Electrochemical device with improved cycle characteristics

1. An electrochemical device comprising:

(a) a composite separator comprising a porous substrate, a first porous coating layer coated on one surface of the porous substrate and composed of a mixture comprising inorganic particles and a binder polymer, and a second porous coating layer coated on the other surface of the porous substrate and composed of a mixture comprising inorganic particles and a binder polymer, wherein the first porous coating layer is thicker than the second porous coating layer, wherein interstitial volumes present between the inorganic particles of the first coating layer form a pore structure, and wherein interstitial volumes present between the inorganic particles of the second coating layer form a pore structure;

(b) an anode disposed to face the first porous coating layer; and

(c) a cathode disposed to face the second porous coating layer, wherein the thicknesses of the first and second porous coating layers satisfy the following relationship 1:

0.11≦( T 1 −T 2 )/ T tot ≦0.56

wherein T 1 and T 2 represent the thicknesses of the first and second porous coating layers, respectively, and T tot represents the total thickness of the first and second porous coating layers, and

wherein the total thickness of the first and second porous coating layers is from 1.0 to 20.0 μm.

2. The electrochemical device according to claim 1 , wherein the porous substrate is a porous polyolefin membrane.

3. The electrochemical device according to claim 1 , wherein the porous substrate has a thickness of 1 to 100 μm.

4. The electrochemical device according to claim 1 , wherein the binder polymers each independently have a solubility parameter of 15 to 45 Mpa 1/2 .

5. The electrochemical device according to claim 1 , wherein the inorganic particles present in each of the first and second porous coating layers have an average particle diameter of 0.001 to 10 μm.

6. The electrochemical device according to claim 1 , wherein the weight ratio of the inorganic particles to the binder polymer in each of the first and second porous coating layers is from 50:50 to 99:1.

7. The electrochemical device according to claim 1 , wherein the cathode comprises particles of at least one cathode active material selected from the group consisting of LiCoO 2 , LiNiO 2 , LiMn 2 O 4 , LiCoPO 4 , LiFePO 4 , LiNiMhCoO 2 , and LiNi 1-x-y-z Co x M1 y M2 z O z (wherein M1 and M2 are each independently selected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg and Mo, and x, y and z represent the atomic fractions of the corresponding constituent elements of the oxide and satisfy the relations of 0≦x<0.5, 0≦y<0.5, 0≦z<0.5).

8. The electrochemical device according to claim 1 , wherein the cathode comprises at least one cathode active material selected from the group consisting of LiCoO 2 and LiNiMnCoO 2 .

9. The electrochemical device according to claim 1 , wherein the anode comprises particles of at least one anode active material selected from the group consisting of natural graphite, artificial graphite, carbonaceous materials, LTO, silicon (Si), and tin (Sn).

10. The electrochemical device according to claim 1 , wherein the electrochemical device is a lithium secondary battery.

11. The electrochemical device of claim 1 , wherein the binder polymers are each independently selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cyanoethylpullulan, cyanoethyl polyvinyl alcohol, cyanoethyl sucrose, pullulan, and mixtures of two or more kinds thereof.

12. The electrochemical device of claim 1 , wherein the inorganic particles are each independently selected from the group consisting of BaTiO 3 , Pb(Zr,Ti)O 3 (PZI), Pb 1-x La x Zr 1-y Ti y O 3 (PLZT, 0<x<1, 0<y<1), Pb (Mg 1/3 Nb 2/3 )O 3 —PbTiO 3 (PMN-PT), hafnia (HfO 2 ), SrTiO 3 , SnO 2 , CeO 2 , NiO, CaO, ZnO, Y 2 O 3 , Al 2 O 3 , TiO 2 , SiC, Li 3 PO 4 , Li x Ti y (PO 4 ) 3 (0<x<2, 0<y<3), Li x Al y Ti z (PO 4 ) 3 (0<x<2, 0<y<1, 0<z<3), (LiAllTiP) x O y (0<x<4, 0<y<13), (Li x La y TiO 3 , 0<x<2, 0<y<3), Li x Ge y P z S w (0<x<4, 0<y<1, 0<z<1, 0<w<5), Li x N y (0<x<4, 0<y<2), Li x Si y S z (0<x<3, 0<y<2, 0<z<4), Li x P y S z (0<x<3, 0<y<3, 0<z<7), and mixtures of two or more kinds thereof.