Method for preparing a sulfonylimide meeting the formula [R-SO 2 -N-SO 2 R' r M, characterized in that it consists of preparing an N-substituted sulfonimide RSO 2 N(R")SO 2 R' from RSO 2 F, and of replacing the R" group via nucleophilic substitution reaction to obtain the sulfonylimide, R" being a benzyl, allyl or trimethylsilyl group, on the understanding that in formulas, and: • R is a group meeting the formula Ar-Z-L-; • R' is a straight-chain or branched perfluoroalkyl group, or a group identical to the Ar-Z-L- group forming R; • Z is a sulfide, sulfinyl or sulfonyl group; • L is a group of formula -(CF 2) n -CFR f -; • n is 0 or 1; • R f is F or perfluoroalkyl group having 1 to 8 carbon atoms; • Ar is an aromatic group selected from the group composed of monocyclic aromatic groups; polycyclic aromatic groups having condensed or non-condensed cores; and heterocyclic, bicyclic aromatic groups with condensed or non-condensed rings, or monocyclic; • M is a cation of valence r, selected from among cations of alkaline metals, alkaline-earth metals, or trivalent or tetravalent metals, and organic cations selected from among ammonium, phosphonium, imidazolium, guanidinium, piperidinium, pyrrolidinium, pyridinium or quinolinium ions, and in that: when the R' group is a group identical to the Ar-Z-L- group forming R, compound meets the formula (IIa) RSO 2 N(SiMe 3)SO 2 R, and it is obtained via reaction of compound R-SO 2 F with a hexamethyldisilazane salt (CH 3) 3 Si) 2 N -A+ (A being an alkaline metal cation or a quaternary ammonium ion), and when group R' is a perfluoroalkyl group, the compound meets the formula (IIb) RSO 2 N(R")SO 2 R', R" being a benzyl group or allyl group, and it is obtained via a compound RSO 2 NH(R") (II'b) with a method comprising the following steps: - reaction of R-SO 2 F with excess benzylamine or allylamine, - neutralisation with HCl and isolation of the compound R-SO 2 NH(R") (II'b); - reaction of compound (II'b) with the anhydride (R'SO 2) 2 O, in the presence of a tertiary amine, to obtain compound R-SO 2 N(R")SO 2 R'. • R is a group meeting the formula Ar-Z-L-; • R' is a straight-chain or branched perfluoroalkyl group, or a group identical to the Ar-Z-L- group forming R; • Z is a sulfide, sulfinyl or sulfonyl group; • L is a group of formula -(CF 2) n -CFR f -; • n is 0 or 1; • R f is F or perfluoroalkyl group having 1 to 8 carbon atoms; • Ar is an aromatic group selected from the group composed of monocyclic aromatic groups; polycyclic aromatic groups having condensed or non-condensed cores; and heterocyclic, bicyclic aromatic groups with condensed or non-condensed rings, or monocyclic; • M is a cation of valence r, selected from among cations of alkaline metals, alkaline-earth metals, or trivalent or tetravalent metals, and organic cations selected from among ammonium, phosphonium, imidazolium, guanidinium, piperidinium, pyrrolidinium, pyridinium or quinolinium ions, and in that: when the R' group is a group identical to the Ar-Z-L- group forming R, compound meets the formula (IIa) RSO 2 N(SiMe 3)SO 2 R, and it is obtained via reaction of compound R-SO 2 F with a hexamethyldisilazane salt (CH 3) 3 Si) 2 N -A+ (A being an alkaline metal cation or a quaternary ammonium ion), and when group R' is a perfluoroalkyl group, the compound meets the formula (IIb) RSO 2 N(R")SO 2 R', R" being a benzyl group or allyl group, and it is obtained via a compound RSO 2 NH(R") (II'b) with a method comprising the following steps: - reaction of R-SO 2 F with excess benzylamine or allylamine, - neutralisation with HCl and isolation of the compound R-SO 2 NH(R") (II'b); - reaction of compound (II'b) with the anhydride (R'SO 2) 2 O, in the presence of a tertiary amine, to obtain compound R-SO 2 N(R")SO 2 R'. when the R' group is a group identical to the Ar-Z-L- group forming R, compound meets the formula (IIa) RSO 2 N(SiMe 3)SO 2 R, and it is obtained via reaction of compound R-SO 2 F with a hexamethyldisilazane salt (CH 3) 3 Si) 2 N -A+ (A being an alkaline metal cation or a quaternary ammonium ion), and when group R' is a perfluoroalkyl group, the compound meets the formula (IIb) RSO 2 N(R")SO 2 R', R" being a benzyl group or allyl group, and it is obtained via a compound RSO 2 NH(R") (II'b) with a method comprising the following steps: - reaction of R-SO 2 F with excess benzylamine or allylamine, - neutralisation with HCl and isolation of the compound R-SO 2 NH(R") (II'b); - reaction of compound (II'b) with the anhydride (R'SO 2) 2 O, in the presence of a tertiary amine, to obtain compound R-SO 2 N(R")SO 2 R'. - reaction of R-SO 2 F with excess benzylamine or allylamine, - neutralisation with HCl and isolation of the compound R-SO 2 NH(R") (II'b); - reaction of compound (II'b) with the anhydride (R'SO 2) 2 O, in the presence of a tertiary amine, to obtain compound R-SO 2 N(R")SO 2 R'.