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How do rechargeable (that is, zinc-alkaline or nickel-cadmium) batteries work and what makes the reactions reversible in some batteries, but not in others?

Michael M. Lerner, an associate professor of chemistry at Oregon State University, responds:

"One of the necessary conditions for a battery to be rechargeable is that the underlying chemical changes that occur during an electrical discharge from the cell must be efficiently reversed when an opposite electrical potential is applied across the cell. In nickel-cadmium (NiCad) batteries, for example, the Cd(OH)2 and Ni(OH)2 that are formed during cell discharge are readily converted back to the original electrode materials (Cd and NiOOH), when the cell is recharged.

"In contrast, nonrechargeable, or primary, batteries can be based on irreversible chemical changes. For example, the carbon-fluoride- lithium primary batteries often used in cameras generate energy by converting (CF) n and Li metal to carbon and LiF. But the starting material at the battery' s cathode, (CF)n, is not reformed when a reverse potential is applied. Instead the cell electrolyte decomposes, and eventually the fluoride is oxidized to form fluorine gas.

"A reversible chemical change is not the only requirement for rechargeable batteries, however. To be classified as rechargeable, the battery must be able to undergo the reverse reaction efficiently, so that hundreds or even thousands of recharging cycles are possible. In addition, there must often be provisions to ensure that the recharging process can occur safely.

"The alkaline batteries (which are generally based on the conversion of MnO2 and Zn to Mn3O4 and ZnO) offer an excellent example of this last point. Although the chemical changes at the electrodes can be reversed, until recently alkaline batteries were manufactured only to function as primary cells. Recharging one of these primary cells could allow the battery to be reused, but the possible number of recharging cycles for such a cell is very limited--it performs more poorly with each recharge. More important, recharging an old-fashioned alkaline battery is not safe. During or after a recharge, the battery might generate enough hydrogen gas to cause an explosion. In their rechargeable form, alkaline cells have undergone several changes. They have been redesigned to allow for a more efficient reverse reaction, they contain a catalyst to minimize hydrogen formation and they have safety vents that prevent the buildup of excess pressure during recharging."

Frank McLarnon is a staff scientist and principal investigator in the Energy & Environment Division of Lawrence Berkeley National Laboratory. He clarifies why some reactions are irreversible:

"All batteries, both rechargeable and nonrechargeable, undergo electrochemical reactions. When a battery is discharged, an electrochemical oxidation reaction proceeds at the negative electrode, and an electrochemical reduction reaction occurs at the positive electrode. When one attempts to recharge a battery by reversing the direction of electric current flow, the opposite takes place: a reduction reaction proceeds at the negative electrode, and an oxidation reaction takes place at the positive electrode.

"In the case of the rechargeable battery, the electrochemical oxidation- reduction reactions are reversible at both electrodes. For example, when the battery is recharged, the overall electrochemical reduction reaction at the negative electrode is identical to the electrochemical oxidation reaction that proceeded at the negative electrode when the battery was discharged, only written in reverse.

"In the case of the nonrechargeable battery, when one attempts to recharge the battery by reversing the direction of electron current flow, at least one of the electrochemical oxidation-reduction reactions is not reversible. When the battery is charged, the overall reduction reaction that proceeds at the negative electrode may not be the true reverse of the oxidation reaction that proceeded when the battery was discharged. For example, metal oxidation might be the sole oxidation reaction during battery discharge, whereas the formation of hydrogen (a highly inflammable and therefore dangerous gas) might be a significant reduction reaction during battery recharging.

"An added requirement for a well-behaved (that is, long-lived) rechargeable battery is that not only must the electrochemical oxidation- reduction reactions be reversible, they must also return the electrode materials to their original physical state. For example, rough or filamentary structures may form in the battery after repeated charge- discharge cycles. These structures can result in unwanted growth of the electrode and subsequent electronic contact between the battery electrodes- -a short circuit.

"Because of these requirements, the development of a well-behaved rechargeable battery is significantly more difficult than the development of a nonrechargeable battery. In the case of the nickel-cadmium battery, the cadmium electrode has two important features. First, the rate of hydrogen formation on cadmium is very slow (compared with that on pure zinc or pure iron, for example). Second, the solubility of cadmium in basic aqueous electrolyte solutions is sufficiently low that the cadmium does not tend to dissolve in the electrolyte and migrate to the positive electrode or elsewhere within the battery. At the same time, however, the cadmium is sufficiently soluble that its reduction can proceed easily, and it can form compact cadmium structures during battery recharge; the solubilities of most other metals are either too high or too low. These and other fundamental factors have allowed the successful development of the rechargeable nickel-cadmium battery

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