Electrolyte solution and lithium ion secondary battery provided with same

The objective of the present invention is to provide an electrolyte solution of which electrolyte salt concentration is high and by which cycle characteristics hardly deteriorate and battery lifetime can be extended, and a lithium ion secondary battery which contains the above electrolyte solution. The electrolyte solution of the present invention comprises an electrolyte salt and a solvent, wherein a concentration of the electrolyte salt is more than 1.1 mol/L, the electrolyte salt contains a compound represented by the following formula (1): (XSO2) (FSO2)NLi (1) (wherein X is a fluorine atom, a C1-6 alkyl group or a C1-6 fluoroalkyl group), and the solvent contains a cyclic carbonate.