Cathode active material, secondary battery comprising the same, and method of manufacturing the positive active material

1. A positive active material comprising:

a core comprising a sodium metal oxide capable of intercalation and deintercalation of sodium ions; and

a non-conductive carbonaceous film containing oxygen on at least one portion of a surface of the core,

wherein the non-conductive carbonaceous film comprises at least one carbon compound having a functional group of the formula —C(═O)Ra, —C(═O)ORa, or —OC(═O)ORa, wherein Ra is hydrogen, a C1-C10 alkyl group, or a substituted or an unsubstituted C6-C20 aryl group, wherein the substituent comprises a hydroxy group, a nitro group, a cyano group, an amino group, an amidino group, a hydrazine, a hydrazone, a carboxyl group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid or a salt thereof, a C1-C20 alkyl group, a C2-C20 alkenyl group, a C2-C20 alkynyl group, a C1-C20 heteroalkyl group, a C6-C20 aryl group, a C6-C20 arylalkyl group, a C6-C20 heteroaryl group, a C6-C20 heteroarylalkyl group, or a C1-C20 alkyl group substituted with a halogen atom, and

wherein the core is represented by Formula 4 below, and the non-conductive carbonaceous film containing oxygen is disposed on at least one portion of the surface of the core

Na x Qa z Qb v O 2+d   Formula 4

wherein in Formula 4,

Qa is Fe, Ru, Os, Cr, Mo, W, or a combination thereof

Qb is at least one of Groups 4, 5, 7, 9, or 10 of the periodic table, and

0.5≤x<1, 0.3≤z≤0.5, 0.5≤v≤0.7, z+v=1, and −0.3≤d<1.

2. The positive active material of claim 1 , wherein

Qa is Fe or Cr,

Qb is Mn, Co, Ni, V, Ti, or a combination thereof, and

0.5≤x<1, 0.3≤z≤0.5, 0.5≤v≤0.7, z+v=1, and −0.3≤d<1.

3. The positive active material of claim 1 , wherein the core is represented by Formula 6 below:

Na x Fe z Qb v O 2   Formula 6

wherein in Formula 6,

Qb is Mn or Ni, and

0.5≤x<1, 0.3≤z≤0.5, z+v=1, and 0.5≤v≤0.7.

4. The positive active material of claim 1 , further comprising an oxygen depletion layer on the at least one portion of the surface of the core and disposed between the core and the non-conductive carbonaceous film.

5. The positive active material of claim 4 , wherein the oxygen depletion layer is a layer formed by reduction of the at least one portion of the surface of the core.

6. A secondary battery comprising:

a positive electrode;

an electrolyte; and

a negative electrode, wherein

the positive electrode comprises the positive active material according to claim 1 .

7. A method of preparing of the positive active material according to claim 1 , the method comprising:

mixing a core comprising a sodium metal oxide capable of intercalation and deintercalation of sodium ions, a carbonaceous precursor having at least one hydroxy group or carboxyl group, and a solvent to chemically adsorb the carbonaceous precursor on at least one portion of the surface of the core; and

heat treating the core on which the carbonaceous precursor is formed to form a non-conductive carbonaceous film containing oxygen on the core to prepare the positive active material,

wherein the non-conductive carbonaceous film comprises at least one carbon compound having a functional group of the formula —C(═O)Ra, —C(═O)ORa, or —OC(═O)ORa, wherein Ra is hydrogen, a C1-C10 alkyl group, or a substituted or unsubstituted C6-C20 aryl group, wherein the substituent comprises a hydroxy group, a nitro group, a cyano group, an amino group, an amidino group, a hydrazine, a hydrazone, a carboxyl group or a salt thereof, a sulfonic acid group or a salt thereof, a phosphoric acid or a salt thereof, a C1-C20 alkyl group, a C2-C20 alkenyl group, a C2-C20 alkynyl group, a C1-C20 heteroalkyl group, a C6-C20 aryl group, a C6-C20 arylalkyl group, a C6-C20 heteroaryl group, a C6-C20 heteroarylalkyl group, or a C1-C20 alkyl group substituted with a halogen atom, and

wherein the core is represented by Formula 4 below, and the non-conductive carbonaceous film containing oxygen is disposed on at least one portion of the surface of the core

Na x Qa z Qb v O 2+d   Formula 4

wherein in Formula 4,

Qa is Fe, Ru, Os, Cr, Mo, W, or a combination thereof,

Qb is at least one of Groups 4, 5, 7, 9, or 10 of the periodic table, and

0.5≤x<1, 0.3≤z≤0.5, 0.5≤v≤0.7, z+v=1, and −0.3≤d<1.

8. The method of claim 7 , wherein the process of chemically adsorbing the carbonaceous precursor on at least one portion of the surface of the core occurs via a chemical bond formed by a dehydration reaction between a hydroxy group of the surface of the core and a hydroxy group or a carboxyl group of the carbonaceous precursor.

9. The method of claim 8 , wherein the dehydration reaction further includes a heat treatment process.

10. The method of claim 9 , wherein the heat treatment process is performed at a temperature of about 200° C. to about 400° C. for about 1 hour to about 5 hours.

11. The method of claim 7 , wherein the heat treating is performed at a temperature of about 400° C. to about 800° C.