The basic principle of a Li-ion battery is that electrical energy is created by an electrochemical reaction between two metals of different affinities.
A Lithium-ion battery is made up of two electrodes (Anode & Cathode), a separator, an electrolyte, and two current collectors (positive and negative).
During discharge, an electrochemical reaction in Anode causes it to release positive lithium ions into the electrolyte. The electrolyte carries the positively charged lithium ions from the Anode to the Cathode.
This reaction in Anode is called reduction, and the Anode is known as the reducing agent because it loses lithium atoms.
The Cathode is known as the oxidizing agent because it accepts lithium ions from the Anode.
When the Anode releases positive Lithium-ions, at the same time, it frees electrons from the lithium atoms of the electrode.
These free electrons congregate inside the anode. As a result, the two electrodes have different charges:
The anode becomes negatively charged as electrons are released, and the cathode becomes positively charged as positive lithium-ions are consumed.
This difference in charge causes the electrons to want to move toward the positively charged cathode. However, they don’t have a way to get there inside the battery because the separator prevents them from doing so. The charge is measured in volts and depends on the chemistry used.
For example, a typical Lithium-ion cell has an electrical charge between 3.2 - 4.2 volts depending on the state of charge (SOC).
In an EV, we can use that desire for the electrons to re-unite with the positive lithium-ions in the Cathode. If we create an external circuit through an electric motor or another electronic component, we can see the flow of electrons drives the motor.
The current collectors work as an electrical conductors between the electrode and external circuits.
If you want to go to the next level, understand how a Lithium Ion battery works. See this genuinely fantastic video from the Limiting Factor.