An electrolyte solution comprising: - a solvent containing a fluorinated diether of formula: CF(R 11)(R 12)-O-CH 2 -CH 2 -O-R 13 wherein R 11 and R 12 each independently are H, CH 3, F, CH 2 F, CHF 2, or CF 3; and R 13 is optionally fluorinated C 1-2 -alkyl; and - a compound of formula: wherein A a+ is a metal ion, a hydrogen ion, or an onium ion; a is an integer of 1-3; b is an integer of 1-3; p is b/a; n 23 is an integer of 1-4; n 21 is an integer of 0-8; n 22 is 0 or 1; Z 21 is a transition metal or an element in groups III--V of the Periodic Table; X 21 is O, S; or C 1-10 -alkylene or C 6-20 -arylene, each optionally halogenated and each optionally containing a substituent and/or a hetero atom in its structure, and multiple X 21 may bind to each other; L 21 is halogen, cyano or -Z 23 Y 23; or C 1-10 -alkyl or C 6-20 -aryl, each optionally halogenated and each optionally containing a substituent and/or a hetero atom in its structure, and multiple L 21 may bind to each other to form a ring; Y 21, Y 22, and Z 23 each individually are O, S, NY 24 or an optionally fluorinated hydrocarbon group; Y 23, Y 24 each individually are H, F, or C 1-10 -alkyl or C 6-20 -aryl, each optionally halogenated and each optionally containing a substituent and/or a hetero atom in its structure, and multiple Y 23 or Y 24 may bind to each other to form a ring. - a solvent containing a fluorinated diether of formula: CF(R 11)(R 12)-O-CH 2 -CH 2 -O-R 13 wherein R 11 and R 12 each independently are H, CH 3, F, CH 2 F, CHF 2, or CF 3; and R 13 is optionally fluorinated C 1-2 -alkyl; and - a compound of formula: wherein A a+ is a metal ion, a hydrogen ion, or an onium ion; a is an integer of 1-3; b is an integer of 1-3; p is b/a; n 23 is an integer of 1-4; n 21 is an integer of 0-8; n 22 is 0 or 1; Z 21 is a transition metal or an element in groups III--V of the Periodic Table; X 21 is O, S; or C 1-10 -alkylene or C 6-20 -arylene, each optionally halogenated and each optionally containing a substituent and/or a hetero atom in its structure, and multiple X 21 may bind to each other; L 21 is halogen, cyano or -Z 23 Y 23; or C 1-10 -alkyl or C 6-20 -aryl, each optionally halogenated and each optionally containing a substituent and/or a hetero atom in its structure, and multiple L 21 may bind to each other to form a ring; Y 21, Y 22, and Z 23 each individually are O, S, NY 24 or an optionally fluorinated hydrocarbon group; Y 23, Y 24 each individually are H, F, or C 1-10 -alkyl or C 6-20 -aryl, each optionally halogenated and each optionally containing a substituent and/or a hetero atom in its structure, and multiple Y 23 or Y 24 may bind to each other to form a ring. A a+ is a metal ion, a hydrogen ion, or an onium ion; a is an integer of 1-3; b is an integer of 1-3; p is b/a; n 23 is an integer of 1-4; n 21 is an integer of 0-8; n 22 is 0 or 1; Z 21 is a transition metal or an element in groups III--V of the Periodic Table; X 21 is O, S; or C 1-10 -alkylene or C 6-20 -arylene, each optionally halogenated and each optionally containing a substituent and/or a hetero atom in its structure, and multiple X 21 may bind to each other; L 21 is halogen, cyano or -Z 23 Y 23; or C 1-10 -alkyl or C 6-20 -aryl, each optionally halogenated and each optionally containing a substituent and/or a hetero atom in its structure, and multiple L 21 may bind to each other to form a ring; Y 21, Y 22, and Z 23 each individually are O, S, NY 24 or an optionally fluorinated hydrocarbon group; Y 23, Y 24 each individually are H, F, or C 1-10 -alkyl or C 6-20 -aryl, each optionally halogenated and each optionally containing a substituent and/or a hetero atom in its structure, and multiple Y 23 or Y 24 may bind to each other to form a ring.