Rechargeable (Secondary) Cells

Introduction to Rechargeable Cells

• Rechargeable cells (also known as secondary cells) are electrochemical cells that can have their chemical reactions reversed by applying an external electrical source.
• This allows the cell to be recharged and used multiple times.
• They function as galvanic cells during discharge (producing electricity) and as electrolytic cells during recharge (storing energy).

KEY TAKEAWAY: Rechargeable cells are galvanic cells during discharge and electrolytic cells during recharge.

Common Design Features

• Two Electrodes: An anode and a cathode, made of materials that can undergo reversible oxidation and reduction.
• Electrolyte: A substance containing ions that facilitates the flow of charge between the electrodes.
• Separator: A porous membrane that prevents physical contact between the electrodes, preventing short circuits, but allows ion flow.
• External Circuit: Allows the flow of electrons during discharge and recharge.
• Casing: Encloses the cell components.

General Operating Principles

Discharging (Galvanic Cell)

• Spontaneous Redox Reaction: A chemical reaction occurs spontaneously, converting chemical energy into electrical energy.
• Electron Flow: Electrons flow from the anode (where oxidation occurs) to the cathode (where reduction occurs) through the external circuit.
• Ion Flow: Ions move through the electrolyte to maintain charge balance.
• Electrode Polarity:
  • Anode: Negative (-) polarity (source of electrons).
  • Cathode: Positive (+) polarity (destination of electrons).
• The cell continues to discharge until the reactants are depleted and the reaction reaches equilibrium.

Recharging (Electrolytic Cell)

• Non-spontaneous Redox Reaction: An external electrical source (e.g., a battery charger) forces a non-spontaneous reaction to occur, converting electrical energy into chemical energy.
• Electron Flow: Electrons are forced to flow from the positive terminal of the external source to the cathode, reversing the redox reaction.
• Ion Flow: Ions move through the electrolyte to maintain charge balance, opposite to the direction during discharge.
• Electrode Polarity:
  • Anode: Positive (+) polarity (connected to the positive terminal of the external source).
  • Cathode: Negative (-) polarity (connected to the negative terminal of the external source).
• The cell is recharged by reversing the original chemical reaction, regenerating the reactants.

EXAM TIP: Be prepared to explain the flow of electrons and ions during both discharge and recharge.

Conditions Required for Reaction Reversal

• External Voltage: The applied voltage from the external source must be greater than the cell’s open-circuit voltage (the voltage when no current is flowing). This overcomes the energy barrier of the non-spontaneous reaction.
• Correct Polarity: The external source must be connected with the correct polarity, matching the anode and cathode to force the reaction in the reverse direction. Reversing the polarity will not recharge the cell and can damage it.
• Current Limit: The charging current must be controlled to prevent overheating, gas formation, and degradation of the electrodes and electrolyte.
• Reversible Chemistry: The chemical reactions must be reversible. The products formed during discharge must be able to readily convert back to the original reactants during recharge.

Electrode Polarities in Each Mode

REMEMBER: “AN OX, RED CAT” (Anode Oxidation, Reduction Cathode). During discharge, electrons flow from the Anode (negative) to the Cathode (positive). During recharge, the external power supply forces electrons to flow in the opposite direction, reversing the polarities.

Examples of Rechargeable Cells (Details not required for this Key Knowledge point)

• Lead-acid batteries (used in cars)
• Nickel-cadmium (Ni-Cd) batteries
• Nickel-metal hydride (Ni-MH) batteries
• Lithium-ion (Li-ion) batteries (used in mobile phones, laptops, and electric vehicles)

APPLICATION: Lithium-ion batteries are ubiquitous in modern portable electronics and electric vehicles due to their high energy density and relatively long lifespan.

Comparison of Primary and Secondary Cells

VCAA FOCUS: VCAA often assesses the understanding of the fundamental differences between primary and secondary cells, particularly focusing on the reversibility of reactions and the implications for sustainability.

Sustainability Considerations

• Resource Depletion: Mining and processing materials for rechargeable batteries can deplete natural resources.
• Energy Consumption: Manufacturing and recycling rechargeable batteries require significant energy input.
• Toxicity: Some battery components (e.g., heavy metals) are toxic and pose environmental risks if not properly managed.
• Recycling: Proper recycling of rechargeable batteries is crucial to recover valuable materials and prevent environmental contamination.

STUDY HINT: Create diagrams illustrating the discharging and recharging processes, labeling the electrodes, electrolyte, and electron flow. This visual aid will help solidify your understanding.