Nonaqueous electrolyte and nonaqueous electrolyte secondary battery

#The present invention relates to an improvement of nonaqueous electrolyte in nonaqueous electrolyte secondary batteries.

#In nonaqueous electrolyte secondary batteries represented by lithium ion secondary batteries, an irreversible reaction occurs in a nonaqueous electrolyte between a nonaqueous solvent and part of a lithium salt during charging and discharging. In light of this fact, it is proposed to use a fluorinated chain carboxylate ester as a nonaqueous solvent in order to prevent the oxidative decomposition of the nonaqueous solvent on the positive electrode (Patent Literature 1).

#PTL 1: Japanese Published Unexamined Patent Application No. 2009-289414

#FIG. 1 is a partially cutout perspective view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention.

#A nonaqueous electrolyte according to an embodiment of the present invention includes a lithium salt and a nonaqueous solvent in which the lithium salt is dissolved. The nonaqueous solvent includes a fluorinated chain carboxylate ester and a dicarbonyl compound having two carbonyl groups in the molecule. The dicarbonyl compound is at least one selected from the group consisting of esters and acid anhydrides and has not more than three atoms between the two carbonyl groups.

#During the charging and discharging process, the dicarbonyl compound and the fluorinated chain carboxylate ester react together to form a lithium ion permeable film on the surface of the negative electrode active material. On the negative electrode, the fluorinated chain carboxylate ester receives electrons and becomes radical species, which react with the dicarbonyl compound. The reaction product (for example, a chain fluorinated dicarbonyl compound such as, for example, 4-hydroxy-4-(3,3,3-trifluoropropanoyloxy)-2-butenoic acid) accumulates on the surface of the negative electrode active material to form a film. The film covering the surface of the negative electrode active material suppresses the reductive decomposition of the fluorinated chain carboxylate ester.

#With the mechanism described above which involves the dicarbonyl compound, a charged battery attains a small decrease in capacity due to the reductive decomposition of the fluorinated chain carboxylate ester on the negative electrode during high-temperature storage. That is, the high-temperature storage characteristics of a battery can be enhanced in spite of the fluorinated chain carboxylate ester being included in the nonaqueous solvent.