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I'm a writer and currently working on a young adult Sci-Fi novel so I obviously don't belong on this stack exchange site. If this question isn't allowed here let me know.

I know that inside of a Li-Ion battery there is an anode, a cathode and a separator. There's probably much more than that involved but those are the elements I'm interested in. Basically I'm just wondering what a larger lithium ion battery looks like (like large enough to power a vehicle.) Is it possible to safely open a lithium ion battery? If so, what would you see looking into it?

I can google what the inside of a lithium ion battery looks like but all I can find is diagrams which is helpful but I don't know what an anode or a cathode or a separator look like in real life. What does an anode look like? What does a cathode look like? What does a separator look like? Are these types of things even things that you can see?

This is only small part of my novel which is why I'm asking rather than taking the time to research thoroughly but I want to make sure my writing is as accurate as possible.

Edit What causes a lithium ion cell to short circuit, overheat, and/or explode? I get that it occurs often when the separator breaks down and allows the anode and cathode to touch but what causes the separator to break down?

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    \$\begingroup\$ This is probably a question that belongs on worldbuilding.SE? In any case, I'm pretty sure clive (bigclivedotcom on youtube) has disassembled some lithium batteries on camera, if you want to look for that. \$\endgroup\$
    – Hearth
    May 23, 2021 at 20:46
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    \$\begingroup\$ It won't look much different inside than a common AA or D cell which (if flat) would be saf*(er)* to take apart (outdoors, don't get the materials on your hands, wash hands thoroughly anyway, safely dispose of waste etc). That might be more hands-on than reading. \$\endgroup\$
    – user16324
    May 23, 2021 at 20:59
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    \$\begingroup\$ Just google images "inside lithium cell" \$\endgroup\$
    – DKNguyen
    May 23, 2021 at 21:00
  • \$\begingroup\$ Being too shy (or, rather, too conventional) to ask someone who has a hands-on experience with lithium ion batteries always wondered what the battery has inside. Good question! And the useful answer from @Jonathan S. \$\endgroup\$
    – V.V.T
    May 24, 2021 at 6:50
  • \$\begingroup\$ I've updated my answer to also include possible reasons for separator breakdown. \$\endgroup\$ May 24, 2021 at 10:29

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It's not safe at all to open lithium ion (or lithium polymer) batteries. They contain volatile, flammable and possibly toxic materials. Opening them is at least a major burn and fire hazard. They can also spontaneously dump their stored energy if you damage them, which causes the battery to get hot and possibly ignite or explode.

Cylindrical Lithium-ion batteries generally consist of a roll of a stack of: a copper anode foil that's coated with black or brown ion absorbing material, a thin white-ish separator made out of a polymer (plastic) that can let lithium ions through but physically separates the electrodes, and an aluminium cathode foil that is coated just like the anode. This roll of electrodes and separator is soaked in liquid electrolyte, which is usually a clear solvent-based liquid with dissolved lithium salts and other additives. It evaporates very quickly and is also highly flammable. The anode and cathode are connected to the battery's terminals via metal strips and the entire thing is tightly packed into a metal tube (the casing).

Flat pouch-type lithium polymer batteries contain the same components (anode, cathode, separator, electrolyte). They're just folded instead of rolled to make a flat stack of alternating electrode and separator layers. This stack is then wrapped in a thick metal foil to provide at least some mechanical protection and to prevent the electrolyte from evaporating.

Larger batteries (like in a car) are made by combining lots (possibly thousands) of those smaller cylindrical or pouch-type cells. A bunch of cells is connected in parallel to provide higher capacity and current-handling capability. Multiple of these "chunks" of paralleled cells are then connected in series to provide the required voltage to the device that has to be powered. For example, a "3S2P" battery is a series connection of three "chunks" of two paralleled cells each. This gives twice the capacity of a single cell at three times the voltage, for a total of six times as much energy (since you have six cells in total). This is a very popular configuration for laptop batteries.

Here's a video of someone disassembling an 18650 lithium cell. Don't try this at home!

To answer the additional question that you edited in: Separator breakdown either happens due to mechanical damage to the cell (i.e. someone whacking, slicing or crushing one of those pouch-type cells) or because of electrical stress. Overcharging and over-discharging a lithium-ion cell can cause metallic lithium dendrites to grow within the cell which can puncture the thin separator and cause a short-circuit, which in turn results in the cell rapidly releasing all of its stored energy as heat. This will then cause the cell to puff up, vent or ignite.

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    \$\begingroup\$ the technical term for the pouch type cells is "Prismatic" (because they ideally have parallel sides) these are the kind that catch fire inside dreamliners and smartphones. \$\endgroup\$ May 24, 2021 at 3:25

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