Based on the discovery that the melting point and solubility of onium salts are affected by the asymmetry of the substitution on cation, and that the branched substituents effectively shield onium cations from electrochemical reduction, new onium salts are synthesized and high performance electrolytes based on these salts for electrochemical capacitor are provided. The composition of the new electrolyte comprises an onium salt or mixture of such onium salts dissolved in aprotic, non-aqueous solvents or mixture of such solvents. The electrolyte is able to perform at high rate of charge/discharge, at low ambient temperatures, and within wide operating voltage, due to the high solubility, low melting temperature, and the improved reduction stability of the new onium cations, respectively.

An aluminum electrolytic capacitor includes an anode sheet of aluminum, a cathode sheet of aluminum, and separators disposed to sandwich either the anode sheet or the cathode sheet. Anode lead tabs of aluminum are connected to the anode sheet by "needling" or welding. Cathode lead tabs formed of an aluminum sheet are connected to the cathode sheet by "needling" or welding. The surface of each of the cathode lead tabs contacting the separator is roughened. Alternatively, an aluminum thin sheet may be disposed to overlie each of smooth-surfaced cathode lead tabs, with its surface contacting the separator roughened.

The present invention is directed to an electrolyte for an electrolytic capacitor. The capacitor has an electrolytic anode and an electrochemical cathode. The electrolyte has water, a water soluble organic salt, and a relatively weak organic acid. This electrolyte is chemically compatible to aluminum and tantalum oxide dielectrics and withstands higher voltage while maintaining good conductivity. This makes the electrolyte especially useful for high voltage applications, such as occur in an implantable cardiac defibrillator.

The electrical properties and high-temperature characteristics of an electrolytic capacitor are rendered satisfactory by addition of a compound with an unsaturated bond-containing chain which serves to absorb hydrogen gas generated by reaction between the aluminum electrode foil and the electrolyte solution. The electrolytic capacitor contains the unsaturated compound which allows hydrogen addition, or the electrolytic solution, which comprises a solvent composed of 10-80 wt % of an organic solvent and 90-20 wt % water and at least one electrolyte selected from the group consisting of carboxylic acids or their salts and inorganic acids or their salts, also comprises one or more unsaturated compounds which are water-soluble or soluble in polar and protic polar solvents.

Electrolytes containing water, phosphoric acid, at least one organic solvent, and at least one alkanolamine can be used for anodizing valve metals prepared from metal powder having a surface area of least 0.35 m.sup.2 /g or 35,000 CV/g. The anodizing electrolytes have relatively high conductivity and are capable of being used at high anodizing currents. The anodic film produced by these electrolytes on valve metals is of substantially uniform thickness and has improved electrical parameters.