Silver-oxide battery

A silver oxide battery (IEC code: S), also known as a silver–zinc battery, is a primary cell (although it may be used as a secondary cell with an open circuit potential of 1.86 volts). Silver oxide batteries have a long life and very high energy/weight ratio, but a prohibitive cost for most applications due to the high price of silver. They are available in either very small sizes as button cells where the amount of silver used is small and not a significant contributor to the overall product costs, or in large custom design batteries where the superior performance characteristics of the silver oxide chemistry outweigh cost considerations. The large cells found some applications with the military. For example in Mark 37 torpedoes or on Alfa class submarines.

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Chemistry

A silver oxide battery is a small-sized primary battery using zinc as the negative electrode (anode), silver oxide as the positive electrode (cathode) plus an alkaline electrolyte, usually sodium hydroxide (NaOH) or potassium hydroxide (KOH). The chemical reaction that takes place inside the battery is the following:

Zinc is the activator in the negative electrode and corrodes in alkaline solution. When this happens, it becomes difficult to maintain the capacity of the unused battery. The zinc corrosion causes electrolysis in the electrolyte, resulting in the production of hydrogen gas, a rise of inner pressure and expansion of the cell. Mercury has been used in the past to suppress the corrosion, despite its harmful effects on the environment.

Characteristics

Compared to other batteries, a silver oxide battery has a higher open circuit potential than a mercury battery, and a flatter discharge curve than a standard alkaline battery.

History

This once attractive technology had the highest energy density (prior to lithium technologies), and was primarily developed for aircraft use. The worldwide rise in silver prices saw its demise. It was used as the power source for the 'moon buggy' on the later Apollo moon missions.

Silver–zinc battery maker