Lithium secondary battery with improved energy density

1. A lithium secondary battery comprising:

a cathode using a cathode active material including a lithium mixed transition metal oxide absorbing and discharging lithium ions,

wherein the lithium mixed transition metal oxide comprises:

Li x (Co y A m D z )O t ,

where 0.8≦x≦1.2, (0.8−m−z)≦y≦(2.2−m−z), 0<m≦0.3, 1.8≦t≦4.2,

said A is selected from Mg and Ca, and

said D is at least one selected from the group consisting of Ti, Zr, Hf; V, Nb, Ta; Cr, Mo, W; Mn, Tc, Re; Fe, Ru, Os, Co, Rh, It, Ni, Pd, Pt; Cu, Au, Ag; Zn, Cd, Hg; B, Al, Ga, In, TI; C, Si, Ge, Sn, Pb; N, P, As, Sb and Bi and

and wherein the lithium mixed transition metal oxide further comprises

α-NaFeO 2 composition having a layered crystal structure,

where the transition metal includes a transition metal mixture of Ni, Mn and Co, an average oxidation number of all transition metals except for lithium is greater than +3, and the lithium mixed transition metal oxide satisfies following Equations 1 and 2

1.1 <m (Ni)/ m (Mn)<1.5  (Eq. 1)

0.4 <m (Ni 2+ )/ m (Mn 4+ )<1  (Eq. 2)

wherein m(Ni)/m(Mn) denotes a molar ratio of Ni to Mn, and

m(Ni 2+ )/m(Mn 4+ ) denotes a molar ratio of Ni 2+ to Mn 4+ ; and

wherein the secondary battery final discharge voltage is in the range of about 1.5 V to about 2.75 V.

2. The lithium secondary battery of claim 1 , wherein the lithium mixed transition metal oxide further comprises at least one or more than two compounds represented by following Eq. 3,

(1 −s−t )[Li(Li a Mn (1−a−x−y) Ni x Co y )O 2 ]*s [Li 2 CO 3 ]*t [LiOH]  (3)

where, 0<a<0.3, 0<x<0.8, 0<y<0.6, 0<s<0.05, and 0<t<0.05.