system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do lithium ion batteries degrade over time?

Why do lithium ion batteries capacity diminishes after each cycle? I'd like to know what happens chemically or structurally.
gpt
I am a battery test engineer. There are many ways lithium batteries can degrade, but since this is ELI5, I'll stick to one main method. 

Batteries have a few main parts: the anode (negative), the cathode (positive), a separator between them, and some stuff in between (usually a liquid) that conducts ions. When you charge a battery, you are cramming a whole bunch of lithium ions into the anode, kind of like absorbing water into a sponge. When you use the battery, these ions flow to the cathode, generating electric current. Over time, by cramming the ions in and out of the anode and cathode, you begin to damage the 'sponge', so it can't hold as many ions any more. So your efficiency goes down.