NOVELTY - Alkyl sulfonate compounds (I) are new.    USE - (I) are useful as an additive in electrolytes for electrochemical or electro-optical devices, preferably lithium batteries, lithium-ion batteries, double-layer capacitors, lithium capacitors, solar cells, electrochromic displays, sensors or biosensors (all claimed).    ADVANTAGE - (I) do not increase the internal resistance of the electrochemical cells, and are non-volatile.    DETAILED DESCRIPTION - Alkyl sulfonate compounds of formula ((K1-(CH2)-(CH2)u-S1-(CH2)v-R12)+(A)-) (I) are new.    u = 0-7 (where at least one CH2 group of -(CH2)u- alkylene chain (when u = 2-7, is optionally replaced by O or optionally contain at least one double bond);    v = 0-4;    S1 = -O-S(=O)2- (IVa) or -S(=O)2-O- (IVb);    R12 = 1-20C alkyl (optionally branched and optionally fluorinated or partially fluorinated), 2-20C alkenyl (optionally branched and contains at least one double bond), 2-20C alkynyl (optionally branched and contains at least one triple bond) or 6-12C aryl (optionally substituted by at least one F, Cl and/or 1-8C alkyl (optionally branched and partially or completely fluorinated);    K1 = cation, preferably (R)3N+-, (R)3P+-, heterocyclic cations of formula (IIa), (IIb), (IIc) or (IId), or piperazine-1,4-diium moieties of formula (IIe), or 1,4-azaphosphinane-1,4-diium moieties of formula (IIf);    R = CH3, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl or tert-butyl, where at least one CH2 group, which is not directly bonded with N or P, is optionally replaced by O;    Y1 = CH2, O, S or NR11;    R11 = CH3, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl or tert-butyl;    A = anion, preferably (F1zB1(C1mF12m+1)4-z)-, (F1yP(C1mF12m+1)6-y)-, ((C1mF12m+1)2P(O)O)-, (C1mF12m+1P(O)O2)2-, (O-C1(O)-CmF12m+1)-, (O-S(O)2-C1mF12m+1)-, (N(C1(O)-CmF12m+1)2)-, (N(S(O)2-C1mF12m+1)2)-, (N(C1(O)-C1mF12m+1)(S(O)2-C1mF2m+1))-, (N(C(O)-C1mF12m+1)(C(O)F1))-, (N(S(O)2-C1mF12m+1)(S(O)2F1))-, (N(S(O)2F1)2)-, (C1(C1(O)-C1mF12m+1)3)-, (C1(S(O)2-C1mF2m+1)3)-, oxo moieties of formula (IIIa) or (IIIb);    m = 1-8, where C1F12 groups of A are partially and optionally replaced by O, S(O)2, NR2 or CH2;    z = 1-4;    y = 1-6;    X = B or Al;    R1, R2 = F, Cl, Br, I, 1-20C perfluoroalkyl (optionally branched), 1-20C alkoxy (optionally branched and optionally fluorinated or partially fluorinated), -O-C(O)-alkyl;    alkyl = 1-20C alkyl (optionally branched and optionally fluorinated or partially fluorinated); and    t1 = bidentate group, preferably 1,2- or 1,3-diol, 1,2- or 1,3-dicarboxylic acid, or 1,2- or 1,3- hydroxycarboxylic acid (in all groups 2 adjacent OH groups of corresponding compound with the central X atom forms a bond).    INDEPENDENT CLAIMS are also included for:    (1) preparing (I); and    (2) an electrolyte comprising (I).