There are provided a novel onium salt and a method for optimizing the interface of a negative electrode by the use of the novel onium salt and/or a conventional onium salt. Such a method is characterized in that an electrolyte comprising novel 2,2,2-trifluoro-N-(trifluoromethanesulfonyl)acetamide•tetraethyl ammonium salt is made to intervene between a negative electrode and a positive electrode in an electrochemical device having the constitution: a negative electrode|an electrolyte|a positive electrode, thereby constructing an electrode structure in the electrochemical device, and applying a voltage between the negative electrode and the positive electrode so as for the negative electrode to have a potential of −1 V to −5 V in terms of the potential relative to a reference electrode of I−/I3−, thereby forming a passive-state layer comprising a decomposition product of the above electrolyte or salt on the surface of the above negative electrode.