Negative electrode active material and method of preparing the same
Since a surface of the artificial graphite of the negative electrode active material according to the present invention is modified with nitrogen, dispersibility in an aqueous system may be improved and accordingly, affinity between a binder and the negative electrode active material is increased to increase the adhesive strength of an electrode. Also, since the surface of the artificial graphite is modified with nitrogen, there is an effect of maintaining a high battery capacity. Furthermore, in the method of preparing a negative electrode active material according to the present invention, since an oxygen-containing functional group is connected to the artificial graphite by a mild oxidation process, the nitrogen may be easily attached to the artificial graphite so that the negative electrode active material becomes an electrical conductor by maintaining original crystallinity while having a hydrophilic property. Thus, excellent battery efficiency may be achieved.
1. A negative electrode active material comprising artificial graphite surface-modified with nitrogen atoms,
wherein the artificial graphite is bonded with 5 wt % to 10 wt % of an oxygen-containing functional group having a portion substituted with the nitrogen atoms;
5 wt % to 15 wt % of an oxygen-containing functional group; and
80 wt % to 90 wt % of a hydrogen atom, based on a total weight of covalent bondable sites of outermost carbon atoms of the artificial graphite, and
wherein the artificial graphite is selected from the group consisting of graphitized mesocarbon microbeads, graphitized mesophase pitch-based carbon fibers, and graphitized cokes, and
wherein the artificial graphite has a diameter of 5 μm to 20 μm.
2. The negative electrode active material of claim 1 , wherein the oxygen-containing functional group is at least one selected from the group consisting of a hydroxyl group, an epoxy group, a carboxyl group, and a lactol group.
3. The negative electrode active material of claim 1 , wherein an interplanar spacing (d 002 ) of a carbon hexagonal plane of the negative electrode active material is in a range of 0.3350 nm to 0.3400 nm.
4. A method of preparing the negative electrode active material of claim 1 , the method comprising steps of:
connecting the oxygen-containing functional group to the artificial graphite by a mild oxidation process (step 1); and
doping with the nitrogen atoms by reducing the oxygen-containing functional group connected the artificial graphite of step 1 in a nitrogen atmosphere (step 2).
5. The method of claim 4 , wherein the mild oxidation process of step 1 is performed by heat treating the artificial graphite at a temperature of 500° C. to 600° C. in an air atmosphere.
6. The method of claim 5 , wherein the mild oxidation process is performed for 1 hour to 1.5 hours.
7. The method of claim 4 , wherein the mild oxidation process of step 1 is performed by immersing the artificial graphite in an acid solution for 4 hours to 6 hours and drying the artificial graphite.
8. The method of claim 7 , wherein a nitric acid solution having a concentration of 30 wt % to 50 wt % and a temperature of 50° C. to 70° C. is used as the acid solution.
9. The method of claim 7 , wherein the drying is performed in a vacuum state at a temperature of 250° C. to 300° C.
10. The method of claim 4 , wherein the oxygen-containing functional group of step 1 is bonded to the covalent bondable sites of outermost carbon atoms of the artificial graphite at a substitution ratio of 10 wt % to 20 wt %.
11. The method of claim 4 , wherein the doping with the nitrogen atoms of step 2 is performed in e the nitrogen gas atmosphere in which hydrogen gas and one gas selected from the group consisting of hydrazine (N2H2), ammonia (NH3), and a mixture thereof are mixed.
12. The method of claim 11 , wherein the doping with the nitrogen atoms of step 2 is performed in the nitrogen gas atmosphere in which the hydrazine (N 2 H 2 ) or the ammonia gas (NH 3 ) and the hydrogen gas are mixed in a ratio of 3:7.
13. The method of claim 11 , wherein the doping with the nitrogen atoms of step 2 is performed in a temperature of 800° C. to 1,000° C.
14. A negative electrode for a secondary battery in which a negative electrode collector is coated with a negative electrode active material slurry including the negative electrode active material of claim 1 .
15. The negative electrode for a secondary battery of claim 14 , wherein the negative electrode active material slurry further is a binder.
16. The negative electrode for a secondary battery of claim 15 , wherein the binder is at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose, starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, an ethylene-propylene-diene monomer (EPDM), a sulfonated EPDM, a styrene-butadiene rubber (SBR), and a fluoro rubber.
17. A secondary battery comprising:
a negative electrode;
a positive electrode;
a separator disposed between the negative electrode and the positive electrode; and
a non-aqueous electrolyte solution,
wherein the negative electrode is the negative electrode for a secondary battery of claim 14 .