NOVELTY - Electrolyte solution (5) comprises nonaqueous solvent, electrolyte, phosphazene compound (I), and 1 or more compounds chosen from phosphorus-containing compound (II-a) (II), compound (II-b) containing alkyne structure, and cyclic ether compound (II-c).    USE - Electrolyte solution is used for nonaqueous secondary battery (claimed) e.g. lithium-ion secondary battery, lead battery and nickel-cadmium battery used as power supply of portable electronic device. Uses include but are not limited to mobile telephone, mobile personal computer, electronic book player, pager, hand-held terminal, portable copier, portable printer, calculator, memory card, electric vehicle, etc.    ADVANTAGE - The nonaqueous secondary battery containing the electrolyte solution has excellent durability in high temperature, high electric potential, and excellent flame-retardant characteristics. The capacity degradation at low temperature/large current discharge of battery is suppressed.    DETAILED DESCRIPTION - Electrolyte solution comprises nonaqueous solvent, electrolyte, phosphazene compound of formula (I), and 1 or more compounds chosen from phosphorus-containing compound (II-a) of formula (II), compound (II-b) containing alkyne structure, and cyclic ether compound (II-c).    R1-R6=monovalent substituents chosen from NRaRb, -N=Rc or azo, and halo;    n=1 or more;    Ra,Rb=H, alkyl, alkenyl, alkynyl, aryl, heterocyclyl, cyano, silyl, -C(=Xa1)-R1a1, -N(-R1b1)-R1b2, -S(=O)2-R1c1, or -P(-R1d2)(=Xd1-R1d3)-R1d1;    Rc==C(-Rx1)(-Rx2), =P(-Rx1)(-Rx3)-Rx2, =S(-Ry1)(-Ry2), =C=O, =C=S, or =S=o;    R1a1,R1c1,R1d1,R1d2=alkyl, alkoxy, aryl, aryloxy, halo or amino;    Xa1=O or S;    Xd1=O, S, or N;    R1d3=alkyl, aryl, silyl, or phosphono when Xd1 is N, or not a substituent when Xd1 is O or S;    R1b1,R1b2=alkyl, aryl, alkoxycarbonyl, aryloxy carbonyl, alkylsulfonyl, arylsulfonyl, phosphono or silyl;    Rx1-Rx3=alkyl, aryl, alkoxy, aryloxy, alkylthio, arylthio, heterocyclyl, halo or silyl;    Ry1,Ry2=halo;    Ra1-Ra3=alkyl, alkoxy, aryloxy, halo, amino, -(CH2)n1C(=O)-(O)mRb4;    n1,m=0 or 1;    Rb4=alkyl, alkenyl, aryl, or aralkyl;    X=O, S or NRa12;and    Ra12=H or monovalent substituent.    An INDEPENDENT CLAIM is included for nonaqueous secondary battery.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of lithium-ion secondary battery.    Positive electrode active material layer (2)    Negative electrode active material layer (4)    Electrolyte solution (5)    Negative electrode (A)    Positive electrode (C)