The basic structure and the charging-discharging processes of rechargeable batteries
are shown in Fig. 12.7. For reference, definitions are provided as below. For more
details, see, for example, Handbook of Batteries .
Cell The basic electrochemical unit converting electrochemical energy to electrical energy.
Battery One or more electrochemical cells connected in series or parallel to provide electrical power.
Primary cells or batteries One-time source of electricity, cannot be recharged after usage. Are discarded after usage.
Secondary (rechargeable) cells or batteries Can be recharged electrically after usage to their original condition.
Oxidation Loss of electron(s).
Reduction Gain of electron(s).
Redox Reduction and oxidation.
Anion Negative ion — after gaining electron(s).
Cation Positive ion — after losing electron(s).
Anode Oxidation takes place. During charge, it is the positive electrode (PE). During discharge, it is the negative electrode (NE).
Cathode Reduction takes place. During charge, it is NE. During discharge, it is PE.
Electrolyte Typically an ionic conducting liquid.
Figure 12.7(a) shows the discharging process. By connecting the cell to an external load, electrons flow from the negative electrode (NE), the anode, which is oxidized, through the external load to the positive electrode (PE), the cathode, where the electrons are accepted and the cathode material is reduced. The electric circuit is completed in the electrolyte by the flow of anions (negative ions) and cations (positive ions) to the anode and cathode, respectively.
Figure 12.7(b) shows the charging process. An external DC power supply is connected to the battery. The external electrical field forces electrons to flow into the negative electrode (NE), where reduction takes place. On the other hand, oxidation takes place at the positive electrode (PE), where the electrons flow out from. As the anode is, by definition, the electrode at which oxidation occurs and the cathode the one where reduction takes place, the positive electrode is the anode and the negative electrode is the cathode.