Illustration: The basic cell is a controlled pathway for ions inside the battery and electrons outside it.
The four essential layers
A lithium-ion cell contains an anode, cathode, separator, and electrolyte. The separator blocks electronic contact while allowing ions to pass.
Discharge
During discharge, lithium ions move through the electrolyte from the anode toward the cathode. Electrons cannot cross the separator, so they move through the external circuit and power a device.
Charge
Charging reverses the direction. External power pushes lithium ions back into the anode host material while electrons are supplied through the charger.
Why voltage exists
Voltage comes from the difference in chemical potential between the two electrodes. Different chemistries tune that potential, trading energy, power, safety, lifetime, and cost.