TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Salt: The electrolyte salt (II) is electrolyte salt (IIa) and/or electrolyte salt (IIb). The electrolyte salt (IIa) is chosen from lithium bis(trifluoromethanesulfonyl)imide and lithium bis(pentafluoroethylsulfonyl)imide, preferably bis(trifluoromethanesulfonyl)imide. The electrolyte salt (IIa) and/or electrolyte salt (IIb) is chosen from lithium hexafluorophosphate and lithium tetrafluoroborate. Preferred Properties: The electrolyte salt (II) has concentration of 0.5-1.5 mol/l. The electrolyte salt (IIa) and electrolyte salt (IIb) have concentration of 0.1-0.9 mol/l. The ratio of electrolyte salt (IIb) concentration and electrolyte salt (IIa) concentration is 1/9-9/1. Preferred Component: A lithium ion secondary cell formed using the electrolyte solution has a positive electrode, a negative material and a separator. The positive electrode is formed using a positive electrode active material chosen from cobalt type complex oxide, nickel series complex oxide, manganese system complex oxide, iron-type complex oxide, and vanadium system complex oxide. The negative electrode is formed using a negative electrode active material such as carbon material.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Composition: The electrolyte solution further contains (in volume%) phosphate ester (D) (1-10), propionate ester (1-30), aromatic compound (0.15), and surfactant (E) (0.01-2 mass%) in the solvent (I). Preferred Compound: The fluorine-containing ether (A) is of formulae: HCF2CF2CH2OCF2CFHCF3, CF3CF2CH2OCF2CFHCF3, HCF2CF2CH2OCF2CF2H and/or CF3CF2CH2OCF2CF2H. The non-fluorine-type cyclic carbonate (C1) is chosen from ethylene carbonate, vinylene carbonate, and propylene carbonate. The non-fluorine-type linear carbonate (C2) is chosen from dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. The phosphate ester (D) is fluorine-containing alkyl phosphate ester (D1). The surfactant (E) is chosen from fluorine-containing carboxylate salt (E1) of formula: Rf7COO-M+, and fluorine-containing sulfonate (E2) of formula: Rf8SO3-M+.Rf7=3-12C ether bond, or fluorine-containing alkyl;M+=Li+, Na+, K+ or NHR'3+;R'=H or 1-3C alkyl;andRf8=3-10C ether bond or fluorine-containing alkyl. Preferred Definitions:    Rf1=3C or 4C fluorine-containing alkyl;and    Rf2=2C or 3C fluorine-containing alkyl.    EXAMPLE - Lithium hexafluorophosphate (1 mol/l) was added to solvent containing 1H,1H,2H',3H-decafluorodipropylether (A1) (in volume%) (40), 1,3-dioxolan-2-one (B1a) (5), ethylene carbonate (10) and dimethyl carbonate (45) and stirred at 25 degrees C, to obtain an electrolyte solution.