Rechargeable, or secondary, batteries use reversible redox reactions. During discharge, a spontaneous reaction produces electrical energy; during charging, an external power supply forces the reverse, non-spontaneous reaction.
This is standard-level and higher-level content in Reactivity 3.2: Electron transfer reactions.
The Mechanism
During discharge, the battery behaves as a voltaic cell:
- Oxidation occurs at the anode, releasing electrons.
- Electrons flow through the external circuit from the anode to the cathode, powering a device.
- Reduction occurs at the cathode, where electrons are accepted.
- Ions move through the electrolyte to maintain electrical neutrality. A separator prevents direct contact between the electrodes while allowing ionic movement.
The spontaneous redox reaction converts chemical energy into electrical energy. As discharge continues, reactants are consumed and products accumulate, so the cell potential eventually becomes too low to power the device effectively.
During recharging, an external voltage greater than the battery's cell potential drives electrons in the opposite direction. This forces the discharge products to reform the original reactants, converting electrical energy back into chemical energy.
| Process | Discharging | Recharging |
|---|---|---|
| Energy conversion | Chemical to electrical | Electrical to chemical |
| Reaction | Spontaneous | Non-spontaneous and externally driven |
| Electron flow | Through the device | Forced in the opposite direction by a power supply |
| Electrode processes | Original redox reactions | Reverse redox reactions |
A common misconception is that the words anode and cathode refer to permanently fixed positive and negative electrodes. They do not: the anode is always where oxidation occurs, and the cathode is always where reduction occurs. Therefore, their roles reverse when the battery changes between discharge and charging.
Exam Technique
For an IB explain question, state the energy conversions, identify oxidation and reduction, describe electron and ion movement, and explain that an external voltage reverses the redox reaction. Do not merely state that the battery “stores electricity”; it stores energy chemically.