NOVELTY - A nonaqueous electrolyte solution comprises phosphonic acid derivative (I) containing silyl ester group.    USE - Nonaqueous electrolyte solution used as additive for lithium secondary battery (both claimed). Uses include but are not limited to mobile telephone, notebook computer, hybrid vehicle, electric vehicle, motor vehicle, mortar, backup power supply, headphone stereo cassette tape recorder, video, liquid crystal television, handheld cleaner, electronic organizer, calculator, radio, electric motor cycle, lighting fixture, bicycle, game machine, timepiece, electric tool, camera, motor bike and electric power assistant bicycle.    ADVANTAGE - The nonaqueous electrolyte solution provides batteries with high output characteristics and low resistance. The performance degradation of the batteries in high temperature environment is suppressed.    DETAILED DESCRIPTION - A nonaqueous electrolyte solution comprises phosphonic acid derivative of formula: R1-P(=O)(-O-Si(-R3)(-R4)(-R5))n-(OR2)m (I) containing silyl ester group.    m=0 or 1;    n=1 or 2;    m+n=2;    R1=H, 1-12C (halo)alkyl, 6-14C aryl (optionally substituted by halo or 1-12C (halo)alkyl), 2-12C alkenyl, 1-6C (halo) alkyl substituted with -SiR18R19R20;    R18-R20=1-6C alkyl, 2-6C alkenyl, 1-6C alkoxy or phenyl, optionally substituted 5- or 6-membered heterocyclyl, or -Ra'-O-Rb', -Ra'-C(=O)-Rb', -Ra'-C(=O)-O-Rb', -Ra'-OC(=O)-ORb', -Ra'-OC(=O)-Rb', -Ra'-S(=O)2O-Rb', -Ra'-S(=O)2-Rb', -Ra'-OS(=O)2O-Rb', -Ra'-OS(=O)2-Rb' or -Ra'-P(=O)(-O-Rb')2';    R2=H, 1-6C (halo)alkyl, 2-6C alkenyl or phenyl optionally substituted by halo or 1-6C (halo) alkyl;    R3=1-6C alkyl, 2-6C alkenyl, 1-6C alkoxy, phenyl, -O-SiR6R7R8;    or two R3=-O-, 1-3C alkylene, or -O-(SiR16R17O)p;    R4-R8,R16,R17=1-6C alkyl, 2-6C alkenyl, 1-6C alkoxy or phenyl;    Ra'=1-12C bivalent hydrocarbyl optionally containing halo;    Rb'=Ra' or -SiR21R22R23;and    R21-R23=R4.    An INDEPENDENT CLAIM is included for lithium secondary battery.    DESCRIPTION OF DRAWING(S) - The drawing shows the sectional view of a coin-type lithium secondary battery.    Positive electrode (1)    Negative electrode (2)    Positive electrode can (3)    Sealing board (4)    Separator (5)