NOVELTY - A nonaqueous electrolyte secondary battery has separator (3) interposed between positive electrode (1) and negative electrode (2), and non-aqueous electrolyte. Negative electrode contains active material comprising particulate silicon and/or silicon alloy, and binder. Product of 100 and ratio of amount (A) of lithium occlusion per unit area of negative electrode in charging state of battery and amount (B) of theoretical maximum lithium occlusion per unit area of negative electrode is 45% or less. Electrolyte contains fluorinated cyclic carbonate and diisocyanate compound.    USE - Nonaqueous electrolyte secondary battery is used as power supply for portable electronic device.    ADVANTAGE - The nonaqueous electrolyte secondary battery has excellent charging and discharging characteristics even in high temperature environment.    DETAILED DESCRIPTION - A nonaqueous electrolyte secondary battery has a separator interposed between a positive electrode and a negative electrode, and a non-aqueous electrolyte containing solute dissolved a non-aqueous solvent. The negative electrode contains active material comprising particulate silicon and/or silicon alloy, and a binder. The product of 100 and ratio of amount (A) of lithium occlusion per unit area of negative electrode in charging state of battery and amount (B) of theoretical maximum lithium occlusion per unit area of negative electrode is 45% or less. The electrolyte contains a fluorinated cyclic carbonate and a diisocyanate compound of formula: OCN-CH2-A-CH2-NCO.    A=chain-shaped saturated hydrocarbyl of formula: -(C(-R1)(-R2))n or aromatic hydrocarbyl of formula (b);    n=natural number;and    R1-R6=H or alkyl.    DESCRIPTION OF DRAWING(S) - The drawing shows the cross-sectional view of flat electrode component.    Positive electrode (1)    Positive electrode current collection tab (1a)    Negative electrode (2)    Negative electrode current collection tab (2a)    Separator (3)    Flat electrode component (10)