Aqueous electrolyte solution for energy storage devices and energy storage device comprising this aqueous electrolyte solution
According to the present invention, an electrolyte is composed of one or more lithium salts of asymmetric imides each having a perfluoroalkyl group, or a mixed salt of a lithium salt of an asymmetric imide having a perfluoroalkyl group and a lithium salt of a symmetric imide having a perfluoroalkyl group. The composition ratio of the one or more lithium salts and the mixed salt is expressed by (lithium salt of asymmetric imide) x (lithium salt of symmetric imide) 1-x (wherein x is from 0.1 to 1.0) in terms of the molar ratio; the asymmetric imide lithium salt is (C 2 F 5 SO 2 )(CF 3 SO 2 )NLi or (C 3 F 7 SO 2 )(CF 3 SO 2 )NLi; the symmetric imide lithium salt is (CF 3 SO 2 ) 2 NLi; and the composition of an electrolyte solution according to the present invention contains 1.0 mole or more but less than 2 moles of a solvent per 1 mole of the one or more lithium salts or the lithium salts of the mixed salt.
1. An aqueous electrolyte solution for an energy storage device containing water as a solvent, wherein:
an electrolyte is a lithium salt of an asymmetric imide having a perfluoroalkyl group, or a mixed salt of a lithium salt of an asymmetric imide having a perfluoroalkyl group and a lithium salt of a symmetric imide having a perfluoroalkyl group,
a composition ratio of the mixed salt is, in a molar ratio, (lithium salt of asymmetric imide) x (lithium salt of symmetric imide) 1-x , where x is 0.1 or more and less than 1.0,
in a case where the electrolyte is the lithium salt of the asymmetric imide, the lithium salt of the asymmetric imide is lithium (heptafluoropropanesulfonyl)(trifluoromethanesulfonyl)imide ((C 3 F 7 SO 2 )(CF 3 SO 2 )NLi),
in a case where the electrolyte is the mixed salt,
the lithium salt of the asymmetric imide is
lithium (heptafluoro-propanesulfonyl) (trifluoromethanesulfonyl)imide ((C 3 F 7 SO 2 )(CF 3 SO 2 )NLi), and
the lithium salt of the symmetric imide is
lithium bis(trifluoromethanesulfonyl)imide ((CF 3 SO 2 ) 2 NLi), and
the composition of the electrolyte solution is 1.0 mole or more and less than 2 moles of an amount of the solvent per 1 mole of the lithium salt of the asymmetric imide or the lithium salts of the mixed salt.
2. The aqueous electrolyte solution for an energy storage device according to claim 1 , wherein a melting point of one kind or two kinds of the lithium salts is 230° C. or lower.
3. The aqueous electrolyte solution for an energy storage device according to claim 1 , wherein a fluorine ion content in the aqueous electrolyte solution is 10 ppm or less, and a hydrogen content in the aqueous electrolyte solution is 10 ppm or less,
the hydrogen in the aqueous electrolyte solution is a hydrogen atom which remains in a molecule of the lithium salt of the imide where the lithium salt is not completely fluorinated,
in a case where the electrolyte is the lithium salt of the asymmetric imide, the lithium salt is the lithium salt of the asymmetric imide, and
in a case where the electrolyte is the mixed salt, the lithium salt is both of the lithium salt of the asymmetric imide and the lithium salt of the symmetric imide.
4. The aqueous electrolyte solution for an energy storage device according to claim 1 , wherein the energy storage device is a lithium ion secondary battery, an electric double layer capacitor or a lithium ion capacitor.
5. An energy storage device containing the aqueous electrolyte device according to claim 1 .