The ability to recharge a cell depends on the reversibility of the electrode reactions. In secondary cells, the products formed during discharge remain in contact with the electrodes in a state that allows them to be converted back to the original reactants.
During discharge, the cell acts as a galvanic (voltaic) cell where chemical energy is converted to electrical energy through spontaneous redox processes.
During recharging, the cell acts as an electrolytic cell. An external power source provides the energy required to drive the non-spontaneous reverse reaction, restoring the chemical potential of the cell.
The Electromotive Force (EMF) of the cell is determined by the difference between the standard electrode potentials () of the reduction and oxidation half-cells:
| Feature | Primary Cells | Secondary Cells |
|---|---|---|
| Reversibility | Irreversible chemical reactions | Reversible via external current |
| Cost | Low initial cost | Higher initial cost, lower long-term |
| Environmental Impact | High waste (single use) | Lower waste (reusable), but toxic components |
| Common Examples | Zinc-Carbon, Alkaline | Lithium-ion, Lead-acid, NiCad |
| Applications | Remote controls, toys | Laptops, EVs, Car batteries |
Equation Reversal: When writing equations for the charging process, remember to flip the discharge equations entirely. The species that was reduced during discharge must be oxidized during charging.
EMF Calculation: Always use the standard reduction potentials. The more positive value identifies the reduction half-cell (cathode) during discharge.
State Symbols: In storage cell questions, state symbols are critical. For example, in lead-acid batteries, is a solid that sticks to the electrodes, which is why the reaction is reversible.
Lithium Advantages: If asked why Lithium is used, focus on its high energy-to-weight ratio (low density) and its very negative standard electrode potential, which allows for high cell voltages.
The Battery Misnomer: Students often use 'battery' and 'cell' interchangeably. In a technical context, a battery is a collection of cells; for instance, a 12V car battery is actually six 2V lead-acid cells connected in series.
Recharging Voltage: A common mistake is assuming any voltage will recharge a cell. Recharging requires an external voltage greater than the cell's EMF to overcome the internal resistance and drive the non-spontaneous reaction.
Memory Effect: Many students believe all rechargeable batteries have a 'memory.' In reality, this is primarily a characteristic of NiCad cells; modern Lithium-ion cells do not suffer from this and can be 'topped up' at any time.
