Positive electrode active material and lithium secondary battery comprising the same

Abstract

The present invention relates to a positive electrode active material which has the structural stability of a lithium composite oxide constituting a positive electrode active material and a lithium secondary battery including the same. The lithium composite oxide constituting the positive electrode active material according to the present invention is able to reduce the surface area and grain boundary of secondary particles having a side reaction with an electrolyte solution, thereby improving high-temperature stability and reducing gas generation caused by the positive electrode active material, and the structural stability of the lithium composite oxide may be improved using a cation-mixing layer covering the surface of a primary particle.

What is claimed is: 
     
         1 . A positive electrode active material, comprising:
 a primary particle, which is lithium-based composite oxide, and a secondary particle in which a plurality of primary particles are aggregated, and   wherein a ratio (x2/x1) of an average particle size (x2) of secondary particles to an average particle size (x1) of primary particles is 1.9 or more and 10.3 or less,   wherein an Ni occupancy in an Li 3a site obtained from the Rietveld analysis of the X-ray diffraction of the secondary particle is more than 0.53% and less than 6.44%.   
     
     
         2 . The positive electrode active material of  claim 1 , wherein a cation-mixing layer is present on the surface of the primary particle, and
 a ratio (d1/x1) of the thickness (d1) of the cation-mixing layer to the average particle size (x1) of primary particles is more than 0.0008 and less than 0.0052.   
     
     
         3 . The positive electrode active material of  claim 2 , wherein a thickness (d1) of the cation-mixing layer, which is present on the surface of the primary particle, is more than 0.00023 μm and less than 0.01204 μm, 
     
     
         4 . The positive electrode active material of  claim 2 , wherein the cation-mixing layer has a crystal structure selected from a layered structure, a rock salt structure and a spinel structure. 
     
     
         5 . The positive electrode active material of  claim 1 , wherein a cation-mixing layer is present on the surface of the secondary particle, and
 a ratio (d2/x2) of the thickness (d2) of the cation-mixing layer to the average particle size (x2) of secondary particles is more than 0.00014 and less than 0.00281.   
     
     
         6 . The positive electrode active material of  claim 5 , wherein a thickness (d2) of the cation-mixing layer, which is present on the surface of the secondary particle, is more than 0.00043 μm and less than 0.01208 μm. 
     
     
         7 . The positive electrode active material of  claim 5 , wherein the cation-mixing layer has a crystal structure selected from a layered structure, a rock salt structure and a spinel structure. 
     
     
         8 . The positive electrode active material of  claim 1 , wherein the positive electrode active material is an assembly of secondary particles having different grain boundary density:
   Grain boundary density=(the number of interfaces between primary particles in secondary particle/the number of primary particles constituting secondary particle)   
     
     
         9 . The positive electrode active material of  claim 8 , wherein the proportion of secondary particles having a grain boundary density of 0.5 or less among the plurality of secondary particles constituting the positive electrode active material is 30% or more:
   Grain boundary density=(the number of interfaces between primary particles in secondary particle/the number of primary particles constituting secondary particle)   
     
     
         10 . The positive electrode active material of  claim 9 , wherein the proportion of secondary particles having a grain boundary density of 0.5 or less among the plurality of secondary particles constituting the positive electrode active material is less than 84%:
   Grain boundary density=(the number of interfaces between primary particles in secondary particle/the number of primary particles constituting secondary particle)   
     
     
         11 . The positive electrode active material of  claim 1 , wherein the positive electrode active material comprises a first aggregate composed of one or two primary particles, a second aggregate composed of three to six primary particles, and a third aggregate composed of seven to ten primary particles,
 wherein the proportion of the first aggregate among the positive electrode active material is 30% or more.   
     
     
         12 . The positive electrode active material of  claim 1 , wherein the primary particle has an average particle size of 1.0 μm to 5.0 μm. 
     
     
         13 . The positive electrode active material of  claim 1 , wherein the secondary particle has an average particle size of 1.0 μm to 20.0 μm. 
     
     
         14 . The positive electrode active material of  claim 1 , wherein the positive electrode active material comprises a small secondary particle and a large secondary particle,
 wherein the small secondary particle has an average particle size of 1.0 μm to 5.0 μm, and the large secondary particle has an average particle size of 10.0 μm to 20.0 μm.   
     
     
         15 . The positive electrode active material of  claim 14 , an average particle of an assembly of the secondary particles in which the small secondary particles and the large secondary particles are mixed is 3.0 μm to 18.0 μm. 
     
     
         16 . The positive electrode active material of  claim 1 , wherein the lithium-based composite oxide is represented by Formula 1 below:
   Li a Ni 1−(b+c+d+e) Co b M1 c M2 d M3 e O f   [Formula 1]
   Wherein, M1 is Mn or Al,   M2 and M3 are each independently selected from Al, Ba, B, Ce, Cr, Mg, Mn, Mo, Na, K, P, Sr, Ti, W, Nb and Zr,   M1 to M3 are different metals, and   0.90≤a≤1.05, 0≤b≤0.20, 0≤c≤0.20, 0≤d≤0.05, 0≤e≤0.05, and 1.0≤f≤2.0.   
     
     
         17 . The positive electrode active material of  claim 1 , a starting temperature at which a weight loss begins, as measured by thermogravimetric (TGA) analysis for the positive electrode active material (under a normal pressure, Ar atmosphere, temperature range from 25 to 350° C. at a heating rate of 10° C./min), is higher than 225° C. 
     
     
         18 . The positive electrode active material of  claim 1 , a temperature at the peak of the weight loss, as measured by thermogravimetric (TGA) analysis for the positive electrode active material (under a normal pressure, Ar atmosphere, temperature range from 25 to 350° C. at a heating rate of 10° C./min), is higher than 232° C. 
     
     
         19 . A positive electrode comprising the positive electrode active material according to  claim 1 . 
     
     
         20 . A lithium secondary battery comprising the positive electrode according to  claim 19 .