The most used Li-ion cell is the cylindrical 18650 cell. Where 18 refers to the cell diameter in millimeters and 65-zero means 65 mm long.
Lithium polymer cells have a different design compared to the 18650 cells. They are pouch shaped. The pouch is a flexible foil (polymer laminate, hence Li-Po). The chemistry is the similar to Li-ion cells. Li-Po cells can be made very thin, typically several mm thick. Compared to Lithium polymer cells, Lithium-ion cells have a higher power density and are mechanically stronger due to their metal based cylindrical shape. The soft package of polymer cells can easily be punctured and has more risk of swelling than the metal cans of Li-ion cells. A punctured cell can cause an internal short circuit, which will cause the cell to get hot. Even if it does not short circuit the cell, a leak may allow moisture in, eventually causing the cell to self-discharge and die.
Over-discharge might result in damage for all Lithium based cells, but the result of gassing in Lithium polymer cells is more obvious (visible swelling because of the soft shape). When the cell voltage drops too low (~1.5V), reactions at the anode start to produce gas. Overcharge is similar, but then gassing occurs at the cathode when the electrolyte starts to decompose at high voltage (~4.6V). All lithium based rechargeable batteries need a Battery Management System (BMS) to prevent overcharging and over discharging, providing battery protection (e.g. temperature, wrong polarity), and to keep individual cells balanced (in case of a battery pack consisting of multiple cells).
Lithium based rechargeable batteries need to be secured within an enclosure to prevent movements during shock and vibration and prevent punctures. They also need to be prevented against short circuiting at all times.
So, the use of lithium based rechargeable batteries comes with risks, they can even catch fire. And when on fire it is almost impossible to stop this process. Protecting the environment by covering the burning battery with dry sand is one of the few options you have.
When implementing lithium based rechargeable batteries in a product, you need to be sure the user stays safe. And even when disposing of the battery, you need to do that in a safe way. Since they can still catch fire, contact employees from the labs, if you want to dispose of these kind of batteries.
If you consider using lithium based rechargeable batteries in a project, a safer option is using a USB power bank. These contain the same kind of batteries but include a Battery Management System (BMS). And given the fact it is a common consumer product, one might expect it to meet certain safety regulations.