NOVELTY - A lithium-ion secondary battery has a positive electrode (21) containing positive-electrode active material comprising lithium complex oxide (I) or (II), negative electrode (22) and an electrolyte provided between positive electrode and negative electrode. The electrolyte contains carbonyl compound (III)-(VI). At least one of positive electrode, electrolyte and negative electrode contains heteropolyacid compound (VII)-(IX).    USE - Lithium-ion secondary battery is used for electric tool, electric vehicle, electric power storage system and electronic device (all claimed). Uses include but are not limited to video camera, digital still camera, mobile telephone, notebook personal computer, cordless telephone apparatus, headphone stereo cassette tape recorder, personal digital assistant, electric shaver, pacemaker and hearing aid.    ADVANTAGE - The lithium-ion secondary battery has high capacity, excellent cycling characteristics and suppressed swelling.    DETAILED DESCRIPTION - A lithium-ion secondary battery has a positive electrode containing positive-electrode active material comprising lithium complex oxide of formula: LiaMO2 (I) or LibNicMn2-cO4 (II), negative electrode and an electrolyte provided between positive electrode and negative electrode. The electrolyte contains carbonyl compound of formula: R1-C(-R2)(-Cl)-C(=O)-R3 (III), R4-C(-Cl)(-R5)-C(=O)-O-R6 (IV), or of formula (V) or (VI). At least one of positive electrode, electrolyte and negative electrode contains heteropolyacid compound of formula: X34Y3Z312O40 (VII), X43Y4Z412O40 (VIII) or X54Y5Z511VO40 (IX).    M=Ni, Co or Mn;    a=0.45 or less in perfect charge condition and 1 in uncharged state;    b=0.1 or less in perfect charge condition, and 1 in uncharged state;    c=0.4-0.6 in uncharged state and perfect charge condition;    R9=-O-, -CH=CH-, -CH=CCl- or -CCl=CCl-;    X1,X2=halo;    m=0-4;    n=0-2m+1;    p=0-4;    q=0-2p-1;    X3-X5=H, Li, Na, K, Rb, Cs or NR4;    Y3=Si or Ge;    Y4,Y5=P or As;    Z3-Z5=Mo or W;    R=CrH2r+1;and    r=0-4.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of the winding electrode component of lithium-ion secondary battery. (Drawing includes non-English language text)    Winding electrode component (20)    Positive electrode (21)    Positive-electrode active material layer (21B)    Negative electrode (22)    Separator (23)