A lithium-ion cell has two electrodes separated by an electrolyte and a porous barrier. During discharge, lithium ions move through the electrolyte from one electrode toward the other. Electrons take a different route through the external circuit, supplying power to a device.
Two paths for charge
Charging pushes the process in the opposite direction using an external power source. The electrodes host lithium in their structures; the cell is not simply a box full of loose metallic lithium. Different electrode chemistries change capacity, voltage, life and safety characteristics.
Why the separator matters
The separator helps prevent direct contact between electrodes while allowing ions to move. Heat, damage, overcharging and poor manufacturing can create risks, so battery packs also use protective electronics and thermal design. The key distinction is that ions move inside the cell while useful electron flow occurs through the circuit outside it.
Related: how a solar cell converts light into electricity.