An exterior material for a lithium ion battery comprising at least a substrate layer, a metal foil layer, and a sealant layer in that order, wherein the sealant layer comprises an anionic functional group-containing layer containing a compound having an anionic functional group, wherein the anionic functional group comprises at least one among a carboxyl group and an acid anhydride group, and wherein the concentration of the anionic functional group contained in the anionic functional group-containing layer is 1.0 mass% or more based on the total amount of the components constituting the anionic functional group-containing layer, characterized in that the sealant layer is composed of a plurality of layers, and at least one layer thereamong is the anionic functional group-containing layer, wherein amongst the plurality of layers of the sealant layer, at least one layer other than the layer furthest from the metal foil layer thereamong is the anionic functional group-containing layer, and wherein the sealant layer comprises a propylene-ethylene random copolymer and a compatible elastomer having compatibility with the propylene-ethylene random copolymer wherein compatible with a propylene-ethylene random copolymer means that the elastomer is dispersed within the propylene-ethylene random copolymer resin at a dispersed phase size of 1 nm or more to less than 200 nm, or comprises polyethylene-based resin and an ethylene-α-olefin-based elastomer being an elastomer which has compatibility with the polyethylene-based resin wherein compatible with a polyethylene-based resin means that the elastomer is dispersed within the polyethylene-based resin at a dispersed phase size of 1 nm or more to less than 200 nm. the sealant layer comprises an anionic functional group-containing layer containing a compound having an anionic functional group, wherein the anionic functional group comprises at least one among a carboxyl group and an acid anhydride group, and wherein the concentration of the anionic functional group contained in the anionic functional group-containing layer is 1.0 mass% or more based on the total amount of the components constituting the anionic functional group-containing layer, characterized in that the sealant layer is composed of a plurality of layers, and at least one layer thereamong is the anionic functional group-containing layer, wherein amongst the plurality of layers of the sealant layer, at least one layer other than the layer furthest from the metal foil layer thereamong is the anionic functional group-containing layer, and wherein the sealant layer comprises a propylene-ethylene random copolymer and a compatible elastomer having compatibility with the propylene-ethylene random copolymer wherein compatible with a propylene-ethylene random copolymer means that the elastomer is dispersed within the propylene-ethylene random copolymer resin at a dispersed phase size of 1 nm or more to less than 200 nm, or comprises polyethylene-based resin and an ethylene-α-olefin-based elastomer being an elastomer which has compatibility with the polyethylene-based resin wherein compatible with a polyethylene-based resin means that the elastomer is dispersed within the polyethylene-based resin at a dispersed phase size of 1 nm or more to less than 200 nm. <EOS>