Silver oxide electrochemical cells are widely used as internal alkaline electrochemical cells (commonly called “button cells”) in watches, measuring instruments, cameras and the like. Silver oxide electrochemical cells are generally constituted using silver oxide (Ag2O and/or AgO) as a cathode active material, zinc powder as an anode active material, and an alkaline solution, e.g., an aqueous solution of KOH or NaOH, as an electrolyte. While silver is an expensive material, silver oxide is nonetheless considered an indispensable cathode active material when high capacity is required despite small size. It is therefore no exaggeration to say that almost all button cells are constituted as silver oxide electrochemical cells.
The proportion of silver oxide electrochemical cell unit price accounted for by the cathode active material is ordinarily very high so that silver oxide is a major factor determining cell unit price. As Ag2O is poor in conductivity, a problem of high internal resistance of the cell lowering the discharge capacity is also a problem. Owing to this, in some cases the cathode active material is prepared by mixing MnO2 with the Ag2O, while it has also been proposed, for example, to improve conductivity by blending with another Ag-system compound such as AgNiO2, as set out in JPA. 60-105170, JPA. 57-849 and JPA. Hei 10-188975.
Similarly, JPA. 52-142241 teaches that addition of an appropriate amount of bismuth oxide powder to a cathode active material composed chiefly of silver oxide enables prediction of the terminal discharge stage and inhibits gassing, without occurrence of excessive deficiency in consumption of the zinc active material and cathode active material.
U.S. Pat. No. 5,389,469, as well as its divisional applications U.S. Pat. No. 5,589,109 and U.S. Pat. No. 6,001,508, discloses a cathode active material consisting of particles composed of an AgO core surrounded by an intermediate layer of Ag2O and an outer layer of a compound containing silver and bismuth (outer layer composed of AgBiO2 or AgBiO3 compound) and teaches that this material can be obtained by reacting AgO and a bismuth compound (bismuth sulfide) in an aqueous alkaline solution under reducing condition.