Rechargeable batteries store electrical energy by using reversible chemical reactions. In a lithium-ion battery, energy moves between electrical form in a circuit and chemical form inside the cell. This matters because rechargeable cells power phones, laptops, electric vehicles, medical devices, and grid storage.
Understanding their design helps explain why batteries have limits on voltage, capacity, charging speed, and lifetime.
A lithium-ion cell has two electrodes, an anode and a cathode, separated by an electrolyte and a thin separator. During discharge, lithium ions move through the electrolyte while electrons travel through the outside circuit to power a device. During charging, an external power source pushes the ions and electrons back toward their higher-energy arrangement.
Good engineering balances energy storage, safety, heat control, material cost, and cycle life.
Key Facts
- Cell voltage is the electric potential difference between the cathode and anode: V = W/q.
- Electrical energy delivered by a battery is E = VQ, where Q is charge in coulombs.
- Battery capacity is often measured in ampere-hours: Q = It.
- Energy in watt-hours is approximately E(Wh) = V × capacity(Ah).
- During discharge in a lithium-ion cell, Li+ ions move through the electrolyte and electrons move through the external circuit.
- Power output is P = IV, so higher current or voltage means faster energy delivery.
Vocabulary
- Anode
- The electrode where oxidation occurs, commonly the graphite electrode during discharge in a lithium-ion cell.
- Cathode
- The electrode where reduction occurs, commonly a lithium metal oxide or phosphate material during discharge.
- Electrolyte
- The ion-conducting material that allows lithium ions to move between electrodes while blocking direct electron flow.
- Separator
- A thin porous barrier that keeps the anode and cathode from touching while allowing ions to pass through.
- Cycle life
- The number of charge and discharge cycles a battery can complete before its usable capacity falls significantly.
Common Mistakes to Avoid
- Saying electrons flow through the electrolyte is wrong because electrons travel through the external circuit while ions move inside the cell.
- Treating voltage and capacity as the same thing is wrong because voltage measures energy per charge, while capacity measures total charge stored.
- Assuming faster charging is always harmless is wrong because high current can cause heating, lithium plating, and faster aging.
- Thinking a rechargeable battery stores electrons like a tank is wrong because it stores energy in chemical arrangements of ions and electrode materials.
Practice Questions
- 1 A lithium-ion cell has a voltage of 3.7 V and a capacity of 2.5 Ah. Estimate its stored energy in watt-hours.
- 2 A phone battery delivers a current of 0.80 A at 3.8 V for 2.0 hours. How much energy does it deliver in watt-hours?
- 3 During discharge, explain why lithium ions must move through the electrolyte while electrons must move through the external circuit instead of directly across the separator.