Sorry for the long post, there are a few things I'd like to add.
My preferred method is to twist the pack, it might break a weld-point or two but it's much safer for both me and the device, than trying to pry it open with tools or cut it with a knife. Twisting the pack will eventually break the plastic welds then you can start prying. Sometimes the pack has glue / tape holding it together and if prying into the cell, only ever use a plastic tool to do so. Cylindrical cells are a little bit more tolerant of poking but you'd damage the insulation on the cells or cause a short potentially.
I have used a dremel with a mini circular saw cutter which I find is good for cutting plastic in brief bursts until the wheel gets hot enough that it melts the plastic.
The real risk is opening up a lithium polymer pack, if a metal tool were to go into the pack, these only have a thin outer layer so puncturing the cell with say a screwdriver is far more hazardous and could cause a short between the cell's layers, which will cause a lithium fire. In which case a burn box could work but the energy output might be great enough that it wouldn't be possible to get it in there in time. If a lithium polymer pack is puffed up then it should really just be disposed of, especially if it was charged. I know of some instances where people dealing with lithium cells say in RC cars or a power wall, have had their houses burn down. Then there's phone shops where a tool slips and punctures a phone's lithium cell, there's lot of videos on Youtube of that happening.
Do not open up a battery pack to replace cells unless there's no other good option and you know what you're doing. A lithium battery pack is meant to be treated as a sealed unit and in my opinion there's a lot of specialist knowledge required to replace cells in a pack safely, which is why you don't really see cell-replacement services anymore.
Replacing cells is only necessary when the pack doesn't function and mix&matching cells in a pack is dangerous. Putting a fresh cell into a used pack could create an imbalance in voltages / capacities where cells could more easily be over-discharged or over-charged, damaging the cells or resulting in thermal runaway (fire). Loose metal swarf left in a pack from soldering or cutting while replacing cells could cause shorts too, it's very tough to replicate factory quality / conditions to achieve the same level of quality and safety that the pack was designed to originally.
Beyond that, lithium batteries have controller chips that perform charge balancing, prevent over charging & discharge and track the battery state. If tampered with, these controller chips can sometimes shut down rendering the pack inoperable and you may need specialist paid tools to fix that.
Mostly I only bother with NiMH packs for my old Toshibas since they're somewhat safer to work on and pre-date such chips.
Laptop battery packs can hold up very well over time if stored at between 20 to 70% charge at a stable temperature. The pack's circuitry will drop this charge over time along with the self-discharge of the cells so they'd need topping up maybe every couple of years. But I have packs that are 20 years old that still charge and can still run properly. Of course some packs have bad cells and that drops the capacity down significantly but unless you intend to use an old computer as a portable I do not recommend messing with the battery pack.
I find that very old lithium cells especially the 17670 type with low energy density (~1200mah per cell) from the 90s do often still hold charge. Unlike NiMH cells which just about always leak, these very old Lithium Ion cells can often still be charged in the laptop and run for a period of time. Out of maybe 10 PA2487u Toshiba Satellite 400-series batteries, only one would throw an error and that pack when dismantled did have something leaking from the cells and was disposed of.
I think that's because they have more parallel cells - laptop batteries that just have 3x batteries in series and none parallel (3s1p) like the Toshiba Libretto don't hold up so well. The PA2487u is I think 3x series and 3x parallel.
Old laptops from the era that I'm interested in (1990 to 2000) only used 17670 and 18650 cylindrical cells and some 103450 prismatic cells. These all seem to be not especially hazardous when stored, though I've seen prismatics damage themselves from over-discharge & poor storage. If they go 100% flat like to 0 volts and have done so over a long period of time, it's usually safe to recharge them. It's only un-safe to recharge them when they won't charge at all, which usually indicates a fault in the pack where one or more cells has an internal short and can't be charged back to its operating voltage safely. But it it can't be charged it's often safe to keep since battery packs are often structural elements 😀
I have Lithium polymer equipped tablets from 2014 that are still going strong and haven't puffed up from use, but I do charge them up / run them every few months.
Phone batteries get run a bit harder and I've had to replace a few of those now but none that have puffed up thankfully.
Modern laptop batteries are a different story, I've had good experiences with Lenovo, my T440's batteries are holding up after 9 years. But my Dell Latitude from 2016 & some at work which have lithium polymer cells did puff up, damaging the laptop casing. Those I'd just dispose of and try to get a replacement original manufacturer battery, cheap OEM battery packs are a lottery and I'm happy to just run that Dell laptop from mains power now.
