A lithium ion battery comprising: a lithium ion battery module provided with a metal tab, wherein the lithium ion battery module includes an electrolyte containing lithium hexa-fluorophosphate; a laminated structure forming a package holding the lithium ion battery module therein so that the tab projects outside from the package, including, in the following order, at least a base layer, a barrier layer made of annealed aluminum, and an innermost layer having heat sealing properties; and an adhesive film disposed between the tab and the innermost layer, the adhesive film having sealing properties to both the tab and the innermost layer; wherein: a peripheral part of the package including a part where the adhesive film is disposed between the tab and the innermost layer such that the tab extends outside, is heat-sealed to seal the lithium ion battery module; at least a surface of the barrier layer facing the innermost layer has been subjected to a chemical conversion treatment; and a front surface, a rear surface and a side surface of at least the heat-sealed portion of the tab has been subjected to a chemical conversion treatment of applying, drying and baking at a temperature not lower than 180°C a solution which is a mixture of a phenolic resin, a chromium fluoride compound and phosphoric acid, or a mixture of a phenolic resin and a metallic salt or metal, wherein the metal is molybdenum, titanium or zirconium. a lithium ion battery module provided with a metal tab, wherein the lithium ion battery module includes an electrolyte containing lithium hexa-fluorophosphate; a laminated structure forming a package holding the lithium ion battery module therein so that the tab projects outside from the package, including, in the following order, at least a base layer, a barrier layer made of annealed aluminum, and an innermost layer having heat sealing properties; and an adhesive film disposed between the tab and the innermost layer, the adhesive film having sealing properties to both the tab and the innermost layer; wherein: a peripheral part of the package including a part where the adhesive film is disposed between the tab and the innermost layer such that the tab extends outside, is heat-sealed to seal the lithium ion battery module; at least a surface of the barrier layer facing the innermost layer has been subjected to a chemical conversion treatment; and a front surface, a rear surface and a side surface of at least the heat-sealed portion of the tab has been subjected to a chemical conversion treatment of applying, drying and baking at a temperature not lower than 180°C a solution which is a mixture of a phenolic resin, a chromium fluoride compound and phosphoric acid, or a mixture of a phenolic resin and a metallic salt or metal, wherein the metal is molybdenum, titanium or zirconium. a peripheral part of the package including a part where the adhesive film is disposed between the tab and the innermost layer such that the tab extends outside, is heat-sealed to seal the lithium ion battery module; at least a surface of the barrier layer facing the innermost layer has been subjected to a chemical conversion treatment; and a front surface, a rear surface and a side surface of at least the heat-sealed portion of the tab has been subjected to a chemical conversion treatment of applying, drying and baking at a temperature not lower than 180°C a solution which is a mixture of a phenolic resin, a chromium fluoride compound and phosphoric acid, or a mixture of a phenolic resin and a metallic salt or metal, wherein the metal is molybdenum, titanium or zirconium.