What do a fiat electric car, a Toshiba laptop, and a Nokia cellular phone have in common? They are all powered by new electrochemical batteries.
Batteries come in various shapes: button cells for cameras and watches, primary cylindrical batteries for toys, car-starting batteries, load-leveling batteries for power plants, or propulsion packs for electric cars.
Batteries are of two types: primary and secondary batteries. Primary batteries are throwaway batteries because the chemicals cannot be brought back to their original form once the batteries have been discharged. They are uneconomical for they produce only 2% of the power used in their manufacture. In secondary batteries, the process is reversible; hence, they are rechargeable. The battery can be returned to its charged state in a reversible chemical reaction.
Inside the Battery
A battery is a group of voltaic cells connected together that converts chemical energy into electricity. The battery allows release of electricity on demand. A voltaic cell produces energy by a spontaneous oxidation-reduction (redox) reaction.
A cell usually consists of an electrolyte (which could be a liquid, paste or solid), a positive and a negative electrode. The electrodes are electrochemically active materials that carry electrons from one to the other, thereby serving as terminals.
In a redox reaction, the electrolyte provides the ions (cations and anions) that migrate towards their respective electrodes. The negative electrode (cathode) attracts the cations and the positive electrode (anode) attracts the anions. At the anode, anions release or give up electrons. This process is called oxidation. The electrons are then picked up or gained by the cations. This happens at the cathode. The process is called reduction. The two processes take place simultaneously provided there is water. Water acts as the transporting agent.
Primary Battery
You feel disappointed when the batteries in your flashlight ran down. The flashlight battery or dry cell is the most common form of primary cell, usually shaped in a cylinder. The electrodes are zinc (anode), which is the can enclosing the battery materials, and carbon (cathode), which is the rod at the center. The electrolyte is not a liquid but a paste of manganese dioxide & ammonium chloride; hence, the name dry cell.
The small flat disk inside your Seiko wristwatch is a button cell. This is another widely used primary cell. It is commonly called the mercury battery whose cathode is mercuric oxide and the electrolyte is a solution of potassium hydroxide. One word of caution: it is dangerous to try to recharge a primary-cell battery.
Storage Battery
The battery in cars is lead-acid; the type most widely used today and is the standard in storage batteries. It consists of six cells connected in series and is also widely used in trucks, aircraft, and other vehicles. Its main advantage is that it can deliver a strong current for starting an engine; however, it discharges quickly.
The battery consists of sets of lead (anode) and lead dioxide (cathode) plates, and a dilute solution of sulfuric acid as the electrolyte. When you turn the car’s ignition switch, a chemical reaction takes place. The lead anode dissociates into free electrons and positive lead ions. The electrons power the starter of your car. The lead ions combine with sulfate ions in the electrolyte to form lead sulfate.
When most of the active material in the battery becomes converted to lead sulfate and water, the chemical reaction stops and the battery becomes discharged. The discharged reaction can be reversed by delivering a current to the batter and restoring the chemicals to their original form. In your car, the generator continually recharges the battery.
Another widely used secondary cell is the nickel-iron battery, also known as the alkaline cell. The anode is iron, the cathode is nickel oxide and the electrolyte is a solution of potassium hydroxide. This battery is used principally in heavy industries.
Another alkaline cell is the nickel-cadmium battery, popularly known as nicad battery. Instead of iron, the anode is cadmium. One of its popular applications is in cordless tools, such as electric drills.
The advent of rechargeable nicad batteries has popularized the use of portable electronic devices to record sound and video, and laptops for information processing. These batteries offer a source of power free from noise and other electrical hazards, though only for a short period of time.
However, ordinary nicad and lead-acid rechargeable batteries can’t meet the demand for small, lightweight batteries that remain charged even if they’re not used for long periods. Companies that make portable electronic products – communication devices, computers, calculators, VCRs, power tools, and even electronic toys use batteries with anodes of lithium. Lithium is preferred because it packs the highest theoretical energy density of any negative battery electrode. These batteries can hold their charge for up to five years. Compared with ordinary batteries, they have five to ten times more energy density (expressed in watt-hr/kg). However, they cannot be recharged.
But new lithium-ion batteries can be recharged up to a thousand times. They have anodes of carbon and cathodes made of a compound of lithium, cobalt, nickel, and manganese, together with an electrolyte made of a mixture of propylene, carbonate & diethyl carbonate. Lithium-ion batteries work by pulling ions back and forth from one end of the battery to the other, causing chemical reactions that generate energy. They have much as 95 watt-hr/kg of energy, compared to 55 watt-hr/kg for nickel-metal hydride and 40 watt-hr/kg for nicad. Batteries using another long-lasting form of lithium, called lithium polymer, are also under development.
Batteries in Cars
The search for alternative sources of energy spurred development of new types of batteries for use in electric vehicles. But further development of these batteries is still needed.
Lead-acid battery has a good power density (expressed in watt/kg) to handle sudden heavy loads, is affordable, and can be easily recycled. But it has a poor cycle life and low energy density, meaning it can power your car for short distances only.
Nickel-cahmium battery delivers a higher electric current and therefore gives better performance than lead-acid. But nicad is just as heavy as lead-acid, ten times more expensive, and is difficult to recycle.
Batteries that have now passed from testing to application for use in electric cars include sodium-nickel-chloride, nickel-iron, nickel-metal hydride, and sodium-sulfur. These batteries are also being developed by electric utilities for use in “load leveling” to compensate for load fluctuations.
Compared to the best commercially available lead-acid batteries, nickel-metal hydride (NiMH) offer specific energy that’s about twice of lead-acid, doubling the driving range and with little degradation of acceleration performance. The cycle life is four times better than other batteries, so they may last the life of the car. Since no hazardous materials are used in this battery, it can by safely disposed of at the end of its life. However, NiMH batteries for use in electric cars are still expensive.
In sodium-sulfur batteries, the active materials are molten sodium (anode) and sulfur (cathode). The electrolyte is a solid membrane or separator that allows ions, but not electrons, to pass. Sodium-sulfur batteries have several advantages – they are light, compact and pack three times the energy density of rival batteries. Their downside is that in order to work, they must first be headed to about 300 0 C, which requires auxiliary equipment. They are, therefore, costly to manufacture.
These facts say that affordable high-energy batteries must be developed or the electric car will likely remain a plaything of few people.
Ernesto Ferreras Jr.
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