Primary cells are disposable, while secondary cells can be recharged. Let us understand representative examples and their reactions.
Basic Knowledge
Primary cells (disposable):
① Manganese dry cell: Zn|NH4Cl,ZnCl2|MnO2·C, 1.5 V
② Alkaline manganese cell: Zn|KOH|MnO2, 1.5 V, high capacity
③ Lithium cell: Li|organic electrolyte|MnO2, 3 V, long life
Secondary cells (rechargeable):
① Lead–acid battery: Pb|H2SO4 aq|PbO2, 2.0 V × 6 cells=12 V. Used in automobiles
② Nickel–metal hydride battery: 1.2 V, hybrid vehicles
③ Lithium-ion battery: approximately 3.7 V, smartphones and EVs — 2019 Nobel Prize in Chemistry(Akira Yoshino)
Manganese dry cell（Zn/MnO2, 1.5 V, inexpensive）
Lead–acid battery（Pb/PbO2, dilute H2SO4, water is decomposed during charging）
Nickel–metal hydride battery（MH/NiOOH, 1.2 V）
Lithium-ion battery（C(Li)/LiCoO2, 3.7 V）
Energy density（Wh/kg, Wh/L）
Cycle life（Number of possible charge–discharge cycles）
Further Exploration (Principles and Applications)
Discharge reactions of a lead–acid battery:Negative electrode: Pb+SO42-→PbSO4+2e-Positive electrode: PbO2+4H++SO42-+2e-→PbSO4+2H2O
Overall reaction: Pb+PbO2+2H2SO4 → 2PbSO4+2H2O. Because the concentration of H2SO4 decreases during discharge, the state of charge can be determined by measuring the specific gravity.
In lithium-ion batteries, Li+ ions intercalate(insert into and leave) between carbon(C6) layers and LiCoO2 crystals. This innovative mechanism achieves low weight and high energy density and is a core technology supporting the spread of smartphones and EVs.
- Primary=disposable; secondary=rechargeable
- Lead–acid battery: 12 V for automobiles
- PbSO4 forms at both electrodes during discharge, and H2SO4 concentration decreases
- Lithium-ion battery: low weight and high energy density
- Akira Yoshino=2019 Nobel Prize in Chemistry
- Charging is the reverse reaction of discharging
- Cycle life=long-term performance
Points to Note (Common Confusions and Frequent Mistakes)
① During discharge of a lead–acid battery, PbSO4 forms at both electrodes. ② Because water is consumed by electrolysis during charging, water must be replenished regularly. ③ A lithium battery(primary) and a lithium-ion battery(secondary) are different. ④ Nickel–cadmium batteries are declining because of the toxicity of Cd.
Practice
- Write the overall reaction when a lead–acid battery discharges.
- Explain why the specific gravity of the electrolyte in a lead–acid battery decreases.
- Give the name of the Japanese scientist who received the 2019 Nobel Prize in Chemistry for the invention of the lithium-ion battery.