Batteries are real-life applications of electrochemistry, directly connecting to IB Chemistry Topic 9 (Redox Processes) and Topic 19 (HL). Every battery—from phone batteries to car batteries—operates using spontaneous redox reactions. Understanding how batteries work helps you connect half-equations, electron flow, electrode potentials, and cell diagrams to something you use every day.
What Is a Battery?
A battery is a device that converts chemical energy into electrical energy through spontaneous redox reactions happening in an electrochemical (galvanic) cell.
A battery contains:
- Two electrodes
- An electrolyte
- A separator
- An external circuit for electrons to flow
These components allow oxidation and reduction to occur in different places, producing a flow of electrons.
The Chemistry Behind a Battery
A battery works by separating the two halves of a redox reaction:
- One material gives up electrons (oxidation).
- Another material accepts electrons (reduction).
- The electrons travel through an external wire.
- Ions move inside the battery to maintain charge balance.
This movement of electrons is what we call electricity.
Anode and Cathode
In a galvanic (voltaic) cell—used in batteries:
Anode (negative electrode)
- Oxidation occurs
- Electrons are released
- Electrons flow away from the anode
Example oxidation:
Zn → Zn²⁺ + 2e⁻
Cathode (positive electrode)
- Reduction occurs
- Electrons are accepted
- Electrons flow into the cathode
Example reduction:
2MnO₂ + H₂O + e⁻ → MnOOH + OH⁻
These processes happen simultaneously.
How Electron Flow Produces Electricity
Electrons released at the anode cannot move through the electrolyte (which only transports ions), so they travel through an external circuit.
This electron flow:
- Powers devices such as phones, flashlights, and laptops
- Continues until one of the reactants is used up
The battery stops working when:
- The anode material is fully oxidized
- The cathode material is fully reduced
- The ions can no longer move freely
The Flow of Ions Inside the Battery
While electrons move through the wire, ions move inside the battery:
- Positive ions travel toward the cathode
- Negative ions travel toward the anode
This keeps charge balanced.
If ions couldn't move, electrons would stop flowing almost immediately.
Electrolytes are essential because they allow ion migration but block electrons.
Types of Batteries and Their Chemistry
1. Alkaline Batteries
- Zinc anode
- Manganese dioxide cathode
- KOH electrolyte
- Produces 1.5 V
These are common in household devices.
2. Lead–Acid Car Batteries
- Pb anode
- PbO₂ cathode
- H₂SO₄ electrolyte
- Produces ~12 V per battery (6 × 2 V cells)
Rechargeable because the reaction is reversible when connected to a charger.
3. Lithium-Ion Batteries
- Graphite anode
- Lithium transition metal oxide cathode
- Organic electrolyte
They work by shuttling lithium ions between electrodes and are highly energy-dense.
Why Batteries Run Out
A battery stops working when:
- Reactants are depleted
- Products build up
- Ion flow is blocked
- Voltage drops below usable levels
Rechargeable batteries can reverse their redox reactions, but non-rechargeable ones cannot.
The Role of Standard Electrode Potentials (E°)
Battery voltage is determined using:
E°cell = E°cathode – E°anode
A more positive E° means stronger reduction.
A more negative E° means stronger oxidation.
By pairing suitable materials, manufacturers design batteries with specific voltages.
Common IB Misunderstandings
“The anode is always positive.”
In galvanic cells (batteries), the anode is negative.
“Electrons travel through the electrolyte.”
False. Only ions travel through the electrolyte; electrons travel through the wire.
“Voltage comes from electrons moving faster.”
Voltage comes from differences in reduction potential, not speed.
FAQs
Why is the anode negative in a battery?
Because oxidation releases electrons, giving the anode excess negative charge.
Why do rechargeable batteries degrade over time?
Side reactions, structural damage, and gradual loss of active material reduce efficiency.
Can a battery be both a galvanic and electrolytic cell?
Yes—rechargeable batteries act as galvanic cells when discharging and electrolytic cells when charging.
Conclusion
Batteries work by separating oxidation and reduction into two electrodes, forcing electrons to travel through an external circuit. This produces electrical energy from chemical reactions. Understanding how batteries function helps explain redox processes, electrode potentials, and electrochemical cell behavior in IB Chemistry.
