Composite cathode active material, cathode and lithium battery including the same, and method of preparing the composite cathode active material

1 . A composite cathode active material comprising a plurality of secondary particles, the secondary particles comprising:

a core comprising an aggregate of a plurality of primary particles; and

a shell on the core, the shell comprising a second composition comprising at least two second metals,

wherein the plurality of primary particles comprise a nickel-containing lithium transition metal oxide represented by Formula 4:

Li a Ni b Co c Mn d M3 e O 2   Formula 4

wherein, in Formula 4, 0.9≤a≤1.1, 0.7<b<1.0, 0<c<0.08, 0<d≤0.1, 0.001<e≤0.0045, b+c+d+e=1, and

M3 is Zr, and

a first composition including Li a Zr b O c disposed in grain boundaries between adjacent primary particles of the plurality of primary particles and on a surface of the core to provide a Li a Zr b O c shell, the Li a Zr b O c having a monoclinic crystalline structure, where 1.9≤a≤2.1, 0.9≤b≤1.1, and 2.9≤c≤3.1,

wherein the second metals comprises Mg and at least one of Co or Ni, and

wherein the second composition is different from the first composition, and the second composition has a crystalline structure that is different from the crystal structure of the first composition and the crystal structure of the nickel-containing lithium transition metal oxide, and

wherein a total amount of the zirconium in the secondary particle is about 0.05 mol % to 0.75 mol % with respect to total moles of the transition metal and the zirconium in the nickel-containing lithium transition metal oxide.

2 . The composite cathode active material of claim 1 , wherein the nickel-containing lithium transition metal oxide of the core comprises a first phase and the monoclinic crystalline structure of Li a Zr b O c as a second phase different from the first phase.

3 . The composite cathode active material of claim 1 , wherein the monoclinic crystalline structure of the Li a Zr b O c belongs to a C2/m, C12/c1, or C2/c space group.

4 . The composite cathode active material of claim 1 , wherein the first composition consists essentially of Li 2 ZrO 3 .

5 . The composite cathode active material of claim 1 , wherein the core comprises a first inner region and a second inner region,

wherein the first inner region extends from a center of the core to halfway between the center of the core and a surface of the core, and the second inner region extends from halfway between the center of the core and the surface of the core to the surface of the core, and

50% or greater of the grain boundaries in the first inner region comprise the Li a Zr b O c , and 50% or greater of the grain boundaries in the second inner region comprise the Li a Zr b O c .

6 . The composite cathode active material of claim 1 , wherein the plurality of primary particles have a polyhedral shape, and the grain boundaries between the plurality of primary particles has a substantially rectilinear form.

7 . The composite cathode active material of claim 1 , wherein at least one grain boundary between adjacent primary particles among the plurality of primary particles, extends in a direction parallel to adjacent surfaces of the adjacent primary particles, and

at least one grain boundary extends in a direction different from a tangential direction of an outer surface of the core of the secondary particle.

8 . The composite cathode active material of claim 1 , wherein the core comprises a first grain boundary and a second grain boundary, and each of the first grain boundary and the second grain boundary are adjacent to a same primary particle, and

wherein the first grain boundary and the second grain boundary intersect at an angle determined by a shape of the primary particle.

9 . The composite cathode active material of claim 1 , wherein the core comprises a plurality of grain boundaries amongst the plurality of primary particles, wherein each grain boundary of the plurality of grain boundaries extends in a direction parallel to a surface of an adjacent primary particle, and

wherein each grain boundary of the plurality of grain boundaries extends in a different direction from each other.

10 . The composite cathode active material of claim 1 , wherein an average grain boundary length is in a range of 50 nanometers to 1000 nanometers and an average grain boundary thickness is in a range of 1 nanometer to 50 nanometers, and

wherein a length direction of the grain boundary is parallel to adjacent surfaces of adjacent primary particles, and thickness direction of the grain boundary is perpendicular to adjacent surfaces of adjacent primary particles.

11 . The composite cathode active material of claim 1 , wherein the core has an average particle diameter of 1 micrometer to 30 micrometers, and the shell has a thickness of 5 nanometers to 40 nanometers.

12 . The composite cathode active material of claim 1 , wherein an amount of the zirconium in the secondary particle is about 0.05 mol % to 0.65 mol % with respect to total moles of the transition metal and the zirconium in the nickel-containing lithium transition metal oxide.

13 . The composite cathode active material of claim 1 , wherein an area of pores in a cross-section of the composite cathode active material is about 1% or less with respect to the total area of the cross-section.

14 . A cathode comprising the composite cathode active material according to claim 1 .

15 . A lithium battery comprising,

the cathode according to claim 14 ,

an anode, and

an electrolyte between the cathode and the anode.

16 . The composite cathode active material of claim 1 , wherein the second metals include Co and Mg.

17 . The lithium battery of claim 15 , wherein the battery exhibits a capacity retention rate of 93.6% to 94.9% as determined in accordance with Equation 2:

Capacity retention rate [%]=[Discharge capacity at 56 th cycle/Discharge capacity at 7 th cycle]×100%.  Equation 2