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

Abstract

The cathode active material for a secondary battery according to the present invention comprises lithium composite oxide particles, wherein the lithium composite oxide particles contain a lithium excess oxide represented by the following chemical formula 1 having a layered crystal structure inside, and the lithium manganese oxide represented by the following chemical formula 2 outside the lithium composite oxide particles, wherein the ratio of the moles of lithium (Li) to the total moles of metals excluding lithium (M) is denoted as Li / M, the lithium excess oxide contained inside and the lithium manganese oxide contained outside have different Li / M values: [Chemical Formula 1] rLi2MnO3·(1-r)LiaNixCoyMnzM11-(x+y+z)O2 (In the above Chemical Formula 1, 0 <r≤0.6, 0<a≤1, 0≤x≤1, 0≤y<1, 0≤z<1, 및 0<x+y+z≤1 이고, 상기 M1은 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 및 Bi 중에서 선택되는 적어도 어느 하나 이상임) [Chemical Formula 2] LibMnPOQ (In the above chemical formula 2, 0.1 ≤ b / p ≤ 2.5 and 0 < q ≤ 15).

Description

Technology Field

[0001] The present invention relates to a positive electrode active material for a lithium secondary battery comprising a lithium excess oxide having a layered crystal structure inside a lithium composite oxide particle, wherein the lithium concentration and metal concentration outside the particle are in excess or deficient, a method for manufacturing the same, and a lithium secondary battery comprising the same. Background Technology

[0003] With the advancement of portable mobile electronic devices such as smartphones, MP3 players, and tablet PCs, the demand for rechargeable batteries capable of storing electrical energy is increasing explosively. In particular, the demand for lithium-ion batteries is rising due to the emergence of electric vehicles, medium-to-large energy storage systems, and portable devices requiring high energy density.

[0004] The material currently receiving the most attention as a cathode active material is lithium nickel manganese cobalt oxide (Li(Ni x Co y Mn z )O2(wherein x, y, and z are each the atomic fractions of independent oxide composition elements, 0 <x≤1, 0<y≤1, 0<z≤1, 및 0<x+y+z≤1)이다. 이 재료는 그동안 양극활물질로서 활발히 연구되고 사용되어 왔던 LiCoO2보다 고전압에서 사용되기 때문에 고용량을 내는 장점이 있고, Co 함량이 상대적으로 적기 때문에 저가격이라는 장점이 있다. 그러나 율특성(rate capability) 및 고온에서의 수명특성이 좋지 않은 단점을 갖고 있다.

[0005] Accordingly, the existing Li(Ni x Co y Mn z Research is underway to apply lithium-excess layered oxides, which exhibit high reversible capacity exceeding that of O2, to lithium secondary batteries.

[0006] However, the cycle life and voltage decay phenomena occurring during cycle life are problematic, as they are caused by a phase transition from a spinel-like structure to a cubic structure due to transition metal migration during cycle life. These cycle life and voltage decay phenomena are problems that must be solved for the commercialization of lithium secondary batteries. The problem to be solved

[0008] In order to solve the above problem,

[0009] By suppressing phase transitions during life cycling, the aim is to increase charge / discharge capacity and resolve issues of life degradation and voltage drop.

[0010] In addition, the lithium ion mobility is increased and the rate capability is improved by a phase formed on the outside of the layered lithium excess oxide.

[0011] In addition, the internal structural stability of the particles is improved by controlling the energy density to increase and the specific surface area of ​​the particles to decrease compared to conventional polycrystalline lithium excess oxide. means of solving the problem

[0013] A cathode active material for a secondary battery according to an embodiment of the present invention comprises lithium composite oxide particles, wherein the lithium composite oxide particles contain a lithium excess oxide represented by the following chemical formula 1 having a layered crystal structure inside, and the lithium manganese oxide represented by the following chemical formula 2 outside the lithium composite oxide particles, wherein the ratio of the number of moles of lithium (Li) to the total number of moles of metals (M) excluding lithium is denoted as Li / M, the lithium excess oxide contained inside and the lithium manganese oxide contained outside have different Li / M values.

[0014] [Chemical Formula 1] rLi2MnO3·(1-r)Li a Ni x Co y Mnz M1 1-(x+y+z) O2

[0015] (In the above Chemical Formula 1, 0 <r≤0.6, 0<a≤1, 0≤x≤1, 0≤y<1, 0≤z<1, 및 0<x+y+z≤1 이고, 상기 M1은 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 및 Bi 중에서 선택되는 적어도 어느 하나 이상임)

[0016] [Chemical Formula 2] Li b Mn P O Q

[0017] (In the above chemical formula 2, 0.1 ≤ b / p ≤ 2.5 and 0 < q ≤ 15).

[0018] In addition, the positive electrode active material for a secondary battery according to an embodiment of the present invention may form a lithium concentration gradient from the inside to the outside of the lithium composite oxide particles.

[0019] In addition, the cathode active material for a secondary battery according to an embodiment of the present invention may form a manganese concentration gradient from the inside to the outside of the lithium composite oxide particles.

[0020] In addition, a method for manufacturing a positive electrode active material for a secondary battery according to an embodiment of the present invention comprises: a step of forming precursor particles to form the interior of the positive electrode active material; a step of mixing the formed precursor particles with a lithium compound and performing a first heat treatment; a step of dispersing the first heat-treated particles in distilled water or an alkaline aqueous solution and then adding a compound containing manganese to coat them to form the exterior of the positive electrode active material; and a step of mixing the lithium compound with the coated particles and performing a second heat treatment.

[0021] In addition, a secondary battery according to an embodiment of the present invention includes the positive electrode active material. Effects of the invention

[0023] The positive electrode active material according to the embodiment of the present invention has increased charge / discharge capacity, and the problems of lifespan degradation and voltage drop are resolved.

[0024] In addition, the lithium ion mobility is increased and the rate capability is improved by the phase formed on the outside of the layered lithium excess oxide.

[0025] In addition, the internal structural stability of the particles is improved. Brief explanation of the drawing

[0027] Figure 1 is an SEM image of a positive electrode active material according to a comparative example and an embodiment of the present invention. FIG. 2 is a conceptual diagram of a positive electrode active material according to Example 1 of the present invention. Figure 3 is a conceptual diagram of a positive electrode active material according to Example 2 of the present invention. Figure 4 is a TEM image of a positive electrode active material according to a comparative example and an embodiment of the present invention. Figure 5 is the result of EDS analysis of the cathode active material according to an embodiment of the present invention. Figure 6 shows the XRD analysis results of the cathode active material according to the comparative example and embodiment of the present invention. FIGS. 7 and 8 compare the charge and discharge capacities of secondary batteries according to comparative examples and embodiments of the present invention. FIG. 9 compares the overvoltage of a secondary battery according to a comparative example and an embodiment of the present invention. FIG. 10 compares the rate characteristics of a secondary battery according to a comparative example and an embodiment of the present invention. FIG. 11 compares the capacity retention rates of secondary batteries according to comparative examples and embodiments of the present invention. FIG. 12 compares the voltage retention rates of secondary batteries according to comparative examples and embodiments of the present invention. Specific details for implementing the invention

[0028] Expressions such as "comprising" as used in this specification should be understood as open-ended terms implying the possibility of including other embodiments.

[0029] As used herein, "preferably" and "preferably" refer to embodiments of the invention that can provide certain advantages under certain conditions, and are not intended to exclude other embodiments from the scope of the invention.

[0031] A positive electrode active material according to an embodiment of the present invention comprises lithium composite oxide particles, and the lithium composite oxide particles contain a lithium excess oxide represented by the following chemical formula 1, having a layered crystal structure.

[0032] [Chemical Formula 1] rLi2MnO3·(1-r)Li a Ni x Co y Mn z M1 1-(x+y+z) O2

[0033] (In the above Chemical Formula 1, 0 <r≤0.6, 0<a≤1, 0≤x≤1, 0≤y<1, 0≤z<1, 및 0<x+y+z≤1 이고, 상기 M1은 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, Ge, Si 및 Bi 중에서 선택되는 적어도 어느 하나 이상임)

[0034] The above-mentioned layered lithium excess oxide may be a solid solution phase in which monoclinic Li2MnO3 and rhombohedral LiMO2 are mixed, and M may be at least one selected from Ni, Co, Mn, and M1.

[0035] The above-mentioned layered lithium excess oxide is on Li2MnO3 in the 4.4 V region of the initial charge-discharge profile. A plateau may appear due to this.

[0036] The above-mentioned positive electrode active material has a layered structure, and may be a layered structure in which a lithium atomic layer and a metal atomic layer of Ni, Co, Mn, or M1 are alternately stacked through an oxygen atomic layer.

[0037] The planes forming the layers of the layered structure of the above-mentioned cathode active material may have crystal orientation in a direction perpendicular to the C-axis. In this case, the mobility of lithium ions contained in the above-mentioned cathode active material is improved, and the structural stability of the above-mentioned cathode active material is increased, so that when applied to a battery, the initial capacity characteristics, output characteristics, resistance characteristics, and long-term life characteristics can be improved.

[0038] When the ratio of the moles of lithium (Li) to the total moles of metal (M) excluding lithium is denoted as Li / M, the Li / M inside the lithium composite oxide particle may be 1.1 to 1.6, 1.2 to 1.6, 1.3 to 1.6, or 1.4 to 1.5.

[0039] In the above chemical formula 1, the value of x may be greater than 0.5, greater than 0.4, greater than 0.3, greater than 0.2, or greater than 0.1.

[0040] In the above chemical formula 1, the value of y may be greater than 0.5, greater than 0.4, greater than 0.3, greater than 0.2, or 0.1 to 0.2.

[0041] As an example, the ratio of the number of moles of manganese to the total number of moles of nickel (Mn / Ni) inside the lithium composite oxide particles may be 1 to 4.5, 2 to 4, or 3 to 4.

[0042] In the above chemical formula 1, M1 is at least one material 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, Ge, Si, and Bi.

[0043] As a more desirable example, the above M1 may be a dopant that acts as a flux for growing the primary particles. Acting as a flux means that it can act as a dopant that increases the size of the primary particles.

[0044] More preferably, at least one selected from Ba, Sr, B, P, Y, Zr, Nb, Mo, Ta, and W can be grown further to more suitably control the size of the primary particles to a specific range, and most preferably, at least one selected from Nb and Ta.

[0045] As an example, the above M1 may be included in an amount of 0.01 to 3 mol%, more preferably 0.1 to 1 mol%, relative to the total lithium-excess layered oxide. If the dopant M1 included as a flux to induce the growth of primary particles exceeds the above range, an excess amount of lithium composite oxide may be produced, which may cause a decrease in capacity and efficiency, and if it is below the above range, the effect of growing primary particles may be insufficient.

[0046] The positive electrode active material according to an embodiment of the present invention comprises lithium manganese oxide represented by the following chemical formula 2 on the outside of the lithium composite oxide particles.

[0047] [Chemical Formula 2] Li b Mn P O Q

[0048] (In the above chemical formula 2, 0.1 ≤ b / p ≤ 2.5 and 0 < q ≤ 15)

[0049] As an example, the b / p value representing Li / M outside the lithium composite oxide particles may be 0.1 to 0.9, more preferably 0.3 to 0.9, and more preferably 0.5 to 0.8.

[0050] In this case, the lithium manganese oxide is Li4Mn5O 12 Or it could be LiMn2O4.

[0051] In addition, as an example, the b / p value may be 1.8 to 2.5, more preferably 1.9 to 2.1.

[0052] In this case, the lithium manganese oxide may be Li2MnO3.

[0053] The present invention distinguishes the interior from the exterior by forming an exterior of lithium manganese oxide having a spinel crystal structure or a lithium-excess layered structure by coating to include Mn and then adding Li.

[0054] In the positive electrode active material according to the embodiment of the present invention, when the ratio of the number of moles of lithium (Li) to the total number of moles of metal (M) excluding lithium is denoted as Li / M, the lithium excess oxide included inside and the lithium manganese oxide included outside have different Li / M values.

[0055] Layered lithium excess oxides have problems with cycle life and voltage decay during cycling, but rate capability can be improved by a phase created by adding or removing lithium and metal concentrations outside the lithium composite oxide particles.

[0056] In addition, by suppressing the leaching of Mn from lithium and manganese-rich oxides and suppressing the lattice change from spinel to rock-salt phase that occurs mainly on the surface of the cathode active material during cycling, there is an effect of improving lifespan characteristics, reducing discharge capacity, and suppressing voltage drop.

[0057] As an example, the outer part of the lithium composite oxide particle may be deficient in lithium compared to the inner part (Fig. 2).

[0058] As an example, the Li / M ratio inside the lithium composite oxide particle may be 1.2 to 1.6, and the Li / M ratio outside the lithium composite oxide particle may be 0.1 to 0.9. In this case, the crystal structure of the outside of the lithium composite oxide particle may be a spinel structure.

[0059] In this way, the mobility of lithium ions can be increased by coating a lithium manganese oxide with a 3D spinel structure on the 2D structural surface inside the lithium composite oxide particles.

[0060] In addition, as an example, the outer part of the lithium composite oxide particle may have an excess of lithium compared to the inner part (Fig. 3).

[0061] As an example, the Li / M ratio inside the lithium composite oxide particle may be 1.2 to 1.6, and the Li / M ratio outside the lithium composite oxide particle may be 1.8 to 2.5. In this case, the crystal structure of the outside of the lithium composite oxide particle may be a layered structure.

[0062] In this way, by coating the outside of the particle with lithium manganese oxide having a higher lithium concentration, the capacity of the coating layer is developed, and the charging and discharging capacities can be increased.

[0063] As an example, the lithium composite oxide particles may form a concentration gradient in which the lithium concentration decreases or increases from the inside to the outside.

[0064] The present invention allows for the formation of a concentration gradient with an external spinel crystal structure or a lithium-excess layered structure by coating to include Mn and then adding Li.

[0065] As an example, the molar concentration of manganese in the coating layer formed on the outside of the lithium composite oxide particles may differ from the molar concentration of manganese inside the particles.

[0066] As an example, the concentration of manganese on the outside of the lithium composite oxide particles may be lower than on the inside.

[0067] As an example, the concentration of manganese may be higher on the outside of the lithium composite oxide particles than on the inside.

[0068] As an example, the lithium composite oxide particles may form a concentration gradient in which the manganese concentration decreases or increases from the inside to the outside.

[0069] As an example, the lithium composite oxide particles may include secondary particles formed by the aggregation of primary particles.

[0070] As a more desirable example, by mixing a dopant acting as a flux with a lithium compound during the calcination step and heat-treating them together, the size of the primary particles can be increased, thereby resolving the problems of reduced discharge capacity and voltage drop and improving the density of the cathode active material.

[0071] As an example, primary particles with a size greater than 300 nm and 10 μm can be controlled to 50 to 100 volume%, 70 to 100 volume%, or 100 volume% of the total primary particles included in the secondary particles.

[0072] As an example, primary particles with a size greater than 500 nm and 10 μm can be controlled to 50 to 100 volume%, 70 to 100 volume%, or 100 volume% of the total primary particles included in the secondary particles.

[0073] As an example, primary particles with a size greater than 1 μm and 10 μm can be controlled to 50 to 100 volume%, 70 to 100 volume%, or 100 volume% of the total primary particles included in the secondary particles.

[0074] At this time, the size of the primary particle refers to the longest length of the particle.

[0076] The average particle size of the primary particles of the above-mentioned cathode active material can be controlled to be greater than 500 nm and 10 μm, or 1 μm to 10 μm.

[0077] The average particle size of the secondary particles of the above positive active material may be 2 to 20 μm.

[0078] At this time, the above average particle size can be defined as the particle size corresponding to 50% of the volume accumulation in the particle size distribution curve.

[0079] As a more desirable example, the primary particle size is increased to increase the portion corresponding to the single-crystal structure; the more the portion corresponding to the single-crystal structure is, that is, the fewer the number of primary particles, the more the problem of voltage drop occurring in polycrystalline materials can be improved. In addition, by controlling the size of the primary particles, the specific surface area of ​​the cathode active material can be reduced, thereby resolving the problem of side reactions with the electrolyte.

[0080] In the present invention, inducing the growth of the primary particles includes the concepts of nucleation, Ostwald ripening, and particle aggregation.

[0081] As an example, by controlling the size of the primary particles through the addition and content control of dopant M1, the full width at half maximum (FWHM(deg.)) at I(104) during XRD analysis of the cathode active material may be 0.1 to 0.25(deg.).

[0082] As an example, by controlling the size of the primary particles through the addition and content control of dopant M1, the energy density per unit volume (Wh / L) of the cathode active material may be 2.7 to 4.0 (Wh / L).

[0083] As an example, by controlling the size of the primary particles through the addition and content control of dopant M1, the specific surface area (BET, m²) of the cathode active material 2 / g) is 0.01 to 2 (BET, m 2 / g) can be.

[0084] However, as the primary particle size increases, the lithium ion diffusion distance increases, which leads to a problem where overpotential occurs due to the concentration polarization of lithium ions during charging and discharging. Consequently, kinetics are degraded, which may actually reduce the capacity of the cathode active material. Accordingly, the present invention can resolve this by creating an excess or deficiency of lithium concentration or metal concentration on the particle surface, or by forming a concentration gradient.

[0086] A method for manufacturing a positive electrode active material for a secondary battery according to an embodiment of the present invention first includes a first step of forming precursor particles to form the interior of the positive electrode active material.

[0087] The formation of the above precursor particles can be carried out by co-precipitation, spray-drying, solid-state method, wet grinding, fluidized bed drying method, and vibration drying method, and is not particularly limited thereto.

[0088] After the first step and before the second step, the method may further include a step of washing and drying the formed precursor particles.

[0089] In addition, after the first step and before the second step, the method may further include a step of roasting the formed precursor particles at 300 to 600°C.

[0090] Next, after the first step, the method includes a second step of mixing the formed precursor particles and the lithium compound and performing a first heat treatment.

[0091] At this time, the first heat treatment temperature may be 700 to 900℃.

[0092] As a more preferable example, the first heat treatment step may be performed by further mixing a compound containing M1 of Formula 1.

[0093] Next, after the second step, the method includes a third step of dispersing the first heat-treated particles in distilled water or an alkaline aqueous solution to form the outer surface of the positive electrode active material, and then adding a compound containing manganese to coat it.

[0094] As an example, after the third step and before the fourth step, a washing and drying step may be further included.

[0095] Next, after the third step, a fourth step is included in which a lithium compound is mixed with the coated particles and subjected to a second heat treatment.

[0096] At this time, the second heat treatment temperature may be 400 to 700℃.

[0097] As an example, after the fourth step, a washing and drying step may be further included.

[0099] A secondary battery according to an embodiment of the present invention comprises the above positive active material.

[0100] The above-mentioned positive active material is as described above, and the binder, conductive material, and solvent are not particularly limited thereto as long as they can be used on the positive current collector of a secondary battery.

[0101] The above lithium secondary battery may specifically include a positive electrode, a negative electrode positioned opposite to the positive electrode, and an electrolyte between the positive electrode and the negative electrode, but is not particularly limited thereto as long as it can be used as a secondary battery.

[0103] Hereinafter, the positive active material according to an embodiment of the present invention will be described in detail.

[0105] <Example 1> Formation of spinel-structured lithium manganese oxide by externally depleting lithium

[0106] Internal synthesis

[0107] Spherical Ni using the co-precipitation method 0.2 Co 0.1 Mn 0.7 A CO3 precursor was synthesized. In a 90L reactor, 25 wt.% NaCO3 and 28 wt.% NH4OH were added to a 2.5 M aqueous complex transition metal sulfuric acid solution prepared by mixing NiSO4·6H2O and CoSO4·7H2O in a molar ratio of MnSO4·H2O (20:10:70). The pH inside the reactor was maintained between 10.0 and 12.0, and the reactor temperature was maintained between 45 and 50 °C. An inert gas, N2, was introduced into the reactor to prevent the oxidation of the prepared precursor. After the synthesis stirring was completed, washing and dehydration were performed using a filter press (F / P). Finally, the dehydrated product was dried at 120 °C for 2 days and sieved through a 75 μm (200 mesh) screen to obtain 18 μm and 4 μm Ni 0.2 Co 0.1 Mn 0.7 CO3 precursor was obtained.

[0108] Baeso

[0109] The above precursor was heated at a rate of 2°C per minute in a Box kiln under an O2 or Air (50L / min) atmosphere, maintained at a firing temperature of 550°C for 1 to 6 hours, and then furnace cooled.

[0110] First heat treatment

[0111] LiOH or Li2CO3 was weighed to achieve a Li / M ratio of 1.45 for the above precursor, and 50.6 mol% of Nb2O was weighed as a FLUX dopant and mixed using a manual mixer (MM). The mixture was heated at a rate of 2°C per minute in a box kiln under an atmosphere of O2 or Air (50 L / min), maintained at a firing temperature of 900°C for 7 to 12 hours, and then furnace cooled.

[0112] External synthesis

[0113] 5 mol.% of Mn was coated on the surface of the above-mentioned calcined product using the coprecipitation method. The active material and distilled water were weighed in a weight ratio of 1:2, and after dispersing the active material in distilled water, an aqueous metal sulfuric acid solution prepared by dissolving MnSO4·H2O in distilled water was added. At this time, the pH was maintained at 10.0–12.0 using NaOH. After coating, washing and dehydration were performed using a Filter Press (F / P) device, followed by drying at 150°C for 14 hours.

[0114] Second heat treatment

[0115] Next, LiOH or Li2CO3 was weighed into the wet-coated product so that the Li / Mn (coating amount) was 0.5 to 0.8, and then mixed using a mixer. The mixture was heated in a box kiln at a rate of 4.4°C per minute while maintaining an O2 or Air atmosphere, and then maintained at a firing temperature of 450°C for 7 to 12 hours, followed by furnace cooling.

[0117] <Example 2> Formation of layered lithium manganese oxide by adding excess lithium externally

[0118] A cathode active material was prepared in the same manner as in Example 1, except that in the second heat treatment step, LiOH or Li2CO3 was weighed so that the Li / Mn (coating amount) of the wet coated product was 2.0, and the second heat treatment was performed at 600°C.

[0120] <Comparative Example> No formation of lithium manganese oxide on the outside

[0121] A positive electrode active material was prepared in the same manner as Example 1, except that the external synthesis and second heat treatment steps of Example 1 were not performed.

[0123] <Manufacturing Example> Manufacturing of a lithium secondary battery

[0124] An anode slurry was prepared by dispersing 90 wt% of the anode active material according to the above examples and comparative examples, 5.5 wt% of carbon black, and 4.5 wt% of PVDF binder in 30 g of N-methyl-2-pyrrolidone (NMP). The anode slurry was coated onto an aluminum (Al) thin film serving as an anode current collector with a thickness of 15 μm, dried, and then roll-pressed to produce an anode. The loading level of the anode was 5.5 mg / cm². 2 and the electrode density is 2.3 g / cm³ 3 It was.

[0125] Metallic lithium was used as the counter electrode for the above anode, and 1M LiPF6, EC / DMC = 1 / 1 (v / v) was used as the electrolyte.

[0126] A battery assembly was formed by interposing a separator made of a porous polyethylene (PE) film between the anode and cathode, and a lithium secondary battery (coin cell) was manufactured by injecting the electrolyte.

[0128] <Experimental Example>

[0129] By SEM analysis of Fig. 1, it can be confirmed that in the cathode active material according to the example, lithium manganese oxide is uniformly coated on the surface of the lithium excess oxide particles of a layered structure in which the size of the primary particles has been grown.

[0130] By TEM analysis of Fig. 4, it can be confirmed that in the cathode active material according to Example 1, a spinel-structured lithium manganese oxide is formed on the surface of the layered lithium excess oxide particles.

[0131] The Line-EDS analysis in Fig. 5 is a method of analyzing changes in metal concentration by applying voltage to the particle surface, and it can be confirmed that a manganese concentration gradient is formed from the inside to the outside of the lithium composite oxide particle. In addition, it can be confirmed that the surface Mn content is higher in Example 2 compared to the Comparative Example.

[0132] By XRD analysis of Fig. 6, it can be confirmed that a spinel-structured lithium manganese oxide was formed in the cathode material according to Example 1.

[0133] Referring to Fig. 7, it can be seen that the discharge capacity of the cathode active material of Example 1 has increased compared to the comparative example. This is because the lithium ion mobility was accelerated by the 3D spinel coating on the 2D structure surface.

[0134] Referring to Fig. 8, it can be seen that the charging capacity of the cathode active material of Example 2 has increased compared to the comparative example. This is because the coated Li2MnO3 exhibits capacity during initial charging. In addition, it can be seen that not only the charging capacity but also the discharging capacity has increased.

[0135] Referring to FIG. 9, it can be seen that the overvoltage of the positive electrode active material of the embodiment of the present invention is significantly reduced compared to the comparative example. This is because the lithium ion conductivity is improved by the material coated on the surface.

[0136] Referring to FIG. 10, it can be seen that the rate characteristics of the cathode active material of the embodiment of the present invention are improved compared to the comparative example. This is because the lithium ion conductivity is improved by the material coated on the surface.

[0137] Referring to FIG. 11, it can be seen that the lifespan characteristics of the cathode active material of the embodiment of the present invention are improved compared to the comparative example. This is because the phase transition during cycling is mitigated by the material coated on the surface.

[0138] Referring to FIG. 12, it can be seen that the voltage drop of the cathode active material of the embodiment of the present invention is suppressed compared to the comparative example. This is because the lithium mobility was increased by the spinel 3D structure coated on the surface in the case of Example 1, and by the Li2MnO3 coated on the surface in the case of Example 2, and the phase transition occurring during cycling was mitigated.

[0140] The above experimental results are shown in Table 1 below.

[0142] ITEM Comparative example Example 1 Example 2 beginning (@25℃) 0.1C 2.0-4.6V CH. mAh / g 261.6 261.8 274.5 DCH. 218.2 230.5 222.6 Eff. % 83.4 88.0 81.1 Rate characteristics 5C / 0.1C Rate 41.9 47.2 44.4 life (@25℃) 1C / 1C 2.0-4.6V Cycle Life (50 cycles) 86.8 88.5 89.3 Voltage Decay (50 cycles) 96.8 97.3 97.8

Claims

Claim 1 A cathode active material for a secondary battery comprising lithium composite oxide particles, wherein the lithium composite oxide particles contain a lithium excess oxide represented by the following Chemical Formula 1 having a layered crystal structure inside the lithium composite oxide particles, and the lithium manganese oxide represented by the following Chemical Formula 2 outside the lithium composite oxide particles, wherein when the ratio of the moles of lithium (Li) to the total moles of metals (M) excluding lithium is denoted as Li / M, the lithium excess oxide contained inside and the lithium manganese oxide contained outside have different Li / M values, the Li / M outside the lithium composite oxide particles is 1.8 to 2.5, and the outside of the lithium composite oxide particles has a layered crystal structure: [Chemical Formula 1] rLi2MnO3·(1-r)Li a Ni x Co y Mn z M1 1-(x+y+z) O2 (in the above chemical formula 1, 0 <r≤0.6, 0<a≤1, 0≤x≤1, 0≤y<1, 0≤z<1, 및 0<x+y+z<1 이고, 상기 M1은 Na, K, Mg, Al, Fe, Cr, Y, Sn, Ti, B, P, Zr, Ru, Nb, W, Ba, Sr, La, Ga, Gd, Sm, Ca, Ce, Ta, Mo, Sc, V, Zn, Cu, In, S, Ge, Si 및 Bi 중에서 선택되는 적어도 어느 하나 이상임)[화학식 2] Li b Mn p O q (In the above chemical formula 2, 0.1 ≤ b / p ≤ 2.5 and 0 < q ≤ 15). Claim 2 A positive electrode active material for a secondary battery, wherein the lithium composite oxide particles form a lithium concentration gradient from the inside to the outside. Claim 3 A positive electrode active material for a secondary battery according to claim 1, wherein the Li / M ratio inside the lithium composite oxide particles is 1.1 to 1.

6. Claim 4 delete Claim 5 A positive electrode active material for a secondary battery according to claim 1, wherein the ratio of the moles of manganese to the total moles of nickel inside the lithium composite oxide particles (Mn / Ni) is 1 to 4.

5. Claim 6 A positive electrode active material for a secondary battery, wherein the lithium composite oxide particles form a manganese concentration gradient from the inside to the outside. Claim 7 A positive electrode active material for a secondary battery according to claim 1, wherein the lithium composite oxide particles comprise secondary particles formed by the aggregation of primary particles, and M1 of Chemical Formula 1 is a dopant acting as a flux for growing the primary particles. Claim 8 A positive electrode active material for a secondary battery according to claim 1, wherein the lithium composite oxide particles include secondary particles formed by the aggregation of primary particles, and the primary particles having a size of 300 nm to 10 μm are controlled to be 50 to 100 volume% of the total primary particles included in the secondary particles. Claim 9 A method for manufacturing a positive electrode active material for a secondary battery according to claim 1, comprising: a step of forming precursor particles to form the interior of the positive electrode active material; a step of mixing the formed precursor particles with a lithium compound and performing a first heat treatment; a step of dispersing the first heat-treated particles in distilled water or an alkaline aqueous solution and then adding a compound containing manganese to coat them to form the exterior of the positive electrode active material; and a step of mixing the lithium compound with the coated particles and performing a second heat treatment. Claim 10 A method for manufacturing a positive electrode active material for a secondary battery, wherein the first heat treatment step comprises further mixing and heat-treating a compound containing M1 of Formula 1. Claim 11 A secondary battery comprising the positive electrode active material of claim 1.

Images