the firing step with a lithium compound, thereby growing the primary particles and reducing the surface area of ​​the positive electrode active material, thereby solving the problems of lifespan and voltage drop.

[0104] In the method for producing a positive electrode active material for a secondary battery according to the present invention, the lithium compound may be, but is not limited to, a lithium-containing sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide, or oxyhydroxide.

[0105] The temperature of the firing step can be 750 to 950°C, 800 to 950°C, or 850 to 950°C.

[0106] In addition, the method may further include a step of roasting the obtained precursor after the precursor preparation step and before the firing step described below, and the temperature of the roasting step may be 300 to 600°C, 400 to 600°C, or 500 to 600°C.

[0107] The step of roasting the precursor may be performed by repeatedly raising and maintaining the temperature, or by raising the temperature, maintaining the temperature, cooling, raising the temperature again, maintaining the temperature, and cooling in this order.

[0108] After the calcination step, the method may further include washing the calcined positive electrode active material with water and drying it.

[0109] Furthermore, after performing the above steps, a step of forming a coating layer on the inside or outside of the positive electrode active material may be included, and the coating layer may be formed by a dry coating, wet coating, CVD coating, or ALD coating method. However, the coating method is not limited thereto as long as it can form a coating layer on a portion of the positive electrode active material.

[0110] Any of the above descriptions regarding the positive electrode active material can be applied to the method for producing the positive electrode active material.

[0111] The secondary battery according to the embodiment of the present invention includes the above-described positive electrode active material.

[0112] The positive electrode active material is as described above, and the binder, conductive material, and solvent are not particularly limited as long as they can be used on the positive electrode current collector of a secondary battery.

[0113] Specifically, the lithium secondary battery may include a positive electrode, a negative electrode facing the positive electrode, and an electrolyte between the positive electrode and the negative electrode, but is not particularly limited as long as it can be used as a secondary battery.

[0114] The present invention will be described in detail below with reference to examples, but the scope of the present invention is not limited to these examples.

[0115] Example 1: Preparation of positive electrode active material synthesis Spherical Ni was prepared using the co-precipitation method. 0.2 Co 0.1 Mn 0.7 The CO3 precursor was synthesized.

[0116] In a 90L reactor, 25 wt% NaCO3 and 28 wt% NH4OH were added to a 2.5M composite transition metal sulfate solution, which was a mixture of NiSO4·6H2O, CoSO4·7H2O, and MnSO4·H2O in a molar ratio of 20:10:70. The pH in the reactor was maintained at 8.0-11.0, and the temperature at 45-50°C. To prevent oxidation of the resulting precursor, N2, an inert gas, was introduced into the reactor. After synthesis and stirring, the mixture was washed and dehydrated using a filter press (F / P). Finally, the dehydrated product was dried at 120°C for 2 days and sieved through a 75μm (200 mesh) sieve to separate NiSO4·6H2O from 4-20μm NiSO4·7H2O. 0.2 Co 0.1 Mn 0.7The precursor of CO3 was obtained.

[0117] Roasting The obtained precursor was heated at a rate of 2°C / min in a box firing furnace while maintaining an atmosphere of O2 or air (50 L / min), and was maintained at 550°C for 1 to 6 hours, after which the furnace was cooled.

[0118] Firing The roasted precursor was mixed with LiOH or Li2CO3 weighed out so that the Li / (Ni+Co+Mn) ratio was 1.45, and 0.3 mol% of Nb2O5 was weighed out as a flux dopant to induce growth between primary particles, using a manual mixer (MM).

[0119] The mixture was heated at a rate of 2°C / min in a box firing furnace while maintaining an atmosphere of O2 or air (50 L / min), and the firing temperature was maintained at 900°C for 7 to 12 hours, followed by furnace cooling to produce a positive electrode active material.

[0120] The composition of the positive electrode active material produced in Example 1 was Li:Ni:Co:Mn:Nb=15.3:15.1:9.3:59.8:0.4 (wt %).

[0121] Example 2: Preparation of positive electrode active material A positive electrode active material was produced in the same manner as in Example 1, except that 0.6 mol % of Nb2O5 was mixed as a flux dopant in the firing step of Example 1.

[0122] The composition of the positive electrode active material produced in Example 2 was Li:Ni:Co:Mn:Nb=15.0:14.8:9.3:60.0:0.8 (wt %).

[0123] <Comparative Example 1> Preparation of Positive Electrode Active Material A positive electrode active material was produced in the same manner as in Example 1, except that Nb2O5 was not mixed as a flux dopant in the firing step of Example 1.

[0124] <Comparative Example 2> Preparation of Positive Electrode Active Material A positive electrode active material was prepared in the same manner as in Example 2, except that ammonium niobate oxalate (C4H4NNbO9xH2O) was mixed as a flux dopant in the firing step of Example 2.

[0125] <Experimental Example> SEM Measurement The positive electrode active materials produced in the above examples and comparative examples were subjected to SEM measurement, and the results are shown in FIG.

[0126] As shown in the figure, the size of the primary particles of the cathode active material according to Example 1 was larger than that of the comparative example, and it was confirmed that the size of the primary particles increased as the amount of NbO added as a flux dopant that induces growth between primary particles increased.

[0127] In the case of the cathode active material prepared according to the embodiment of the present invention, the size of the primary particles was measured to be 300 nm to 5 μm, and it was confirmed that it can be adjusted to sub-micron size primary particles instead of the existing nano-sized primary particles.

[0128] In Comparative Example 2, the Nb compound was also added at 0.6 mol %, but unlike the Nb compound used in Comparative Example 1, it was confirmed that the particle size was smaller than that of Example 1, in which a smaller amount of Nb was added at 0.3 mol %.

[0129] <Experimental Example> Cross-sectional SEM measurement The cross-sections of the positive electrode active materials prepared in the above examples and comparative examples were measured by SEM, and the results are shown in FIGS. 2a and 2b.

[0130] FIG. 2a is a cross-sectional SEM image of the cathode active material prepared in Comparative Example 1, which confirms that not only the primary particles present outside the secondary particles of the cathode active material but also the primary particles present inside the secondary particles are small in size.

[0131] FIG. 2b is a cross-sectional SEM image of the cathode active material prepared in Example 2, which shows that not only the primary particles present outside the secondary particles of the cathode active material but also the primary particles present inside the secondary particles have grown in size.

[0132] <Experimental Example> EDX Measurement EDX photographs of the positive electrode active materials prepared in the above examples and comparative examples were taken, and the results are shown in FIGS. 3a and 3b.

[0133] FIG. 3a shows the EDX result of the cathode active material containing the flux dopant prepared in Example 2. It was confirmed that not only the elements Ni, Co, and Mn but also Nb, which was added as a flux dopant to induce growth between primary particles, was uniformly contained within the particles.

[0134] FIG. 3b shows the EDX result of the cathode active material prepared in Comparative Example 1 without adding any flux dopant. It was confirmed that only Ni, Co, and Mn elements were uniformly contained within the particles.

[0135] <Experimental Example> XRD analysis The results of XRD analysis of the positive electrode active materials produced in the examples of the present invention and the comparative examples are shown in Figures 4 to 6. XRD analysis was performed using CuKα radiation at a wavelength of 1.5406 Å.

[0136] As shown in Figure 4, when a flux dopant is added, the major peak of the lithium-rich layered oxide, I(003), shifts to a lower angle during XRD analysis. This is evidence that the flux dopant, Nb, is doped into the lattice of the lithium-rich layered oxide.

[0137] From FIG. 5, it was confirmed that when Nb was mixed according to Example 2 of the present invention, a peak due to Li3NbO4 appeared during XRD analysis.

[0138] As shown in FIG. 6, during XRD analysis of the positive electrode active material according to the above-described example, the full width at half maximum (FWHM (deg.)) at I(104) was found to be smaller than that of the comparative example, and it was confirmed that the full width at half maximum decreased as the content of the flux dopant that induces interparticle growth increased.

[0139] More specifically, it was confirmed that adding 0.3 mol% of Nb as a flux dopant to induce growth between primary particles resulted in a 7.5% reduction rate, and adding 0.6 mol% resulted in a 17.3% reduction rate.Even though the firing temperature was the same, it was confirmed that the size of the primary particles could be adjusted by adding a dopant, and therefore the half-width at I(104) could be adjusted.

[0140] <Experimental Example> Packing density measurement From FIG. 7, it was confirmed that the packing density (g / cc) of the positive electrode active material according to the above-described Examples was increased compared to the above-described Comparative Example, and that the packing density increased as the content of the flux dopant increased.

[0141] <Experimental Example> BET Measurement From FIG. 8, it can be seen that the specific surface area (BET, m 2 / g) decreased compared to the comparative example, and the specific surface area (BET, m 2 / g) was confirmed to increase. More specifically, it was confirmed that the addition of 0.3 mol% Nb as a flux dopant reduced the specific surface area by 60%, and the addition of 0.6 mol% reduced the specific surface area by more than 80%.

[0142] <Experimental Example> Electrochemical property measurement 9, it was confirmed that the Examples in which a flux dopant that induces primary particle growth was added during the calcination step exhibited superior voltage characteristics compared to the Comparative Examples in which no flux dopant was added. The reason why the initial charge / discharge capacity of the cathode active material containing a lithium-excess layered oxide is higher than that of the Comparative Examples is that the flux dopant increases inter-slabs and the presence of an ion conductor coating layer increases the kinetics of lithium ions (Li ions).

[0143] 10, it was confirmed that the energy density per volume (Wh / L) of the positive electrode active material according to the above-described Examples was higher than that of the Comparative Example, and that this increased as the content of the flux dopant increased. More specifically, it was confirmed that the addition of 0.3 mol% of Nb as the flux dopant increased the energy density by 9.1%, and the addition of 0.6 mol% of Nb increased the energy density by 14.9%.

[0144] As can be seen from FIG. 11, the examples in which the flux dopant was added during the firing step exhibited superior voltage characteristics compared to the comparative examples in which the flux dopant was not added, and it was confirmed that the capacity retention rate with the number of cycles increased as the flux dopant content increased.

[0145] As can be seen from FIG. 12, the example in which the flux dopant was added during the firing step exhibited superior voltage characteristics compared to the comparative example in which the flux dopant was not added, and it was confirmed that the capacity was maintained over the number of cycles as the flux dopant content increased.

[0146] As can be seen from FIG. 13, the examples in which the flux dopant was added during the firing step exhibited superior voltage characteristics compared to the comparative examples in which the flux dopant was not added, and it was confirmed that the voltage retention rate as a function of the cycle number was maintained as the flux dopant content increased.

[0147] As can be seen from FIG. 14, the examples in which the flux dopant was added during the firing step exhibited superior voltage characteristics compared to the comparative examples in which the flux dopant was not added, and it was confirmed that the nominal voltage was maintained over the number of cycles as the flux dopant content increased.

[0148] The results of the above experiments are shown in Table 1 below. [Table 1]

Claims

1. A positive electrode active material for a secondary battery, comprising an overlithiated layered oxide (OLO) represented by the following [Chemical Formula 1]: [Chemical formula 1] (In the formula, 0<r≦0.6, 0<a≦1, 0≦x≦1, 0≦y<1, 0≦z<1, and 0<x+y+z<1, and M1 is at least one selected from Na, K, Mg, Al, Fe, Cr, Y, Sn, Ti, B, P, Zr, Ru, Nb, W, Ba, Sr, La, Ga, Mg, Gd, Sm, Ca, Ce, Fe, Al, Ta, Mo, Sc, V, Zn, Nb, Cu, In, S, B, and Bi.) The specific surface area (BET, m ２ / g) is 0.1 to 1.5 (BET, m ２ / g).

2. 2. The positive electrode active material for a secondary battery according to claim 1, wherein the average particle size of the secondary particles of the positive electrode active material for a secondary battery is 2 μm to 20 μm.

3. 2. The positive electrode active material for a secondary battery according to claim 1, wherein M1 in the formula (1) is a dopant that acts as a flux for growing primary particles.

4. 4. The positive electrode active material for a secondary battery according to claim 3, wherein M1 in the [Chemical Formula 1] is at least one selected from Ba, Sr, B, P, Y, Zr, Nb, Mo, Ta, and W.

5. 2. The positive electrode active material for a secondary battery according to claim 1, wherein M1 in the [Chemical Formula 1] is contained in an amount in the range of 0.001 to 10 mol % with respect to the total number of moles of metals constituting the positive electrode active material for a secondary battery.

6. 2. The positive electrode active material for a secondary battery according to claim 1, wherein the OLO further contains a substance represented by the following Chemical Formula 2: [Chemical 2] (In the formula, 0<a≦7, 0<b≦15, and M1′ is at least one selected from Ba, Sr, B, P, Y, Zr, Nb, Mo, Ta, and W.)

7. 10. The positive electrode active material for a secondary battery according to claim 1, wherein the packing density of the positive electrode active material for a secondary battery is 2.0 to 4.0 (g / cc).

8. 2. The positive electrode active material for a secondary battery according to claim 1, wherein in the lithium-excess layered oxide, a ratio of the number of moles of lithium to the total number of moles of at least one metal selected from Ni, Co, and Mn (Li / (Ni+Co+Mn)) is 1.1 to 1.

6.

9. 2. The positive electrode active material for a secondary battery according to claim 1, wherein in the lithium-excess layered oxide, the ratio of the number of moles of Mn to the total number of moles of Ni (Mn / Ni) is 1 to 4.

5.

10. A step of preparing a positive electrode active material precursor containing at least one element selected from Ni, Co, and Mn; and a step of mixing the positive electrode active material precursor with a lithium compound and a compound containing M1 in Chemical Formula 1, and calcining the mixture; The method for producing a positive electrode active material for a secondary battery according to claim 1 , comprising:

11. 10. The method of claim 8, further comprising roasting the prepared precursor at 300 to 600°C after the step of preparing the precursor and before the step of calcining.

12. A secondary battery comprising the positive electrode active material for secondary batteries according to claim 1 .

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