Silver oxide battery
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| Specific energy | 130 Wh/kg |
|---|---|
| Energy density | 240 Wh/L |
| Specific power | High |
| Charge/discharge efficiency | N/A |
| Energy/consumer-price | Low |
| Self-discharge rate | Negligible |
| Time durability | High |
| Cycle durability | N/A |
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.
Chemistry
A silver oxide battery is a small-sized primary battery using silver oxide as the negative electrode (cathode), zinc as the positive electrode (anode) plus an alkaline electrolyte, usually sodium hydroxide (NaOH) or potassium hydroxide (KOH). The silver is reduced at the cathode from Ag+ to Ag0 and the zinc is oxidized from Zn0 to Zn2+. 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 seemed to spell its demise. It was the power source in all of the Apollo spacecraft: the command module re-entry batteries, the lunar module and the lunar rover. (The Apollo Service Module used fuel cells as a primary power source.)
More recently, the company ZPower has introduced a new line of rechargeable silver-zinc batteries to compete with lithium-ion batteries. The claims include an energy density of 200Wh/kg, complete non-flammability, and no toxic chemicals. To offset the high price of silver, the company plans to offer a trade-in policy to recycle all of it.[1][2]. Intel's venture arm, Intel Capital, has provided the company with financial backing.[3]