Secondary (Rechargeable) Cells

Specification Reference R3.2.7

Quick Notes

Secondary cells (rechargeable batteries) work by reversible redox reactions.

Discharging: chemical energy turned into electrical energy (like a primary cell).
Charging: electrical energy turned into chemical energy (requires external power source).
Reactions reverse during charging.

Full Notes

What Are Secondary Cells?

Secondary (or rechargeable) cells are electrochemical cells in which:

The redox reactions are reversible.
The cell can be recharged by passing an electric current in the opposite direction to when the cell is in normal use.

Common examples include lead–acid car batteries and lithium-ion batteries (such as used in mobile phones and laptops).

Discharge vs. Charging Reactions

Discharge:
During discharging, the cell works like a voltaic cell – spontaneous redox reaction occurs and electron flow from the anode to the cathode produces a current.

Charging:
During charging, the cell works like an electrolytic cell – a non-spontaneous redox reaction (the opposite direction to the discharge reaction) is forced to happen by the use of external energy.

To work out a charging reaction:

Reverse the discharge half-equations
Switch anode and cathode reactions

Example Lead–Acid Battery

IB Chemistry diagram of a lead–acid battery showing discharge and charging reactions.

Discharge reactions:

Anode (oxidation): Pb(s) + SO₄²⁻(aq) → PbSO₄(s) + 2e⁻
Cathode (reduction): PbO₂(s) + 4H⁺(aq) + SO₄²⁻(aq) + 2e⁻ → PbSO₄(s) + 2H₂O(l)

Charging reactions: Reverse both reactions using an external voltage source.

Example Lithium-Ion Cell

Another common example of a rechargeable cell is a lithium-ion cell, used to make lithium-ion batteries.

Reactions in a lithium-ion battery:

Positive electrode (cathode, where reduction happens):
Negative electrode (anode, where oxidation happens):

When the cell is in use, Li is oxidised, and CoO₂ is reduced. Recharging reverses the reaction.

Advantages and Disadvantages of Electrochemical Cell Types

Cell Type	Advantages	Disadvantages
Primary cells	Convenient, portable, inexpensive	Non-rechargeable, waste issues
Secondary cells	Rechargeable, long-term cost effective	Limited charge cycles, environmental impact
Fuel cells	Continuous supply of energy, high efficiency	Require constant fuel supply, infrastructure issues

Summary

Secondary cells store energy reversibly.
Charging reverses the redox reactions seen during discharge.
Examples include lead–acid and lithium-ion batteries.
Primary cells are single-use

Linked Course Question

Reactivity 2.3 — Linked Course Question

Secondary cells rely on electrode reactions that are reversible. What are the common features of these reactions?

Reversible Redox Reactions: The chemical reactions at both electrodes can be reversed by applying an external current. During discharge, one redox direction occurs to produce electrical energy. During recharge, the reverse reactions restore the original materials.

No Permanent Change in Electrodes: The electrodes must remain chemically stable and structurally intact through multiple cycles of oxidation and reduction.

Efficient Energy Conversion: The reactions ideally occur with minimal side reactions and energy loss so that the cell can be recharged many times with little capacity loss.

No Gas Production: Secondary cell reactions are ideally designed to avoid gaseous products. This prevents pressure build-up in sealed cells, improving safety and maintaining performance over many cycles.