A high density capacitor 12, a method of manufacturing it, and applications of it are described. The capacitor 12 is an electrochemical capacitor using a metal ion accepting cathode 22 and a metal ion accepting anode 26 and a amorphous solid electrolyte 24 between. The cathode and anode may be of amorphous lithium ion intercalating material such as suitable transition metal oxides with multiple oxidation states.