Separator and method of manufacturing the same and rechargeable lithium battery including the same

1. A separator for a lithium battery comprising:

a porous substrate; and

a coating layer on at least one side of the porous substrate, the coating layer having a first side adjacent to the porous substrate, and a second side opposite the first side, the coating layer comprising:

an inorganic compound; and

a polymer binder comprising a first polymer binder and a second polymer binder, and

wherein each of the first and second polymer binders is an emulsion type polymer binder that exists as particulates in a solvent, wherein the emulsion type polymer binder is a urethane-based polymer, a styrene-butadiene rubber, an acrylated styrene-butadiene rubber, an acrylonitrile-butadiene rubber, an acrylonitrile-butadiene-styrene rubber, an acrylic rubber, a butyl rubber, a fluorine rubber, polyethylene, polypropylene, an ethylene propylene copolymer, an ethylene propylene diene copolymer, polystyrene, or a combination thereof,

wherein the first polymer binder has a first average particle diameter of about 50 nm to about 500 nm and the second polymer binder has a second average particle diameter of about 20 nm to about 400 nm, the second average particle diameter being less than or equal to 80% of the first average particle diameter,

the polymer binder comprises a greater amount of the first polymer binder than the second polymer binder, and

wherein an amount of the second polymer binder at the second side is greater than an amount of the second polymer binder at the first side.

2. The separator of claim 1 , wherein the first polymer binder is present in the polymer binder in an amount of about 70 to 99 parts by weight based on the total weight of the polymer binder, and the second polymer binder is present in the polymer binder in an amount of about 1 to 30 parts by weight based on the total weight of the polymer binder.

3. The separator of claim 1 , wherein the polymer binder is present in the coating layer in an amount of about 1 to 30 parts by weight based on 100 parts by weight of the inorganic compound.

4. The separator of claim 1 , wherein the inorganic compound comprises a material selected from the group consisting of metal oxides, semi-metal oxides, metal fluorides, metal hydroxides, and combinations thereof.

5. The separator of claim 4 , wherein the inorganic compound is selected from the group consisting of Al 2 O 3 , SiO 2 , TiO 2 , SnO 2 , CeO 2 , MgO, NiO, CaO, ZnO, ZrO 2 , Y 2 O 3 , SrTiO 3 , BaTiO 3 , Mg(OH) 2 , MgF, and combinations thereof.

6. The separator of claim 1 , wherein the inorganic compound has a particle diameter of about 1 nm to 800 nm.

7. The separator of claim 1 , wherein the emulsion type polymer binder is an acrylated styrene-butadiene rubber, an acrylonitrile-butadiene rubber, an acrylonitrile-butadiene-styrene rubber, an acrylic rubber, a butyl rubber, polyethylene, polypropylene, an ethylene propylene copolymer, an ethylene propylene diene copolymer, polystyrene, or a combination thereof.

8. A rechargeable lithium battery comprising:

a positive electrode comprising a positive active material;

a negative electrode comprising a negative active material; and

the separator of claim 1 between the positive electrode and the negative electrode.

9. A method of forming the separator for a lithium battery according to claim 1 , the method comprising:

mixing the inorganic compound and the polymer binder to form a slurry;

applying the slurry to at least one side of the porous substrate; and

drying the slurry at a rate of about 30 to 70%/min to form the coating layer.

10. The method of claim 9 , wherein the drying the slurry comprises drying the slurry at a rate of about 40 to 60%/min.