The lead-acid battery has been around for a century and a half, and lithium-ion cells for only 15 years. But battery technology lags the spiraling energy demands spurred by the portable electronics boom. Today's consumer wants power "to go," and the race for the best way to power portable devices is on.
Basically, batteries are containers that hold chemicals, and the technology hasn't changed much since the 1940s. Different chemicals have increased battery life but it's still not satisfactory.
One problem is the fixed size and shape of AA and other standard batteries make them incompatible with modern portable consumer gear. Further complicating matters, the shapes of proprietary batteries conform only to specific applications such as powering cell phones. Bottom line: researchers are constantly looking for ways of squeezing more power from ever-smaller batteries.
POWERING UP UNCLE SAM
A driving force behind better battery technology is the U.S. military. While consumers have been okay with battery-powered appliances that run for 4 hr, the military has demanded batteries with enough power for 12-hr missions. Now, 72 hr is the goal.
A recent Army-sponsored study by the National Research Council recommended hybrid energy systems to support mobile warriors of the Future Force (the Army's initiative to develop high-tech capabilities in soldier systems). Portable battery rechargers; laser target-designator devices for guiding rockets, missiles, or bombs; and cooling systems for protective garments consume an average of 100 W. The report concluded that hybrid systems combining fuel cells and batteries offer the best approach, because they can provide power for varying levels of energy use.
One result: A soldier on a three-day mission would need only a 10-lb portable fuel cell (including fuel) to get the same amount of power available from nearly 30 lb of batteries.
All lithium cells use a nonaqueous electrolyte. Their nominal open-circuit voltages (OCVs) range from 2.1 to 3.9 V. Lithium cells can operate over large temperature ranges, with some lithium-based cells capable of working at up to 150°C.
One type of lithium cell is the bobbin-type lithium thionyl chloride cell from Tadiran Batteries, Port Washington, N.Y. These cells are used in wireless applications such as automatic water and gasmeter-reading devices. Many of these nonrechargeable devices have been in service for 20 yr.
Tadiran Vice President and General Manager Sol Jacobs points out that lithium does not harm the environment like nickel-cadmium and lead-acid batteries, and lithium cells don't need recy-cling. The company also makes batteries for pressure-monitoring systems in automobile tires.
Tadiran's hybrid lithium batteries, which include what's called a hybrid-layer capacitor (HLC), handle the high-current pulses of wireless sensors and remote devices. The combined HLC and bobbin cell exhibits the qualities of a capacitor and battery, Jacobs explains. The capacitor cell eliminates the voltage drop that would otherwise accompany loads drawing significant pulse currents. The battery portion supplies long-term, relatively low currents. Pulsed loads initially draw current from the capacitor side. The capacitor and the battery are electrically in parallel.
Cellular phones and laptop computers owe their existence to the advent of rechargeable lithium-ion batteries. "But the next challenge for the industry is supplanting older chemistries like NiCad or nickel-metal-hydride in devices that need high power," says Vice President Business Development and Marketing Ric Fulop of A123Systems, Watertown, Mass., a maker of lithium-ion batteries.
Battery developers have turned to nanomaterials as a way of reaching much greater power densities. Specially prepared nanomaterials can have a super-large surface area per square inch. When used as anode or cathode material, this leads to much faster kinetics, which is useful in increasing power.
Thanks to these advances in conductivity, A123Systems li-ion batteries now drive powerful 36-V power tools with higher currents than are available from nickel-based chemistry. And Fulop expects li-ion chemistry will improve continuously over the next decade.
The chemistry lets A123 Systems make much larger batteries, and "you couldn't do that without a safe chemistry," Fulop says. "Our lithium technology could power a Toyota Prius with 80% less weight than nickel-metal-hydride and do it longer and more cost effectively," he claims.
Most lithium-ion batteries use carbon as the cathode and alternating layers of cobalt oxide and lithium as the anode. The exchange of lithium ions between the cathode and anode recharges the battery. But cobalt oxide is sensitive to heat spikes generated during high-current demands. A heat spike or short inside a cobalt or metal-oxide battery could trig-ger thermal runaway. In turn, this could cause a fire fueled by the device's own oxygen.
In recent years, the power capacity of lithium-ion batteries has improved at a rate of 8.5% annually. However, according to international strategy and general management consulting firm Boston Consulting Group, the power demand of portable applications is expected to grow 26% per year.
Austin-based Valence Technology replaces the metal-oxide materials typically used in lithium-ion solutions with a phosphate-based cathode. This, the company claims, provides a safer, more stable, and longer-lasting alternative to cobalt oxide. And phosphate costs less than cobalt. Valence offers two forms of what it calls Saphion technology: N-Charge and U-Charge power systems.
N-charge is a small-format system providing 130-Whr in a thin tablet (about 9 X 12 X 1 /2 in.) that fits easily in a briefcase. The N-charge can power any number of laptops or notebook power PCs for up to 10 hr. It comes with a brand-specific power cable and can be recharged with the laptop's ac adapter.
The U-Charge Power Systems are 12.8-V batteries that come in several of the standard shapes used by deep-cycle lead-acid batteries. Only they pack a lot more punch. Although they are the same size, they provide twice the runtime, four to five times as many discharge cycles, and require no maintenance. The 12.8-V models weigh from 13.4 to 41 lb.
Another fuel-cell-based portable power system (or "hydrogen battery") comes from Millennium Cell, Eatontown, N.J., and its partners. Millennium licenses its hydrogen battery technology to Protonex Technology Corp., Southborough, Mass., and Jadoo Power Systems, two private fuel-cell developers for military, medical, and industrial markets. The companies claim their hydrogen batteries offer performance and cost advantages over traditional battery chemistries, in a variety of applications under 500 W.
Millennium and Protonex are working on a 30-W fuel-cell power system for the U.S. Air Force and Army. They will also market the product to industrial and medical markets.
Millennium's Hydrogen on Demand system stores and delivers hydrogen from sodium borohydride, an energy-rich derivative of borax. The sodium borohydride is mixed with a stabilizer and water to create a nonflammable, energy-rich fuel. The fuel is then passed through a proprietary catalyst chamber where pure hydrogen is released as needed. The hydrogen can then be sent to a fuel cell (such as those developed by Protonex and Jadoo) to create electricity for portable devices.
Millennium hopes its hydrogen batteries will replace the lithium sulfur dioxide (BA 5590) batteries presently used by the military.
Today's lithium batteries can put out about 160-Whr/kg. The Protonex 30-W fuel-cell-based power product (the P2), which uses Millennium Cell's hydrogen storage, has an energy density of 425-Whr/kg. The P2 system could save about $200 per soldier, per 72-hr mission, the company claims. The companies are working on next-generation technologies based on sodium borohydride that could increase that advantage by 50 to 80% in the next few years, according to a Millennium spokesman.
Toshiba, on the other hand, recently unveiled a prototype of what it's calling a direct methanol fuel cell, or DMFC. An external cartridge contains the alternative fuel in a highly concentrated form. Commercial products based on the technology should appear on store shelves next year, the company says. Toshiba's 100-mW version is similar in shape and size to a pack of gum and can power a flash-based player for about 35 hr on a single 3.5-ml charge. A 300-mW version, about the size of a pack of playing cards, has enough juice to run an HDD-based player for about 60 hr on a single 10-ml charge. Methanol works best at a 3 to 9% concentration, normally requiring a tank 10 times larger than a cell phone. But Toshiba claims to have solved the problem by using a 99.5% methanol solution and diluting it with the water by-product from the fuel cell. In the laboratory, some methanol fuel cells get 1,500 Whr/kg.
Many battery manufacturers are working on microsized fuel cells. And industry-analyst NanoMarkets LC, Glen Allen, Va., expects the market for portable fuel cells to reach $2.6 billion by 2012.
Researchers at the Oak Ridge National Laboratory in Tennessee have created their own version of a thin, solid-state lithium-ion battery using phosphate glass as an electrolyte. Current applications for thin-film rechargeable lithium batteries include implantable medical devices, remote sensors, miniature transmitters, smart cards, and microelectronic devices, says Nancy Dudney, group leader of the Ceramic Thin Films Group in ORNL's Condensed Matter Sciences Div.
Thin-film lithium batteries last longer, hold their charge better during storage, recharge faster with no memory effects, can be recharged many times, and can be made much smaller, lighter, and more flexile than traditional batteries. Also, since they are entirely solid, potential problems and hazards from leakage, corrosion, and freezing are eliminated, Dudney claims.
However, thin-film lithium batteries are more expensive than other small-capacity batteries because they are more complex to manufacture and because they are produced in much smaller numbers. And they are not available in standard cell sizes.
Cymbet Corp., Elk River, Minn., is looking at using the thin-film technology to make microbatteries that graft directly onto microchips. Unlike lithium-ion batteries, which typically break down after 500 recharging cycles, these microbatteries could be recharged thousands of times.
Most micro fuel cells create power through the reaction of diluted liquid methanol and a catalyst. The reaction releases protons and electrons. The protons on the fuel-cell side pass through a membrane into an air chamber, where they bond with oxygen atoms, pulling the electrons along with them. The flow creates a charge, and the only byproducts are water vapor, heat, and a small amount of carbon dioxide.
How fuel cells work
A fuel cell is an electrochemical device that produces electricity by separating the fuel (usually hydrogen gas) via a catalyst. The protons flow through a membrane and combine with oxygen to form water again with the help of a catalyst. The electrons flow from the anode to the cathode through the load to create electrical current. As long as the reactants pure hydrogen and oxygen are supplied to the fuel cell, it will produce electrical energy.
A single fuel cell is basically a piece of plastic between a couple of pieces of carbon plates that are sandwiched between two end plates acting as electrodes. These plates have channels that distribute the fuel and oxygen.
A factor that draws interest to the fuel cell is that it can operate at efficiencies two to three times that of the internal combustion engine, and it requires no moving parts. Because it converts the fuel, hydrogen, and oxygen directly to electrical energy, the only byproducts are heat and water. Without combustion, fuel cells are virtually pollution-free.
Alkaline fuel cells (AFC): First used in the Gemini-Apollo space program to produce drinking water and electrical energy; operate on compressed hydrogen; generally use a solution of potassium hydroxide (KOH) in water as electrolyte; output of alkaline fuel cell ranges from 300 W to 5 kW.
Direct-methanol fuel cells (DMFC): Use methanol instead of hydrogen; operating temperatures in the same range as PEM fuel cells 50 to 100°C (122 to 212°F); transportation-industry focus.
Moltencarbonate fuel cells (MCFC): Use a liquid solution of lithium, sodium, and/or potassium carbonates soaked in a matrix; units with output up to 2 MW have been constructed, and designs exist for units up to 100 MW; nickel electrode-catalysts of molten carbonate fuel cells are inexpensive compared to those used in other cells, but high temperatures limit the materials and safe uses of MCFCs.
Phosphoric-acid fuel cells (PAFC): Use phosphoric acid as the electrolyte; efficiency ranges from 40 to 80% and operating temperature is 150 to 200°C (about 300 to 400°F); existing phosphoric-acid cells have outputs up to 200 kW; and 11-MW units have been tested.
Proton-exchange-membrane fuel cells (PEM): The most common type of fuel cell being developed for transportation use; react quickly to changes in electrical demand and will not leak or corrode; use inexpensive manufacturing materials (plastic membrane).
Regenerative fuel cells (RFC): Separate water into hydrogen and oxygen by a solar-powered electrolyzer; hydrogen and oxygen are fed into regenerative fuel cells, generating electricity, heat, and water; water is then recirculated back to the electrolyzer of the regenerative fuel cell and the process repeats.
Solid-oxide fuel cells (SOFC): Use a hard ceramic compound of metal (like calcium or zirconium) oxides (chemically, O2) as electrolyte; output for solid oxide fuel cells is up to 100 kW; reformer is not required to extract hydrogen from the fuel due to high temperature.
Nanomaterial-based catalysts are usually heterogeneous catalysts divided into nanoparticles to speed up the catalytic process. The extremely small size of the particles increases the surface area exposed to the reactant, allowing more reactions to occur at the same time, thus speeding up the process. Much research on nanomaterialbased catalysts concerns maximizing the effectiveness of the catalyst coating in fuel cells. Platinum, the most common catalyst for this application, is rare and costly. A great deal of research hopes to maximize the catalytic properties of less expensive metals by shrinking them to nanoparticles.