NOVELTY - Non-aqueous electrolyte secondary battery comprises: flat electrode assembly (14) including positive electrode, negative electrode and separator interposed between positive and negative electrodes; outer can that stores the flat electrode assembly and a non-aqueous electrolyte; and sealing plate that seals an opening portion of outer can. The flat electrode assembly has a portion other than the side facing the sealing plate, and is covered with an insulating sheet. The non-aqueous electrolyte contains at least one of lithium salt having an oxalate complex as anion or lithium difluorophosphate.    USE - Used as non-aqueous electrolyte secondary battery.    ADVANTAGE - The non-aqueous electrolyte secondary battery: has a capacity of greater than or equal to 5 Ah, preferably greater than or equal to 20 Ah (claimed); improves the penetration speed of the non-aqueous electrolyte into the electrode assembly, and shortens the time to pore the non-aqueous electrolyte; exhibits excellent insulating properties between the flat electrode assembly and the outer can; and can be prepared by a method, which enables the resistance welding to be performed in an accurate and stable state, and curbs variation in the welding strength.    DETAILED DESCRIPTION - Non-aqueous electrolyte secondary battery comprises: a flat electrode assembly (14) including a positive electrode, a negative electrode and a separator interposed between positive and negative electrodes; an outer can that stores the flat electrode assembly and a non-aqueous electrolyte; and a sealing plate that seals an opening portion of the outer can. The flat electrode assembly has a portion other than the side facing the sealing plate, and is covered with an insulating sheet. The non-aqueous electrolyte contains at least one of a lithium salt having an oxalate complex as an anion or lithium difluorophosphate at the time of making the non-aqueous electrolyte secondary battery, where the wettability of the insulating sheet to the non-aqueous electrolyte is lower than that of the outer can to the non-aqueous electrolyte.    DESCRIPTION OF DRAWING(S) - The figure shows a fragmentary sectional view of the non-aqueous electrolyte secondary battery.    Flat electrode assembly (14)    Wound positive electrode substrate exposed portion (15)    Wound negative electrode substrate exposed portion (16)    Positive electrode collector (17)    Negative electrode collector (19)