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 n — 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 .