An electrolyte solution comprising - a solvent containing a fluorinated acyclic carbonate having a fluorine content of 33-70 mass-%, - 0.001-10 mass-%, based on the electrolyte solution, of a borate, and - 0.5-3 mol/1, in the electrolyte solution, of electrolyte salt other than the borate; wherein the borate is a compound of formula : wherein A a+ is a metal ion or an onium ion; a is an integer of 1-3; b is an integer of 1-3; p is b/a; m is an integer of 1-2; n is an integer of 0-8; q is 0 or 1; Y 1 is C 1-10 -alkylene, C 1-10 -haloalkylene, C 6-20 -arylene, or C 6-20 -haloarylene, each optionally having a substituent or a hetero atom in the structure, and m Y 1 s may be bonded to each other when q = 1 and m = 2; Y 2 is C 1-10 -alkyl, C 1-10 -haloalkyl, C 6-20 -aryl, or C 6-20 -haloaryl, each optionally having a substituent or a hetero atom in the structure, or halogen or -Z 3 Y 3 , and n Y 2 s may be bonded to each other to form a ring when n is 2-8; Z 1 , Z 2 , and Z 3 each individually are O, S, or NY 4 ; and Y 3 and Y 4 each individually are H, or C 1 - 10 -alkyl, C 1-10 -haloalkyl, C 6-20 -aryl, or C 6-20 -haloaryl, each optionally having a hetero atom in the structure, and if there are multiple Y 3 s or Y 4 s, the multiple Y 3 s or Y 4 s may be bonded to each other to form a ring, and the optional substituent in Y 1 and Y 2 is selected from halogen, linear or cyclic alkyl, aryl, alkenyl, alkoxy, aryloxy, sulfonyl, amino, cyano, carbonyl, acyl and amide. - a solvent containing a fluorinated acyclic carbonate having a fluorine content of 33-70 mass-%, - 0.001-10 mass-%, based on the electrolyte solution, of a borate, and - 0.5-3 mol/1, in the electrolyte solution, of electrolyte salt other than the borate; wherein the borate is a compound of formula : wherein A a+ is a metal ion or an onium ion; a is an integer of 1-3; b is an integer of 1-3; p is b/a; m is an integer of 1-2; n is an integer of 0-8; q is 0 or 1; Y 1 is C 1-10 -alkylene, C 1-10 -haloalkylene, C 6-20 -arylene, or C 6-20 -haloarylene, each optionally having a substituent or a hetero atom in the structure, and m Y 1 s may be bonded to each other when q = 1 and m = 2; Y 2 is C 1-10 -alkyl, C 1-10 -haloalkyl, C 6-20 -aryl, or C 6-20 -haloaryl, each optionally having a substituent or a hetero atom in the structure, or halogen or -Z 3 Y 3 , and n Y 2 s may be bonded to each other to form a ring when n is 2-8; Z 1 , Z 2 , and Z 3 each individually are O, S, or NY 4 ; and Y 3 and Y 4 each individually are H, or C 1 - 10 -alkyl, C 1-10 -haloalkyl, C 6-20 -aryl, or C 6-20 -haloaryl, each optionally having a hetero atom in the structure, and if there are multiple Y 3 s or Y 4 s, the multiple Y 3 s or Y 4 s may be bonded to each other to form a ring, and the optional substituent in Y 1 and Y 2 is selected from halogen, linear or cyclic alkyl, aryl, alkenyl, alkoxy, aryloxy, sulfonyl, amino, cyano, carbonyl, acyl and amide. A a+ is a metal ion or an onium ion; a is an integer of 1-3; b is an integer of 1-3; p is b/a; m is an integer of 1-2; n is an integer of 0-8; q is 0 or 1; Y 1 is C 1-10 -alkylene, C 1-10 -haloalkylene, C 6-20 -arylene, or C 6-20 -haloarylene, each optionally having a substituent or a hetero atom in the structure, and m Y 1 s may be bonded to each other when q = 1 and m = 2; Y 2 is C 1-10 -alkyl, C 1-10 -haloalkyl, C 6-20 -aryl, or C 6-20 -haloaryl, each optionally having a substituent or a hetero atom in the structure, or halogen or -Z 3 Y 3 , and n Y 2 s may be bonded to each other to form a ring when n is 2-8; Z 1 , Z 2 , and Z 3 each individually are O, S, or NY 4 ; and Y 3 and Y 4 each individually are H, or C 1 - 10 -alkyl, C 1-10 -haloalkyl, C 6-20 -aryl, or C 6-20 -haloaryl, each optionally having a hetero atom in the structure, and if there are multiple Y 3 s or Y 4 s, the multiple Y 3 s or Y 4 s may be bonded to each other to form a ring, <EOS>