US-12620587-B2 - Positive electrode active material and lithium secondary battery including the same

US12620587B2US 12620587 B2US12620587 B2US 12620587B2US-12620587-B2

Abstract

The present invention relates to a positive electrode active material and a lithium secondary battery including the same, and more particularly, to a positive electrode active material including a lithium composite oxide containing at least nickel and cobalt, wherein since the cobalt in the lithium composite oxide has a concentration gradient having at least different slopes from a surface portion toward a central portion, it is possible to improve the stability of particles not only in a surface portion of the lithium composite oxide but also in a central portion thereof, a positive electrode including the positive electrode active material, and a lithium secondary battery using the negative electrode.

Inventors

Yu Gyeong CHUN
Moon Ho Choi
Yoon Young Choi
Jong Seung Shin
Jin Ho BAE
Jin Won Lee

Assignees

ECOPRO BM CO., LTD.

Dates

Publication Date: 20260505
Application Date: 20221025
Priority Date: 20211026

Claims (9)

1 . A positive electrode active material comprising a lithium composite oxide containing at least nickel and cobalt, wherein: the cobalt in the lithium composite oxide has a concentration gradient that decreases from a surface portion of the lithium composite oxide toward a central portion thereof; and the concentration gradient of the cobalt has at least different slopes, and signs of the different slopes are the same, wherein, in Energy Profiling-Energy Dispersive X-Ray Spectroscopy (EP-EDS) analysis which measures a cumulative concentration of the cobalt from a surface of the lithium composite oxide to a depth to which an electron beam penetrates using the electron beam radiated with an acceleration voltage increasing from 1 kV to 30 kV with respect to the surface of the lithium composite oxide, an inflection point at which the slope of the concentration gradient of the cobalt in the lithium composite oxide changes in a region where the acceleration voltage is 7.5 kV to 12.5 kV is present, and wherein, when s1 represents the slope of the concentration gradient of the cobalt in the lithium composite oxide in a region where the acceleration voltage is 1 kV to 10 kV, s1 satisfies the following Equation 1, 2.0≤ s 1≤3.6. [Equation 1]
2 . The positive electrode active material of claim 1 , wherein an absolute value of a slope of the cobalt in the lithium composite oxide relatively close to the surface portion of the lithium composite oxide is greater than an absolute value of a slope of the cobalt relatively close to the central portion of the lithium composite oxide.
3 . The positive electrode active material of claim 1 , wherein: the lithium composite oxide is a secondary particle in which a plurality of primary particles are aggregated; cobalt in the secondary particle has a concentration gradient that decreases from a surface portion of the secondary particle toward a central portion thereof; and in EP-EDS analysis which measures a cumulative concentration of the cobalt from a surface of the secondary particle to a depth to which an electron beam penetrates using the electron beam radiated with an acceleration voltage increasing from 1 kV to 30 kV with respect to the surface of the secondary particle, an inflection point at which the slope of the concentration gradient of the cobalt in the secondary particle changes in a region where the acceleration voltage is 7.5 kV to 12.5 kV is present.
4 . The positive electrode active material of claim 1 , wherein, when s2 represents the slope of the concentration gradient of the cobalt in the lithium composite oxide in a region where the acceleration voltage is 10 kV to 30 kV, s2 satisfies the following Equation 2, 0.2≤ s 2≤0.7. [Equation 2]
5 . The positive electrode active material of claim 1 , wherein, when s1 represents the slope of the concentration gradient of the cobalt in the lithium composite oxide in a region where the acceleration voltage is 1 kV to 10 kV and s2 represents the slope of the concentration gradient of the cobalt in the lithium composite oxide in a region where the acceleration voltage is 10 kV to 30 kV, s1 and s2 satisfy Equation 3 below: 1.7≤ s 1− s 2≤3.0. [Equation 3]
6 . The positive electrode active material of claim 1 , wherein the lithium composite oxide is represented by the following Chemical Formula 1, Li w Ni 1−(x+y+z+z′) Co x M1 y M2 z B z O 2 [Chemical Formula 1] where M1 is at least one selected from Mn and Al, M2 is at least one selected from P, Sr, Ba, Ti, Zr, Mn, Al, W, Ce, Hf, Ta, Cr, F, Mg, Cr, V, Fe, Zn, Si, Y, Ga, Sn, Mo, Ge, Nd, Gd, and Cu, M1 and M2 are different from each other, 0.5≤w≤1.5, 0≤x≤0.50, 0≤y≤0.20, 0≤z≤0.20, and 0≤z′≤0.20.
7 . The positive electrode active material of claim 3 , wherein: the lithium composite oxide further includes a coating layer covering at least a portion of an interface between the primary particles and the surface of the secondary particle; and at least one metal oxide represented by the following Chemical Formula 2 is present in the coating layer: Li a M3 b O c [Chemical Formula 2] where M3 is at least one selected from Ni, Mn, Co, Fe, Cu, Nb, Mo, Ti, Al, Cr, Zr, Zn, Na, K, Ca, Mg, Pt, Au, B, P, Eu, Sm, W, Ce, V, Ba, Ta, Sn, Hf, Ce, Gd, and Nd, 0≤a≤10, 0≤b≤8, and 2≤c≤13, provided that a and b are not simultaneously 0.
8 . A positive electrode comprising the positive electrode active material of claim 1 .
9 . A lithium secondary battery using the positive electrode of claim 8 .

Description

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority to and the benefit of Korean Patent Application No. 10-2021-0143300, filed on Oct. 26, 2021, the disclosure of which is incorporated herein by reference in its entirety. BACKGROUND 1. Field of the Invention The present invention relates to a positive electrode active material and a lithium secondary battery including the same, and more particularly, to a positive electrode active material including a lithium composite oxide containing at least nickel and cobalt, wherein since the cobalt in the lithium composite oxide has a concentration gradient having at least different slopes from a surface portion toward a central portion, it is possible to improve the stability of particles not only in the surface portion of the lithium composite oxide but also in the central portion thereof, a positive electrode including the positive electrode active material, and a lithium secondary battery using the positive electrode 2. Discussion of Related Art Batteries store electrical power by using materials facilitating an electrochemical reaction at a positive electrode and a negative electrode. As a representative example of such batteries, there is a lithium secondary battery storing electrical energy due to a difference in chemical potential when lithium ions are intercalated/deintercalated into/from a positive electrode and a negative electrode. The lithium secondary battery uses materials enabling reversible intercalation/deintercalation of lithium ions as positive electrode and negative electrode active materials, and is manufactured by charging an organic electrolyte solution or a polymer electrolyte solution between the positive electrode and the negative electrode. A lithium composite oxide is used as a positive electrode active material of the lithium secondary battery, and composite oxides such as LiCoO2, LiMn2O4, LiNiO2, LiMnO2, etc. Are being studied. Among the positive electrode active materials, LiCoO2 is most widely used due to excellent lifetime characteristics and charge/discharge efficiency, but it is expensive because of the limited resource of cobalt, which is used as a raw material, and thus has a disadvantage of limited price competitiveness. Lithium manganese oxides such as LiMnO2 and LiMn2O4 have advantages of excellent thermal safety and low costs, but also have problems of small capacity and poor high-temperature characteristics. In addition, while a LiNiO2-based positive electrode active material exhibits a battery characteristic such as a high discharging capacity, due to cation mixing between Li and a transition metal, it is difficult to synthesize the LiNiO2-based positive electrode active material, thereby causing a big problem in rate characteristics. In addition, depending on the intensification of such cation mixing, a large amount of Li by-products is generated, and since most of the Li by-products consist of compounds of LiOH and Li2CO3, they become a cause of gelation in preparation of a positive electrode paste and gas generation according to charge/discharge progression after the preparation of an electrode. Residual Li2CO3 increases the swelling phenomenon of a cell and thus reduces cycles and also leads to the swelling of a battery. Meanwhile, the lithium composite oxide included in the positive electrode active material involves a change in volume according to intercalation/deintercalation of lithium ions with respect to the lithium composite oxide during charging and discharging. Usually, the lithium composite oxide is in the form of a secondary particle in which a plurality of primary particles are aggregated, and there is a problem in that a rapid change in volume of the primary particles occurs during charging and discharging, cracks are generated in the secondary particle when a stress due to repeated charging and discharging is accumulated, or a collapse of a crystal structure or a change in crystal structure (phase transition) occurs. Since this problem in turn acts as a cause of degrading the stability and reliability of the positive electrode active material, various studies have been made to mitigate the change in volume of the lithium composite oxide during charging and discharging, or minimize the occurrence of stress due to the change in volume, thereby preventing damage to the particles. SUMMARY OF THE INVENTION In the lithium secondary battery market, while the growth of lithium secondary batteries for electric vehicles is playing a leading role in the market, the demand for positive electrode materials used in lithium secondary batteries is also continuously changing. For example, in the past, lithium secondary batteries using LFP have been mainly used from the viewpoint of ensuring safety, but in recent years, the use of nickel-based lithium composite oxides having a larger energy capacity per weight compared to LFP has been expanding. Accordingly, a positive electrode active material used in a