This excellent video from BASF covers what happens inside lithium ion batteries when they are charging and discharging. Although its focus is on batteries for electric vehicles the basic principles remain the same for all lithium ion units.
VIDEO – How a lithium battery is made
Why don’t lithium ion batteries last forever?
What are lithium batteries?
How to calculate the Watt Hours (Wh) of a lithium battery
How to calculate the lithium content in a battery
Shipping lithium batteries