New Battery Technology
The need for better batteries is a recurring theme in the effort to reduce energy consumption and in the effort to make electricity increasingly portable. As the world depends more and more on portable devices and turns to electric vehicles to reduce pollution, it becomes important that lightweight, long-lived batteries be developed. Additional desirable features of batteries include safety, dependability, environmental friendliness, and cost. This article considers battery technology currently used in transportation, including in electric vehicles and spacecraft.
A battery is a collection of one or more electrochemical cells that convert chemical energy into electrical energy via electrochemical reactions ( oxidation-reduction reactions ). These reactions take place at the battery's anode and cathode. The electrochemical cells are connected in series or in parallel depending on the desired voltage and capacity. Series connections provide a higher voltage, whereas parallel connections provide a higher capacity, compared with one cell.
A cell typically consists of a negative electrode, a positive electrode, and an electrolyte. A cell discharges when a load such as a motor is connected between the negative and positive electrodes. The negative electrode, the anode, produces electrons that flow in an external circuit. The positive electrode, the cathode, consumes the electrons from the external circuit. The uniform flow of electrons around the circuit results in an electric current. Within the cell, the electrons received at the positive electrode react with the active material of this electrode, in reduction reactions that continue the flow of charge by sending ions through the electrolyte to the negative electrode. At the negative electrode, oxidation reactions between the active material of this electrode and the ions flowing through the electrolyte results in a surplus of electrons that are donated to the external circuit. For every electron generated in an oxidation reaction at the negative electrode, there is an electron consumed in a reduction reaction at the positive electrode. As the electrode reactions continue spontaneously, the active materials become depleted and the reactions slow down until the battery is no longer capable of supplying electrons; the battery is said to be fully discharged.
A battery is either a primary or a secondary battery. Primary batteries, such as those used in a flashlight, are used once and replaced. The chemical reactions producing the current in such batteries are too difficult to make it worth trying to reverse them. Secondary batteries, such as car batteries, can be recharged and reused because the chemical reactions are easily reversed. By reversing the flow of electricity (i.e., putting current in rather than taking it out), the chemical reactions are reversed to restore active material that had been depleted. Secondary batteries are also known as rechargeable or storage batteries and are used in transportation applications.
Battery performance is measured in terms of voltage and capacity. The voltage is determined by the chemistry of the metals and electrolytes used in the battery. The capacity is the number of electrons that can be obtained from a battery. Since current is the number of electrons released per unit time, cell capacity is the current supplied by a cell over time and is normally measure in ampere-hours. Battery specialists experiment with many different redox combinations and try to balance the energy output with the costs of manufacturing the battery. Other factors, such as battery weight, shelf life, and environmental impact also factor into the battery's design.
Present-Day Battery Technology
Lead-acid batteries are used in gasoline-driven automobiles and in electric and hybrid vehicles. They have the best discharge rate of secondary battery technology, they are the cheapest to produce, and they are rechargeable. The chemical reactions are:
The positive electrode is made of lead dioxide (PbO 2 ) and is reduced to lead sulfate (PbSO 4 ), while sponge metallic lead (Pb) is oxidized to lead sulfate at the negative electrode. The electrolyte is sulfuric acid (H 2 SO 4 ), which provides the sulfate ion (SO 4 2− ) for the discharge reactions.
The nickel-cadmium battery (Ni-Cd) is the most common battery used in communication satellites, in Earth orbiters, and in space probes. The chemical reactions are:
Nickel hydroxide, NiO(OH), is the active cathode material, cadmium, Cd, is the active anode material, and aqueous potassium hydroxide, KOH, is the electrolyte.
There is considerable interest in the development of nickel-metal hybrid (Ni/MH) batteries for electric and hybrid vehicles. These batteries operate in concentrated KOH electrolyte. The electrode reactions are:
Ni/MH batteries use nickel hydoxide, NiO(OH), as the active material for the cathode, a metal hydride, MH, as the anode, and a potassium hydroxide, KOH, solution as the electrolyte. The metal hydride is a type of alloy (hydrogen absorption alloy) that is capable of undergoing a reversible hydrogen absorbing-desorbing process while the battery is discharged and charged. Current research is directed at improving the performance of the metal hydride anode and making the battery rechargeable.
Lithium ion (Li-ion) batteries are environmentally friendly batteries that offer more energy in smaller, lighter packages and thus are promising candidates for electric and hybrid vehicle applications. The electrode reactions are:
Li-ion batteries use various forms of carbon (C) as anode material because carbon can reversibly accept and donate significant amounts of lithium (as Li x C 6 . Li-intercalation compounds (such as LiCoO 2 , LiMn 2 O 4 , and LiNiO 2 ) are used as cathode materials. Electrolyte mixtures include a lithiated salt (LiPF 6 or LiClO 4 ) dissolved into a nonaqueous solvent (ethylene carbonate, propylene carbonate, or dimethyl carbonate). Because Li is a highly reactive metal in aqueous solution , Li-ion batteries are constructed to keep Li in its ionic state, and nonaqueous solvents are used. The next step in lithium-ion battery technology is believed to be the lithium polymer battery, in which a gelled or solid electrolyte will replace the liquid electrolyte.
Unlike the batteries described in the previous section, a fuel cell does not run down or require recharging; it will produce energy in the form of electricity and heat as long as fuel is supplied. Additionally, the electrode materials (usually platinum) serve only as a site for the reactions to occur (i.e., as a catalyst ) and are not involved in the chemical reactions. As hydrogen flows over the anode, it is oxidized to hydrogen ions and electrons in a proton-exchange membrane or PEM fuel cell. The hydrogen ions pass through the membrane to the cathode, where they combine with oxygen from the air and with the electrons flowing in the external circuit from the anode to form water, which is expelled from the cell. A fuel cell system that includes a "fuel reformer" utilizes hydrogen from any hydrocarbon fuel, such as natural gas or methanol. This also makes a fuel cell quiet, dependable, and very fuel-efficient. Fuel cell reactions include:
O 2 + 4H + + 4e → 2H 2 O
2H 2 → 4H + + 4e
Fuel cells are lighter and more compact, compared with batteries that make available the same amount of energy.
Solar cells (photovoltaic cells) convert sunlight to electricity. Photovoltaic cells are made of semiconductor materials such as silicon and gallium arsenide. When light strikes the cell, photons are absorbed within the semiconductor and create electron-hole pairs that move within the cell. This generates the energy that is used to power space vehicles.
Electric and Hybrid Vehicles
Electric vehicles have an electric motor rather than a gasoline engine. The electric motor is usually powered by two banks of twenty-five 12-volt lead-acid rechargeable batteries, providing a total of 300 volts for each battery bank. Problems with lead-acid battery technology include battery weight (a typical lead-acid battery pack weighs 1,000 pounds or more), limited capacity (a typical battery pack holds about 15 kilowatt-hours of electricity, giving the car a range of approximately 80 kilometers [50 miles]), long recharging times (typically between four and ten hours for full charge), short life (three to four years) and cost (about $2,000 for each battery pack). The hybrid vehicle, in which a small gas engine is combined with an electric motor, is a compromise between gas-powered and electric vehicles.
The car of the future will likely be an electric or hybrid vehicle that gets its electricity from a fuel cell. It is unlikely that these vehicles will ever be solar powered, because solar cells produce too little power to make using them to run a full-size car practical.
Spacecraft and space stations are powered by solar cells or collections of solar cells called solar panels. To get the most power, solar panels must be pointed directly at the Sun. Spacecraft are built so that the solar panels can be pivoted as the spacecraft moves, so that they can always stay in the direct path of the rays of light.
Solar cells generate electricity in the sunshine but not in the dark. Thus space stations and spacecraft run on power from batteries during dark periods. As of June 2003, solar power has been practical for spacecraft operating no farther from the Sun than the orbit of Mars. For example, Magellan, Mars Global Surveyor, Mars Observer, and the Earth-orbiting Hubble Space Telescope operate on solar power.
A Look to the Future
Exciting research is underway to improve the performance and longevity of batteries, fuel cells, and solar cells. Much of this research is directed at enhancing the chemistry in these systems through the use of polymer electrolytes, nanoparticle catalysts, and various membrane supports. Additionally, considerable effort is being put into the construction of three-dimensional microbatteries.
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