An electrochemical battery cell with Cu.sub.xO, where x is from 0.9 to 1.3, as a positive electrode active material. The specific surface area of the Cu.sub.xO is from 1.0 to 4.0 m.sup.2/gram, preferably from 1.0 to 2.9 m.sup.2/g to provide an increase in the high voltage discharge capacity of the Cu.sub.xO.