Silver & Batteries
August 28, 1998
For more than 60 years silver's superb conductivity has played an important role in the high performance battery market, and today provides power to small electronic products as well as immense military systems and satellites.
Silver's entrance into the battery market came during World War II when it was tapped for its ability to provide high energy with minimum weight and volume. Initially, two systems were developed: silver oxide/zinc and silver chloride/magnesium systems. Neither type of battery was rechargeable at first, but since that time, a silver-zinc battery has been developed as a secondary, or rechargeable, battery.
In today's extremely fragmented battery market, silver's high energy density is still its major advantage. Silver's disadvantage is its relative short life and higher cost. For example, silver-zinc batteries provide about 40 watt-hours per pound compared with nickel-cadmium systems which have an output of 14 watt-hours per pound. But nickel-cadmium systems last almost 10 times longer than silver-zinc batteries.
By far, the most common silver system today is the small silver oxide-zinc battery that powers miniature electronic devices. Called gbutton cells,h these batteries are used for a variety of consumer products including watches, calculators, cameras and hearing aids. Although silver batteries face competition from lithium batteries, the use of button cells continues to expand at a steady pace. In 1993, 838 million batteries were sold worldwide, according to industry estimates. By 1997, that number had grown to approximately 1.2 billion, a 43 percent increase in four years.
In addition to button cells, silver remains the industry standard for systems requiring absolute reliability. gThere's always a trade-off,h explains battery consultant Robert Morey. gBut silver has a proven track record for over 50 years, and in products where safety and reliability are important, silver is worth any premium you might pay.h
Silver-zinc batteries, with high energy, low mass and low volume, have the specific requirements for spacecraft, satellites, missiles, rockets, space launch vehicles, torpedoes, underwater vehicles and life support systems. On the Mars Pathfinder mission, the rover and the cruise system were all powered by solar cells. The energy storage requirements of the lander were met with modified silver-zinc batteries which have about three times the energy density of the standard nickel-cadmium battery.
On board the Space Shuttle, the lighter weight, higher power silver oxide batteries run the instrumentation, open and close bay doors, and power on-board and ejected experiments. Astronauts use silver oxide batteries in their space suits for space walk life support systems and power tools.
The U.S. Navy is currently completing an upgrade of its underwater SEAL Delivery Vehicles designed to carry combat swimmers and their cargo in fully flooded compartments. The vehicles use an all-electric propulsion system powered by rechargeable silver-zinc batteries. The current program calls for 10 of the boats to be completed by the end of 1998 at a cost of $2.2 million per vehicle.
And just three months ago, a Pegasus Rocket delivering NASA's gTrace Spacecrafth to its low-earth orbit was powered by two 28-volt silver-zinc aerospace batteries providing reliable electrical power for the rocket's operations.
Other Silver Batteries
In addition to silver-zinc batteries, other systems couple silver with cadmium, magnesium and hydrogen, but such batteries have far narrower applications. Silver chloride-magnesium batteries are activated at the time of use by immersion in either seawater or fresh water and are used primarily for military applications. Silver oxide-cadmium has been called the gcompromise battery,h with energy density and life that lie between those of the nickel-cadmium and silver oxide-zinc systems. Both commercial and military use of these batteries has been limited. Silver-hydrogen batteries, also limited in use, are found in systems requiring large power sources such as high-energy laser weapons.
Solar Cells
Solar cells are not electrochemical, but they do convert the energy of sunlight into electrical energy. Silver paste is used in 90 percent of all crystalline silicon photovoltaic cells, which are the most common solar cell. And all silicon cells used in space to power satellites use silver in the form of evaporated metal to make the electrical contact.
The electricity generated by photovoltaic cells is highly reliable. As soon as sunlight strikes, power begins to flow. Sunlight striking silicon cells generates electrons, which the silver conductors collect to become a useful electric current.
Fuel Cells
While there has been speculation about the use of silver in fuel cells, experts at the U.S. Department of Energy and throughout the industry generally view silver as an unlikely metal for this application. Fuel cells differ from other chemical batteries in two major respects. One is that they can operate continuously, without being stopped for recharging, so long as their active materials are supplied from an external source. The other is that their electrolyte remains chemically unchanged during operation.
Fuel cells require catalysts that become active at or near room temperature. Current research focuses on platinum, which is active at low temperatures, but is extremely costly. Silver requires higher temperatures, which makes it less likely for this application.
Conclusion
The United States is one of the largest manufacturer of batteries worldwide and currently consumes approximately 4.2 million ounces of silver for that purpose. Japan is also a leading producer of batteries, and is estimated to consume an additional 2.0 million ounces for their manufacture.
Product manufacturers searching for batteries to power the broad range of new consumer products, such as notebook computers and cellular/digital communications devices, must constantly balance the trade-offs of power, energy, cycle life and cost. While silver's cost and weight may preclude its use for some of these devices, silver's superior power-to-weight characteristics are expected to remain an important technological component of the overall battery industry.
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