Lithium composite oxide and lithium secondary battery comprising the same

A lithium composite oxide, comprising:

• a primary particle enabling lithium intercalation and deintercalation and a secondary particle in which a plurality of primary particles are aggregated,
• wherein a niobium (Nb)-containing oxide is present in at least a part of a region selected from the interface between the primary particles and the surface of the secondary particle,
• wherein a c-axis length obtained from a Rietveld analysis of X-ray diffraction is more than 14.177 Å and less than 14.218 Å, and
• wherein a ratio of a c-axis length to an a-axis length obtained from a Rietveld analysis of X-ray diffraction (c/a) is 4.934 to 4.939.