The invention concerns novel ionic compounds with low melting point whereof the onium type cation having at least a heteroatom such as N, 0, S or P bearing the positive charge and whereof the anion includes, wholly or partially, at least an ion imidide such as (FXI0)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.