[PROBLEMS] increase the thermal stability without changing the structure of LiPF _6, a method of stabilizing a solution containing LiPF _6, with improved heat stability, non-aqueous secondary battery with improved electrolyte and the thermal stability provided if the non-aqueous secondary battery .
[MEANS FOR SOLVING PROBLEMS] to the presence of 0.001 to 2-fold molar ratio with respect to the phosphate ester to the amide is a solution containing LiPF ₆ represented by the general formula (I) in LiPF _6:

In the formula, m represents an integer of 1 or 2, R ¹ and R ² are a hydrocarbon group with an alkyl group and / or unsaturated bond in the straight-chain or branched chain of 1 to 6 carbon atoms. However, R ¹ and R ² are an alkoxy group, an import amino group, an alkyl group, a saturated heterocyclic group, optionally have a substituent selected from an unsaturated heterocyclic group, and a fluorine atom consisting of the group and, R ¹ and R ² may be bonded to each other 5 to 8-membered or it may constitute a cyclic structure of the ring. R ³ represents a fluorine-containing alkyl group of straight chain or branched chain of 1 to 6 carbon atoms or an alkyl group having 1 to 6 carbon atoms in the straight chain or branched chain.