What is claimed is:1. A battery module comprising:one or more cell assemblies;a main housing comprising a main boundary portion surrounding an accommodation space in which the one or more cell assemblies are positioned;a first plate comprising a first boundary portion coupled with the main boundary portion and a first exposure portion exposed from the main boundary portion and defining a side of a cooling fluid passage for cooling the accommodation space; anda second plate facing the first plate and comprising a second boundary portion coupled with the main boundary portion and a second exposure portion exposed from the main boundary portion and defining an other side of the cooling fluid passage,wherein the second boundary portion includes a first portion located along an edge of the second plate and a second portion located around a slit provided in the second plate,wherein the main boundary portion and the second boundary portion are coupled with each other by a first portion of a second coupling line located along the edge of the second plate and a second portion of the second coupling line located around the slit in the second plate,wherein the second coupling line includes a sealing portion located between the main boundary portion and the second boundary portion,wherein the sealing portion includes:a first sealing portion extending between the main boundary portion and the second boundary portion; anda second sealing portion extending from a location touching an outside of the main boundary portion across the first sealing portion in a direction crossing the first sealing portion.2. The battery module of claim 1, wherein the first sealing portion and the second sealing portion cross each other in a middle location between an uppermost location and a lowermost location in the direction in which the second sealing portion extends.3. The battery module of claim 1, whereinan outer sealing portion for coupling between a main outer boundary portion of the main boundary portion and the second boundary portion includes the first sealing portion extending from a side of the second sealing portion and has an asymmetrical cross-section, andan inner sealing portion for coupling between a main inner boundary portion of the main boundary portion and the second boundary portion includes the first sealing portion extending from both sides of the second sealing portion and has a symmetrical cross-section.4. The battery module of claim 1, wherein the second boundary portion includes a flat plate portion coupled with the main boundary portion with the sealing portion between the flat plate portion and the main boundary portion, and a protrusion step portion provided between the flat plate portion and the second exposure portion.5. The battery module of claim 4, wherein the protrusion step portion includes an upper surface upwardly protruding and a lower surface that is concave and indented from below.6. The battery module of claim 5, wherein an upper surface of the protrusion step portion is aligned with the main boundary portion by being inserted into a pair of bump portions, which protrude from both sides of the protrusion step portion in the main boundary portion such that the protrusion step portion is between the pair of bump portions.7. The battery module of claim 5, wherein a lower surface of the protrusion step portion has a jig groove having a concave shape.8. A battery module comprising:one or more cell assemblies;a main housing comprising a main boundary portion surrounding an accommodation space in which the one or more cell assemblies are positioned;a first plate comprising a first boundary portion coupled with the main boundary portion and a first exposure portion exposed from the main boundary portion and defining a side of a cooling fluid passage for cooling the accommodation space; anda second plate facing the first plate and comprising a second boundary portion coupled with the main boundary portion and a second exposure portion exposed from the main boundary portion and defining an other side of the cooling fluid passage,wherein the second boundary portion includes a first portion located along an edge of the second plate and a second portion located around a slit provided in the second plate,wherein the main boundary portion and the second boundary portion are coupled with each other by a first portion of a second coupling line located along the edge of the second plate and a second portion of the second coupling line located around the slit in the second plate,wherein the second coupling line includes a sealing portion located between the main boundary portion and the second boundary portion,wherein the sealing portion includes:a first sealing portion extending between the main boundary portion and the second boundary portion; anda second sealing portion extending from a location touching an outside of the main boundary portion across the first sealing portion in a direction crossing the first sealing portion.9. The battery module of claim 8, wherein the first sealing portion and the second sealing portion cross each other in a middle location between an uppermost location and a lowermost location in the direction in which the second sealing portion extends.10. The battery module of claim 8, whereinan outer sealing portion for coupling between a main outer boundary portion of the main boundary portion and the second boundary portion includes the first sealing portion extending from a side of the second sealing portion and has an asymmetrical cross-section, andan inner sealing portion for coupling between a main inner boundary portion of the main boundary portion and the second boundary portion includes the first sealing portion extending from both sides of the second sealing portion and has a symmetrical cross-section.11. The battery module of claim 8, wherein the second boundary portion includes a flat plate portion coupled with the main boundary portion with the sealing portion between the flat plate portion and the main boundary portion, and a protrusion step portion provided between the flat plate portion and the second exposure portion.12. The battery module of claim 11, wherein the protrusion step portion includes an upper surface upwardly protruding and a lower surface that is concave and indented from below.13. The battery module of claim 12, wherein an upper surface of the protrusion step portion is aligned with the main boundary portion by being inserted into a pair of bump portions, which protrude from both sides of the protrusion step portion in the main boundary portion such that the protrusion step portion is between the pair of bump portions.14. The battery module of claim 12, wherein a lower surface of the protrusion step portion has a jig groove having a concave shape.