Battery chemistry is the study of how chemical reactions are arranged to produce electric current. A battery works by separating oxidation and reduction into two electrodes so electrons are forced to travel through an external circuit. This makes batteries useful because stored chemical energy can be converted into electrical energy on demand.
Alkaline, lithium-ion, and lead-acid batteries all use the same electrochemical principles, but their materials, voltages, and reusability are different.
In a discharging cell, oxidation occurs at the anode and reduction occurs at the cathode, while ions move through an electrolyte to keep charge balanced. Alkaline cells are usually primary batteries, meaning their reactions are not designed to be reversed many times. Lithium-ion and lead-acid cells are rechargeable because an external power source can drive the reactions backward during charging.
Understanding the half-reactions, ion movement, and cell voltage helps explain why different batteries are chosen for flashlights, phones, cars, and grid storage.
Key Facts
- Cell voltage is the electric potential difference between the cathode and anode: Ecell = Ecathode - Eanode.
- During discharge, oxidation occurs at the anode and reduction occurs at the cathode.
- Electrons flow through the external circuit from anode to cathode during discharge.
- In an alkaline cell, Zn is oxidized and MnO2 is reduced in a basic electrolyte, often KOH.
- In a lithium-ion cell, Li+ ions move through the electrolyte while electrons move through the circuit: LiC6 + CoO2 ⇌ C6 + LiCoO2.
- In a lead-acid cell during discharge: Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O.
Vocabulary
- Anode
- The electrode where oxidation occurs, releasing electrons during a battery reaction.
- Cathode
- The electrode where reduction occurs, gaining electrons during a battery reaction.
- Electrolyte
- A substance that allows ions to move between electrodes while usually preventing direct electron flow.
- Primary cell
- A battery designed for one main discharge cycle because its chemical reactions are not easily reversed.
- Secondary cell
- A rechargeable battery whose chemical reactions can be driven backward by an external voltage.
Common Mistakes to Avoid
- Confusing electron flow with ion flow: electrons travel through the external wire, while ions move through the electrolyte to maintain charge balance.
- Assuming the anode is always positive: during discharge in a galvanic battery, the anode is negative, but during charging of a rechargeable cell the electrode roles can reverse.
- Writing only the overall reaction: half-reactions are needed to identify oxidation, reduction, electron transfer, and the source of cell voltage.
- Treating all batteries as rechargeable: alkaline cells are usually primary cells, while lithium-ion and lead-acid cells are designed for many charge and discharge cycles.
Practice Questions
- 1 A battery delivers a current of 2.0 A for 30 minutes. How much charge passes through the circuit in coulombs? Use Q = It.
- 2 A lithium-ion cell has a voltage of 3.7 V and moves 5400 C of charge during discharge. How much electrical energy is delivered? Use E = QV.
- 3 Compare an alkaline battery and a lithium-ion battery in terms of reversibility, ion movement, and why one is commonly used as a primary cell while the other is rechargeable.