Cathode active material, method for preparing the same, and lithium secondary batteries including the same

1. A cathode active material comprising:

a lithium-excess lithium metal composite including monoclinic Li 2 MnO 3 having a layered structure and rhombohedral LiMO 2 , wherein M is one or more of Ni, Co, or Mn, and

a fluoro compound,

wherein an FWHM (half value width) value is within a range from 0.164 degree to 0.185 degree,

wherein the lithium-excess lithium metal composite is represented by Formula Li 1+α Ni x Co y Mn z O 2 , wherein 0.1<α≦1 and 0<x+y+z≦1,

wherein the lithium-excess metal composite comprises Li 2 MnO 3 in an amount of 20% or more,

wherein a length of the c-axis in the cathode active material is within a range from 14.241 Å to 14.2429 Å.

2. The cathode active material of claim 1 , wherein the fluoro compound is one selected from the group consisting of LiF, NaF, KF, MgF 2 , CaF 2 , CuF 2 , CdF 2 , FeF 2 , MnF 2 , MgF 2 , NiF 2 , PbF 2 , SnF 2 , SrF 2 , XeF 2 , ZnF 2 , AlF 3 , BF 3 , BiF 3 , CeF 3 , CrF 3 , FeF 3 , InF 3 , LaF 3 , MnF 3 , NdF 3 , VOF 3 , YF 3 , CeF 4 , GeF 4 , HfF 4 , SiF 4 , SnF 4 , TiF 4 , VF 4 , ZrF 4 , VF 5 , NbF 5 , SbF 5 , TaF 5 , BiF 5 , MoF 6 , ReF 6 , SF 6 , WF 6 , NH 4 F, or a mixture thereof.

3. A method for preparing a cathode active material comprising a lithium-excess lithium metal composite including monoclinic Li 2 MnO 3 having a layered structure and rhombohedral LiMO 2 , wherein M is one or more of Ni, Co, or Mn, wherein the lithium-excess lithium metal composite is represented by Formula Li 1+α Ni x Co y Mn z O 2 , wherein 0.1<α≦1 and 0<x+y+z≦1, wherein an FWHM (half value width) value of the cathode active material is within a range from 0.164 degree to 0.185 degree and a length of the c-axis in the cathode active material is within a range from 14.241 Å to 14.2429 Å, the method comprising:

synthesizing a transition metal hydroxide, the transition metal hydroxide being represented by Formula Ni a Co b Mn c (OH) 2 , wherein 0<a<0.4, 0<b≦0.7, 0<c≦0.7, a+b+c=1, and a<b; and

mixing the transition metal hydroxide, a lithium supply source, and a fluoro compound, and then subjecting the mixture to heat treatment at 800° C. or less,

wherein the amount of the fluoro is 0.02 to 0.06 mol per 2.0 mol of a total of the transition metal hydroxide and the lithium supply source.

4. The method of claim 3 , wherein the heat treatment is performed at 700° C.

5. The method of claim 3 , wherein the fluoro is one selected from the group consisting of LiF, NaF, KF, MgF 2 , CaF 2 , CuF 2 , CdF 2 , FeF 2 , MrIF 2 , MgF 2 , NiF 2 , PbF 2 , SnF 2 , SrF 2 , XeF 2 , ZnF 2 , AlF3, BF 3 , BiF 3 , CeF 3 , CrF 3 , FeF 3 , InF 3 , LaF 3 , MnF 3 , NdF 3 , VOF 3 , YF 3 , CeF 4 , GeF 4 , HfF 4 , SiF 4 , SnF 4 , TiF 4 , VF 4 , ZrF 4 , VF s , NbF 5 , SbF 5 , TaF 3 , BiF 5 , MoF 6 , ReF 6 , SF 6 , WF 6 , NH 4 F, or a mixture thereof.

6. The method of claim 3 , wherein the fluoro compound is LiF and the lithium supply source is Li 2 CO 3 .

7. The method of claim 3 , wherein the transition metal hydroxide is synthesized at a pH from 10 to 12.

8. A lithium secondary battery comprising:

a cathode comprising the cathode active material of claim 1 ;

an anode comprising an anode active material; and

an electrolyte present between the cathode and the anode.