Low temperature non-aqueous electrolyte

1. A non-aqueous electrolyte for electrochemical devices that function below −50° C. which comprises a mixture, eutectic or tereutectic of at least two low viscosity aprotic solvents which includes 20 to 75% acetonitrile and a mixture of conductive salts having a molecular weight of up to 240 selected from the group consisting of;

a) an imidazolium compound of the formula:

wherein R 1 , R 2 , R 3 , R 4 , and R 5 are the same or different and are selected from the group consisting of hydrogen, an alkyl group of 1-4 carbon atoms, or fluoroalkyl, with the proviso that at least one of R 1 , R 2 , R 3 , R 4 , and R 5 being hydrogen;

b) an ammonium compound of the formula:

wherein R 5 , R 6 , R 7 , and R 8 are the same or different and consists of alkyl group of 1-4 carbon atoms, or fluoroalkyl, and

c) a pyrrolidinyl compound of the formula;

wherein R 8 , and R 9 are the same or different and consists of alkyl group of 1-4 carbon atoms, and an anion selected from the group consisting of tetrafluoroborate (BF 4 − ), hexafluoroarsenate (AsF6) − , hexafluorophosphate (PF 6 − ), fluoro hydrogen fluoride F(HF)x − ,

wherein x=1 to 4,

(SbF 6 − ), (CF 3 BF 3 − ), trifluoromethylsulfonate

(triflate), and bis (trifluoromethylsufonyl) imide.

2. The electrolyte of claim 1 wherein one of said salts is 1,3-dimethylimidazolium tetrafluoroborate.

3. The electrolyte of claim 1 wherein the solvent is ethylene carbonate and gamma butyrolactone.

4. The electrolyte of claim 3 wherein said solvent comprises 30 to 75% by weight of acetonitrile.

5. The electrolyte of claim 3 which includes about 1 to 20% by weight of a member selected from the group consisting of methyl acetate, methylene chloride, and methyl formate.

6. The electrolyte of claim 1 where the salts comprise 1,3-dimethylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium tetrafluoroborate.

7. The electrolyte of claim 6 wherein the salt concentration is 2.5 to 4.5 M.

8. The electrolyte of claim 7 including about 1 to 15% of a solvent selected from the group consisting of methylene chloride, methyl acetate and methyl formate.

9. The electrolyte of claim 1 including two members selected from the group consisting of methyl triethylammonium tetrafluoroborate, diethyldimethylammonium tetrafluoroborate and trimethylethylammonium tetrafluoroborate.

10. Electrolyte of claim 1 including 2 members from the group 1-ethyl-methylpyrrolidinyl, boron tetrafluoride; 1,1-dimethylpyrrolidinyl boron tetrafluoride; bix (1,1-tetramethylene ammonium boron tetrafluoride and spiro.

11. The electronic device of claim 10 in which the conductive salts of the electrolyte comprise inimidazolium tetrafluoroborate and tetraalkyl ammonium tribluoroborate.

12. The electronic device of claim 10 which is a double layer capacitor.

13. In an electronic device containing a non-aqueous electrolyte, the improvement which comprises the electrolyte of claim 1 .