Electric double film capacitor has (a) an anode with a current collector, combined with a material for a polarisable electrode, consisting mainly of activated charcoal; (b) a cathode with a current collector of porous metal, which cannot alloy with Li and is combined with a carbonaceous material contg. Li ions, occluded in the material by an (electro)chemical process and can be released; and (c); a non-aq. electrolyte contg. a Li salt. The capacitor has a ratio bv/3.6d in the 0.05-090 range (b = the capacity, in F, of the anode; d = the Li ion release capacity, in mA.hr, of the cathode; and v = the p.d., in V, in the operating voltage range) (claimed).