During charging and discharging processes, the direction of electron flow in a battery or electrochemical cell is reversed. This reversal causes the cathode and anode to switch roles.
Let's consider a typical rechargeable battery, such as a lithium-ion battery, as an example:
Charging: During the charging process, an external power source is connected to the battery. This power source applies a higher voltage than the battery's voltage, causing a flow of electrons from the external source into the battery. The electrons enter the battery through the positive terminal, which is connected to the cathode. At the same time, positive ions (e.g., lithium ions) within the battery move from the cathode to the anode, driven by the applied voltage. As a result, the cathode becomes the positive electrode during charging, attracting electrons, while the anode becomes the negative electrode, releasing electrons.
Discharging: When a charged battery is being used or providing power, it goes through the discharging process. The battery's stored energy is released, generating an electric current. In this case, the battery acts as the power source itself. Electrons flow from the negative terminal (anode) through the external circuit, providing power to the connected device. Simultaneously, positive ions (e.g., lithium ions) flow from the anode to the cathode within the battery, completing the internal circuit. Hence, during discharging, the cathode becomes the negative electrode (electron source), and the anode becomes the positive electrode (electron receiver).
The flipping of cathode and anode roles during charging and discharging occurs because the direction of electron flow and ion movement is determined by the external circuit and the applied voltage. In each mode, the battery operates as a chemical system where the flow of electrons and ions is driven by the electrical potential difference across the cell.