NOVELTY - A composition comprises silyl group-containing compound (A) in which hydrogen atom(s) of acid chosen from carboxylic acid, proton acid having phosphorus atom and/or boron atom, and sulfonic acid is substituted by silyl group (I), and basic compound(s) chosen from Lewis base and aromatic compound (II), and/or silicon compound(s) (III). The content of basic compound and/or silicon compound is 1-100 %mass with respect to 100 %mass silyl group-containing compound.    USE - Composition is used for electrolyte solution for lithium-ion secondary battery (all claimed) for hybrid vehicle and electric vehicle.    ADVANTAGE - The composition has reduced viscosity, and enables fall of generated gas amount in battery containing the composition. The electrolyte composition containing the composition has excellent storage stability. The battery has excellent input-output characteristics and cycle characteristics.    DETAILED DESCRIPTION - A composition comprises silyl group-containing compound (A) in which hydrogen atom(s) of acid chosen from carboxylic acid, proton acid having phosphorus atom and/or boron atom, and sulfonic acid is substituted by silyl group of formula -Si(Ra1)(Ra2)(Ra3) (I), and basic compound(s) chosen from Lewis base and aromatic compound of formula Q+Y- (II), and/or silicon compound(s) of formula Rc2-Si(RC1)(Rc3)-X1 (III). The content of basic compound and/or silicon compound is 1-100 %mass with respect to 100 %mass silyl group-containing compound.    Ra1-Ra3=optionally substituted 1-20C hydrocarbyl;    Q+=quaternary ammonium, quaternary phosphonium, alkali metal or alkaline-earth metal;    Y'=alkoxy or aryloxy;    Rc1-Rc3=1-20C hydrocarbyl or 1-20C alkoxy (both optionally substituted);    X1=OR1 or halo;and    R1=H, optionally substituted 1-20C hydrocarbyl, 1-20C silyl, SO2CH3 or SO2CF3.    INDEPENDENT CLAIMS are included for the following:    (1) electrolyte solution, which contains nonaqueous solvent, lithium salt and the composition; and    (2) lithium-ion secondary battery (100), which contains positive electrode (120) containing positive electrode active material, negative electrode (130) containing negative electrode active material, and electrolyte solution.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the battery containing the composition.    Lithium-ion secondary battery (100)    Separator (110)    Positive electrode (120)    Negative electrode (130)    Anode collector (150)