The invention provides an encapsulating material for a lithium-ion cell. The encapsulating material is provided with a substrate layer comprising a resin film, and a first adhesive layer, a metal foil layer, a corrosion-prevention process layer, a second adhesive layer, and a sealant layer which are laminated on one surface of the substrate layer in order. The resin film has a water-absorption rate of 0.1% to 3% when tested according to JIS K 7209:2000. The test sample, when stretched 10% in a tensile test (performed at a tensioning rate of 300 mm/min on a 6-mm-wide test sample that has been stored for 24 hours in a 23 DEG C/40%RH environment, the distance between reference points on the resin film being 35 mm), has a stress value of no greater than 110 MPa in either the MD direction or the TD direction, at least one of the stress value in the MD direction or TD direction of the test sample being at least 70 MPa.