Electrode for secondary battery, secondary battery and cable-type secondary battery comprising the same

1. A cable-type secondary battery having a horizontal cross section and extending longitudinally, and comprising:

a core for supplying lithium ions, which comprises an electrolyte;

an inner electrode comprising an open-structured inner current collector surrounding the outer surface of the core for supplying lithium ions, an inner electrode active material layer surrounding the outer surface of the inner current collector, a graphite-based coating layer formed on the outer surface of the inner electrode active material layer and comprising graphite, a conductive material and a first polymer binder, and a porous coating layer formed on the outer surface of the graphite-based coating layer and comprising a second polymer binder;

a separation layer surrounding the outer surface of the inner electrode to prevent a short circuit between electrodes; and

an outer electrode surrounding the outer surface of the separation layer and comprising an outer current collector and an outer electrode active material layer.

2. The cable-type secondary battery according to claim 1 , wherein the graphite, the conductive material and the first polymer binder in the graphite-based coating layer are present in a weight ratio of 50:10:40 to 90:1:9.

3. The cable-type secondary battery according to claim 1 , wherein the graphite-based coating layer has pores having a size of 0.1 to 5 μm, and a porosity of 10 to 70%.

4. The cable-type secondary battery according to claim 1 , wherein the conductive material comprises any one selected from the group consisting of carbon black, acetylene black, ketjen black, carbon fiber, carbon nanotube, graphene and a mixture thereof.

5. The cable-type secondary battery according to claim 1 , wherein the first polymer binder is selected from the group consisting of polyvinylidene fluoride (PVDF), hexafluoro propylene (HFP), polyvinylidene fluoride-co-hexafluoro propylene, polyvinylidene fluoride-co-trichloroethylene, polybutyl acrylate, polymethyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, styrene-butadiene rubber, acrylonitrile-styrene-butadiene copolymer, polyimide and a mixture thereof.

6. The cable-type secondary battery according to claim 1 , wherein the porous coating layer has pores having a size of 0.01 to 10 μm, and a porosity of 5 to 95%.

7. The cable-type secondary battery according to claim 1 , wherein the porous coating layer further comprises inorganic particles.

8. The cable-type secondary battery according to claim 7 , wherein the inorganic particles and the second polymer binder are present in a weight ratio of 10:90 to 95:5.

9. The cable-type secondary battery according to claim 7 , wherein the inorganic particles are inorganic particles having a dielectric constant of 5 or higher, inorganic particles having the ability to transport lithium ions, or a mixture thereof.

10. The cable-type secondary battery according to claim 9 , wherein the inorganic particles having a dielectric constant of 5 or higher are selected from the group consisting of BaTiO 3 , Pb(Zr x , Ti 1-x )O 3 (PZT, 0<x<1), Pb 1-x La x Zr 1-y Ti y O 3 (PLZT, 0<x<1, 0<y<1), (1−x)Pb(Mg 1/3 Nb 2/3 )O 3 -xPbTiO 3 (PMN-PT, 0<x<1), hafnia (HfO 2 ), SrTiO 2 , SnO 2 , CeO 2 , MgO, NiO, CaO, ZnO, ZrO 2 , Y 2 O 3 , Al 2 O 3 , SiC, SiO 2 , AlOOH, Al(OH) 2 , TiO 2 and a mixture thereof.

11. The cable-type secondary battery according to claim 9 , wherein the inorganic particles having the ability to transport lithium ions are selected from the group consisting of lithium phosphate (Li 3 PO 4 ), lithium titanium phosphate (Li x Ti y (PO 4 ) 3 , 0<x<2, 0<y<3), lithium aluminum titanium phosphate (Li x Al y Ti z (PO 4 ) 3 , 0<x<2, 0<y<1, 0<z<3), (LiAlTiP) x O y type glass (0<x<4, 0<y<13), lithium lanthanum titanate (Li x La y TiO 3 , 0<x<2, 0<y<3), lithium germanium thiophosphate (Li x Ge y P z S w , 0<x<4, 0<y<1, 0<z<1, 0<w<5), lithium nitride (Li x N y , 0<x<4, 0<y<2), SiS 2 type glass (Li x Si y S z , 0<x<3, 0<y<2, 0<z<4), P 2 S 5 type glass (Li x P y S z , 0<x<3, 0<y<3, 0<z<7) inorganic particles, and a mixture thereof.

12. The cable-type secondary battery according to claim 7 , wherein the inorganic particles have an average diameter of 10 nm to 5 μm.

13. The cable-type secondary battery according to claim 1 , wherein the second polymer binder is selected from the group consisting of polyvinylidene fluoride (PVDF), hexafluoro propylene (HFP), polyvinylidene fluoride-co-hexafluoro propylene, polyvinylidene fluoride-co-trichloroethylene, polybutyl acrylate, polymethyl methacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, styrene-butadiene rubber, acrylonitrile-styrene-butadiene copolymer, polyimide and a mixture thereof.