Nonaqueous electrolyte for secondary battery and nonaqueous-electrolyte secondary battery employing the same

A nonaqueous electrolyte, comprising:

• (A) an electrolyte;
• (B) a nonaqueous solvent dissolving the electrolyte;
• (C) a salt of a monofluorophosphate ion of formula PO3F, a difluorophosphate ion of formula PO2F2, or a mixture thereof with at least one metal ion selected from a metal of Group 1 of the periodic table selected from lithium, sodium, potassium, and cesium, a metal of Group 2 of the periodic table selected from magnesium, calcium, strontium, and barium and a metal of Group 13 of the periodic table selected from aluminum, gallium, indium, and thallium;
• and
• (D) at least one compound selected from the group consisting of:
• wherein R1, R2, and R3 are each independently a fluorine atom, an alkyl group comprising 1-12 carbon atoms and optionally substituted with a fluorine atom, or an alkoxy group comprising 1-12 carbon atoms and optionally substituted with a fluorine atom;
• (ii) a nitrile compound;
• (iii) an isocyanate compound;
• (iv) a phosphazene compound;
• (v) a disulfonic acid ester compound;
• (vi) a sulfide compound;
• (vii) a disulfide compound;
• (viii) an acid anhydride;
• (ix) a lactone compound comprising a substituent in the α-position;
• and
• (x) a compound comprising a carbon-carbon triple bond.