This invention provides an electrochemical energy storage device comprising a negative electrode comprising a carbon material and having a negative electrode potential of not more than 1.4 V with respect to a lithium standard during charge, and a nonaqueous electrolyte. In order to enhance high energy density of an electrochemical energy storage device, the negative electrode charge potential should be rendered less noble. This, however, poses a problem, for example, that, upon deep charge, the decomposition of a nonaqueous electrolyte proceeds significantly. In the electrochemical energy storage device, the nonaqueous electrolyte is a solution of at least one fluorinated benzene, selected from lithium salts, ammonium salts, hexafluorobenzene, pentafluorobenzene, 1,2,3,4-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, 1,2,4,5-tetrafluorobenzene, and 1,2,3-trifluorobenzene, dissolved in a nonaqueous solvent. According to this constitution, the decomposition of the electrolyte during charge can be suppressed, and enhanced energy density can be realized.