The invention concerns novel ionic compounds with low melting point whereof the onium type cation having at least a heteroatom such as N, O, S or P bearing the positive charge and whereof the anion includes, wholly or partially, at least an ion imidide such as (FX1O)N−(OX2F) wherein X1 and X2 are identical or different and comprise SO or PF, and their use as solvent in electrochemical devices. Said composition comprises a salt wherein the anionic charge is delocalised, and can be used, inter alia, as electrolyte.