NOVELTY - A lithium ion battery sheath material (1) comprises metal foil layer (15), inner side contact bonding layer (17), and sealant layer (18) which are laminated on one surface side of base material layer (10) in this order and corrosion-prevention process layer (16) on surface at the side of contact bonding layer of metal foil layer. Base material layer consists of biaxially stretched film obtained by co-extruding thermoplastic resin (a) (10a) and thermoplastic resin (b) (10b) and arranging layer of thermoplastic resin (b) at the metal foil layer side of base material layer.    USE - A lithium ion battery sheath material for electric vehicle.    ADVANTAGE - The lithium ion battery sheath material has excellent moldability and long-term reliability and safety.    DETAILED DESCRIPTION - A lithium ion battery sheath material comprises metal foil layer, inner side contact bonding layer, and sealant layer which are laminated on one surface side of base material layer in this order and corrosion-prevention process layer on surface at the side of contact bonding layer of metal foil layer. Base material layer consists of biaxially stretched film obtained by co-extruding thermoplastic resin (a) and thermoplastic resin (b) and arranging layer of thermoplastic resin (b) at the metal foil layer side of base material layer in which layer of thermoplastic resin (b) contacts metal foil layer, where thermoplastic resin (a) is aromatic polyester resin or polyamide resin, and thermoplastic resin (b) is modified thermoplastic resin graft modified with acid anhydride of unsaturated carboxylic acid and/or unsaturated carboxylic acid derivative e.g. ester of unsaturated carboxylic acid.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional diagram of the lithium ion battery sheath material.    Lithium ion battery sheath material (1)    Base material layer (10)    Thermoplastic resin a layer (10a)    Thermoplastic resin b layer (10b)    Metal foil layer (15)    Corrosion-prevention process layer (16)    Inner side contact bonding layer (17)    Sealant layer (18)