NOVELTY - An electrolyte composition comprises pyrrolidinium compound (I) and benzyl compound (II) which are different from each other.    USE - Electrolyte composition for use in lithium ion battery (claimed) that is utilized in storing energy for mobile applications such as portable electronic device, electric tool, or electric vehicle, power tool, cell phone, laptop computer, pure battery electric vehicle, hybrid electric vehicle, plug-in hybrid electric vehicle and electric bicycle.    ADVANTAGE - The electrolyte composition has improved overcharge protection efficiency, increased safety of the battery, high electrochemical performance and high purity of the battery.    DETAILED DESCRIPTION - An electrolyte composition comprises pyrrolidinium compound of formula (I) and benzyl compound of formula (II) which are different from each other.    R11 = alkyl in which methylene groups may be replaced by oxygen atoms unless the methylene group is directly bound to the nitrogen atom;    X = sulfur-containing moiety of formula (a) or (b);    R13 = alkyl group in which methylene groups may be replaced by oxygen atoms unless bond to an oxygen atom;    A = alkylene group in which methylene groups may be replaced by oxygen atoms unless the methylene group is directly bound to the nitrogen atom and/or which may contain double bonds, preferably ethylene;    Y1 = CH2, O, S or NR14, preferably CH2;    R14 = alkyl;    Z = bidentate group derived from oxalic acid, glycolic acid, 1,2-dihydroxyethane, malonic acid, 3-hydroxypropionic acid or 1,3-dihydroxypropane, preferably oxalic acid, where the bidentate group is bound to M through two of its oxygen atoms;    p = 0-3, preferably 2;    M = B or P, preferably B;    R12 = alkyl, alkoxy or acyloxy (all partially or fully halogenated), F, Cl, Br or I;    q = 0-6, preferably 0; and    R21, R22, R23, R24, R25, R26 = H, F, Cl, Br, I, alkyl, preferably phenyl or cyclohexyl, which may be substituted by halogen atoms and/or in which methylene groups may be replaced by an oxygen atom, or aryl which may be optionally substituted by halogens or alkyl groups which may be independent of each other substituted by halogen atoms and/or in which methylene groups may be replaced by an oxygen atom.    INDEPENDENT CLAIMS are also included for the following:    (1) an use of a mixture of a compound (I) and/or compound (II) as overcharge protection additive in an electrolyte composition    (2) a lithium ion battery comprising the electrolyte composition.