Patent ID: 8148017

Claim:
An electrochemical energy storage device comprising: a negative electrode which contains a carbon material and has a negative electrode potential of 1.4 V or less relative to a lithium reference when being charged; and a non-aqueous electrolyte solution dissolving a lithium salt, an ammonium salt, and at least one kind of fluorinated benzene selected among hexafluorobenzene, pentafluorobenzene, 1,2,3,4-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, 1,2,4,5-tetrafluorobenzene and 1,2,3-trifluorobenzene, in a non-aqueous solvent, wherein the ammonium salt is an ammonium salt containing a quaternary ammonium ion having a straight-chain alkyl group with 4 or less carbon atoms as a cation, and a ratio of the content of the non-aqueous solvent to the total content of the lithium salt and the ammonium salt in the non-aqueous electrolyte solution, (non-aqueous solvent/(lithium salt+ammonium salt)), is from 1/2 to 4/1 in terms of a molar ratio.