a) placing a plate, a cell assembly, a first elastic substrate, and a second elastic substrate into a bag such that the second elastic substrate is adjacent the plate and the cell assembly is between the first elastic substrate and the second elastic substrate to create a filled bag, wherein the cell assembly comprises a cell pouch, a cell stack in the cell pouch, and one or more tabs extending from inside the pouch to outside the pouch;

b) placing the filled bag into a chamber filled with a working fluid; and

c) loading a working pressure to the working fluid, which applies an isostatic compression to the filled bag, thereby compressing the cell stack in the cell pouch, leading to a densified cell assembly,