NOVELTY - Stabilization of lithium hexafluorophosphate involves coexisting 0.001-2 times of phosphoric acid esteramide (I) in solution containing lithium hexafluorophosphate by molar ratio with respect to lithium hexafluorophosphate.    USE - Stabilization of lithium hexafluorophosphate used for nonaqueous electrolyte for nonaqueous secondary battery (all claimed) e.g. coin-shaped lithium ion secondary battery used as power supply for mobile telephone, notebook personal computer, electric power storage, and electric vehicles.    ADVANTAGE - The method efficiently stabilizes solution containing lithium hexafluorophosphate by increasing thermal stability of lithium hexafluorophosphate without changing its structure.    DETAILED DESCRIPTION - Stabilization of lithium hexafluorophosphate involves coexisting 0.001-2 times of phosphoric acid esteramide of formula: (N(-R1)(-R2))m-P(=O)-(OR3)3-m (I) in solution containing lithium hexafluorophosphate by molar ratio with respect to lithium hexafluorophosphate.    M=1 or 2;    R1,R2=hydrocarbyl having unsaturated bond and/or branched or linear 1-6C alkyl (both optionally substituted by alkoxy, amino, alkylthio, saturated heterocyclyl, unsaturated heterocyclyl and fluoro);    or R1+R2=5-8 membered ring;and    R3=branched or linear 1-6C alkyl or branched or linear 1-6C fluoroalkyl.    INDEPENDENT CLAIMS are included for the following:    (1) nonaqueous electrolyte, which contains lithium hexafluorophosphate as electrolyte, cyclic carbonate and/or linear carbonate as solvent, and 0.001-2 times of phosphoric acid esteramide (I) with respect to content of lithium hexafluorophosphate; and    (2) nonaqueous secondary battery, which has a positive electrode (1) which can occlude and discharge lithium, negative electrode (4) and nonaqueous electrolyte.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of coin-shaped lithium ion secondary battery.    Positive electrode (1)    Positive electrode stainless steel cap (2)    Negative electrode stainless steel cap (3)    Negative electrode (4)    Gasket (7)