A battery module comprising: a cell stack comprising a plurality of unit stacks each comprising a plurality of battery cells; a module housing accommodating the cell stack therein and comprising a plurality of insertion slits respectively formed in a top surface and a bottom surface; and at least one partition wall assembly located parallel to each of the plurality of battery cells, and located between adjacent unit stacks and deformed due to volume expansion due to swelling of the plurality battery cells to absorb the swelling, wherein the at least one partition wall assembly comprises: a pair of partition walls spaced apart from each other to face each other; and a pair of connectors respectively formed at a position downwardly spaced apart by a certain distance from upper ends of the pair of partition walls and at a position upwardly spaced apart by a certain distance from lower ends of the pair of partition walls, and configured to connect the pair of partition walls; wherein the upper ends and the lower ends of the pair of partition walls are configured to be inserted and fixed through the plurality of insertion slits, such that the upper and lower ends of the pair of partition walls are configured to pass through the plurality of insertion slits to be exposed to the outside of the module housing; wherein each of the pair of partition walls has a length and a width corresponding to the respective insertion slit, and wherein the number of insertion slits corresponds to the number of the partition walls. a cell stack comprising a plurality of unit stacks each comprising a plurality of battery cells; a module housing accommodating the cell stack therein and comprising a plurality of insertion slits respectively formed in a top surface and a bottom surface; and at least one partition wall assembly located parallel to each of the plurality of battery cells, and located between adjacent unit stacks and deformed due to volume expansion due to swelling of the plurality battery cells to absorb the swelling, wherein the at least one partition wall assembly comprises: a pair of partition walls spaced apart from each other to face each other; and a pair of connectors respectively formed at a position downwardly spaced apart by a certain distance from upper ends of the pair of partition walls and at a position upwardly spaced apart by a certain distance from lower ends of the pair of partition walls, and configured to connect the pair of partition walls; wherein the upper ends and the lower ends of the pair of partition walls are configured to be inserted and fixed through the plurality of insertion slits, such that the upper and lower ends of the pair of partition walls are configured to pass through the plurality of insertion slits to be exposed to the outside of the module housing; wherein each of the pair of partition walls has a length and a width corresponding to the respective insertion slit, and wherein the number of insertion slits corresponds to the number of the partition walls. a pair of partition walls spaced apart from each other to face each other; and a pair of connectors respectively formed at a position downwardly spaced apart by a certain distance from upper ends of the pair of partition walls and at a position upwardly spaced apart by a certain distance from lower ends of the pair of partition walls, and configured to connect the pair of partition walls; wherein the upper ends and the lower ends of the pair of partition walls are configured to be inserted and fixed through the plurality of insertion slits, such that the upper and lower ends of the pair of partition walls are configured to pass through the plurality of insertion slits to be exposed to the outside of the module housing; wherein each of the pair of partition walls has a length and a width corresponding to the respective insertion slit, and wherein the number of insertion slits corresponds to the number of the partition walls.