| specific energy | 130 Wh/kg[1] |
|---|---|
| energy density | 500 Wh/L[1] |
| 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) is a primary cell with relatively very high energy/weight ratio. They are costly 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. Spent batteries may be processed for their silver content.
Silver-oxide primary batteries account for 21% of all primary battery sales in Japan (14% of all portable batteries) but by contrast only 0.05% of all battery sales in the UK including secondary types. [2] [3]
A related rechargeable device usually called a silver–zinc battery uses a variation of silver–zinc chemistry. It shares most of the pros and cons of a silver-oxide battery, in addition to being able to withstand the largest loads of all known secondary power sources. Long used in specialist applications, they are now geared to enter the mainstream markets, for example as laptop batteries.
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A silver oxide battery uses silver oxide as the positive electrode (cathode), zinc as the negative electrode (anode) plus an alkaline electrolyte, usually sodium hydroxide (NaOH) or potassium hydroxide (KOH). The silver is reduced at the cathode from Ag(I) to Ag and the zinc is oxidized from Zn to Zn(II). 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.
The silver–zinc battery, on the other hand, uses the opposite electrode composition, the cathode being made of pure silver, while the anode is made from a mixture of zinc oxide and pure zinc powders. The electrolyte used is the pure caustic potash solution without any added NaOH. Chemical processes during the discharge are similar to the silver oxide cell, while the different starting electrode composition makes it possible to recharge such a cell.
During the charge process, silver is first oxidized to silver(I) oxide: 2Ag + 2OH– → Ag2O + H2O + 2e– and then to silver(II) oxide: Ag + 2OH– → 2AgO + H2O + 2e–, while the zinc oxide is reduced to metallic zinc: 2Zn(OH)2 + 4e– = 2Zn + 4OH–. This process continues until the cell potential reaches the level where the electrolysis of the hydroxide ion is possible, about 1.55 V. This is usually taken as the sign of the end of the charge, as at this point no other charge is taken, and the oxygen generated poses a mechanical and fire hazard for the cell.
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.
It provides up to 40 percent more run time than lithium-ion batteries and also feature a water-based chemistry that is free from the thermal runaway and flammability problems that have plagued the lithium-ion alternatives.[4]
This technology had the highest energy density prior to lithium technologies. Primarily developed for aircraft, they have long been used in space launchers and crewed spacecraft where their short cycle life is not a drawback. Non-rechargeable silver–zinc batteries powered the Saturn launch vehicles, the Apollo Lunar Module, lunar rover and life support backpack. The primary power sources for the command module were the hydrogen/oxygen fuel cells in the service module. They provided greater energy densities than any conventional battery, but peak power limitations required supplementation by silver–zinc batteries in the CM that also became its sole power supply during re-entry after separation of the service module. Only these batteries were recharged in flight. After the Apollo 13 near-disaster, an auxiliary silver–zinc battery was added to the service module as a backup to the fuel cells. The Apollo service modules used as crew ferries to the Skylab space station were powered by three silver–zinc batteries between undocking and SM jettison as the hydrogen and oxygen tanks could not store fuel cell reactants through the long stays at the station.
Silver oxide batteries become hazardous on the onset of leakage; this generally takes five years from the time they are put into use (which coincides with their normal shelf life). Until recently, all silver oxide batteries contained up to 0.2% mercury. The mercury was incorporated into the zinc anode to inhibit corrosion in the alkaline environment. Sony started producing the first silver oxide batteries without added mercury in 2004. [5]