A battery is a device that uses a redox reaction to extract chemical energy as electrical energy.
Basic Knowledge
Basic structure of a battery:
・Negative electrode: A metal with a high ionization tendency. Oxidation occurs (electrons are released).
・Positive electrode: A metal with a low ionization tendency. Reduction occurs (electrons are received).
・Electrolyte: Carries ions and completes the circuit.
In the external circuit, electrons flow from the negative electrode to the positive electrode; this is the source of electric current (the direction of current is opposite to that of electron flow).
Representative batteries:
・Voltaic cell (Zn|dilute H2SO4|Cu): electromotive force of about 1.1 V (theoretical); polarization is a problem.
・Daniell cell (Zn|ZnSO4||CuSO4|Cu): the electrolytes are separated by a porous plate, allowing stable operation. About 1.1 V.
・Lead-acid battery (Pb|H2SO4|PbO2): automobile battery. Rechargeable (secondary battery), about 2.0 V/cell.
Battery（A device that converts chemical energy into electrical energy）
Negative electrode（The electrode where oxidation occurs. Releases electrons. Has a high ionization tendency）
Positive electrode（The electrode where reduction occurs. Receives electrons. Has a low ionization tendency）
Electrolyte（A solution that conducts ions and completes the circuit）
Primary battery（Not rechargeable. Examples include manganese and alkaline dry cells）
Secondary battery（Rechargeable. Examples include lead-acid and lithium-ion batteries）
Further Study (Background・Significance)
Reactions at each electrode of a Daniell cell (cell notation (-) Zn | ZnSO4 aq || CuSO4 aq | Cu (+)):
・Negative electrode: Zn → Zn2+ + 2e- (oxidation)
・Positive electrode: Cu2+ + 2e- → Cu (reduction)
・Overall reaction: Zn + Cu2+ → Zn2+ + Cu
Lead-acid battery (during discharge):
・Negative electrode: Pb + SO42- → PbSO4 + 2e-
・Positive electrode: PbO2 + 4H+ + SO42- + 2e- → PbSO4 + 2H2O
In recent years, lithium-ion batteries (high energy density and light weight) and fuel cells (H2 + O2 → H2O, producing only water) have attracted attention. Battery technology is key to EVs and a renewable-energy society.
- Negative electrode = oxidation = large ionization tendency
- Positive electrode = reduction = small ionization tendency
- Electrons flow from the negative electrode → outside → positive electrode
- The direction of current is opposite to that of electron flow
- Daniell cell: Zn(-)/Cu(+), about 1.1 V
- Lead-acid battery: Pb(-)/PbO2(+), a secondary battery
- Primary battery = disposable; secondary battery = rechargeable
Points to Note (Easily Confused)
① Electrons flow from the negative electrode to the positive electrode, while current flows in the opposite direction (positive electrode → negative electrode in the external circuit). ② Oxidation occurs at the negative electrode, and reduction occurs at the positive electrode (note that the terminology differs from electrolysis). ③ Polarization in a Voltaic cell: a phenomenon in which H2 is produced at the positive electrode and the voltage drops. ④ Distinguish between primary batteries (manganese dry cells, etc.) and secondary batteries (lead-acid and lithium-ion batteries).
Practice
- Name the metal that becomes the negative electrode in a Daniell cell and write the reaction equation that occurs.
- What substance is present at the positive electrode of a lead-acid battery?
- What is a rechargeable battery generally called?