Reactivity 3.2.7 — Secondary (Rechargeable) Cells

A secondary cell (or rechargeable cell) is an electrochemical cell in which the redox reactions that occur during discharge can be reversed by applying an external voltage. This contrasts with a primary (non-rechargeable) cell, which cannot be reused once the chemicals are spent.

Characteristics of Secondary Cells

• They undergo spontaneous redox reactions to produce electrical energy during discharge.
• During charging, electrical energy is supplied to reverse the redox reaction and regenerate the original reactants.
• These cells are widely used in portable electronics, electric vehicles, and energy storage systems.

General Discharge and Charging Reactions

During discharge, oxidation occurs at the anode and reduction at the cathode — just like in primary cells. During charging, an external power source forces the reaction to proceed in the reverse direction.

Example: Lead–Acid Battery (Used in Cars)

Discharge reactions:

• • At the anode: \( \text{Pb (s)} + \text{HSO}_4^- \rightarrow \text{PbSO}_4 (s) + \text{H}^+ + 2e^- \)
  • At the cathode: \( \text{PbO}_2 (s) + \text{HSO}_4^- + 3\text{H}^+ + 2e^- \rightarrow \text{PbSO}_4 (s) + 2\text{H}_2\text{O} \)

Overall discharge:

• \( \text{Pb (s)} + \text{PbO}_2 (s) + 2\text{HSO}_4^- + 2\text{H}^+ \rightarrow 2\text{PbSO}_4 (s) + 2\text{H}_2\text{O} \)

Charging: Reactions are reversed with an external power supply.

Example: Lithium-Ion Cell (Used in Phones, Laptops)

Discharge:

• • At anode: \( \text{LiC}_6 \rightarrow \text{Li}^+ + e^- + \text{C}_6 \)
  • At cathode: \( \text{LiCoO}_2 + \text{Li}^+ + e^- \rightarrow \text{Li}_2\text{CoO}_2 \)

Charging: External voltage reverses the electron flow and lithium ions move back to the graphite anode.

 Overview