Highly reversible lithium intercalating electrode active material, preparation method thereof, electrode and secondary battery comprising the same
Disclosed are an electrode active material, having a composition of SnPx (0.9≦x≦0.98), an electrode comprising the same, and a lithium secondary battery comprising the electrode. Also disclosed is a method for preparing an electrode active material having a composition of SnPx (0.9≦x≦0.98), the method comprising the steps of: preparing a mixed solution of a Sn precursor, trioctyl phosphine (TOP) and trioctyl phosphine oxide (TOPO); and heating the solution. The application of the teardrop-shaped single-crystal SnP0-94 particles as an anode active material for lithium secondary batteries can provide an anode having very excellent cycling properties because the active material has a reversible capacity, which is about two times as large as that of a carbon anode, along with a very low irreversible capacity, and it is structurally very stable against Li ion intercalation/deintercalation in a charge/discharge process, indicating little or no change in the volume thereof.
1. A negative electrode active material having a composition of SnP x , wherein x is 0.94, and the active material particles are monocrystalline.
2. The negative electrode active material according to claim 1 , which is obtained by selective anisotropic growth of crystals along the c-axis of a hexagonal crystal lattice.
3. The negative electrode active material according to claim 1 , wherein the active material particles are teardrop-shaped and have a length of 100 nm to 1 μm and an aspect ratio (long-axis length/short-axis length) of 1.0-10.0.
4. The negative electrode active material according to claim 3 , wherein the long-axis connecting the tail portion with the head portion of the teardrop shape is the c-axis of a hexagonal crystal lattice.
5. The negative electrode active material according to claim 1 , wherein the electrode active material has a hexagonal layered crystal structure, in which tin atoms are hexagonally packed in the unit cell and closely linked with two kinds of diphosphorus atomic pairs, and the layered slab consists of a polymeric network of —[Sn—P—P—Sn]— that exhibits zigzag bonding.
6. The negative electrode active material according to claim 1 , which undergoes reversible lithium intercalation/deintercalation in a charge/discharge process.
7. The negative electrode active material according to claim 1 , wherein the crystal structure of the active material is not changed, even by lithium intercalation/deintercalation.
8. The negative electrode active material according to claim 1 , wherein the oxidation state of Sn is not changed, even by the reversible intercalation/deintercalation of lithium, and charge compensation occurs due to local structural variation around Sn ions.
9. The negative electrode active material according to claim 1 , wherein lithium is located in the interstitial sites of the crystal lattice.
10. The negative electrode active material according to claim 1 , which has a capacity of 600-900 mAh/g between 0V and 1.2V.
11. The negative electrode active material according to claim 1 , which has a capacity retention of more than 90% after 40 cycles.
12. The negative electrode active material according to claim 1 , which has a Coulombic efficiency of more than 80% in the first charge/discharge cycle.
13. An electrode comprising a negative electrode active material as defined in claim 1 .
14. A lithium secondary battery comprising an electrode as defined in claim 13 .
15. A method for preparing a negative electrode active material having a composition of Snp x , wherein 0.9≦x≦0.98 comprising the steps of: preparing a mixed solution of a Sn precursor, trioctyl phosphine (TOP) and trioctyl phosphine oxide (TOPO); and heating the solution.
16. The method according to claim 15 , wherein the Sn precursor is Sn acetate.