The protections I know for typical 1 standalone cell are against overcharge and overdischarge situations, overcurrent protection might be just a fuse.
For the negative voltage, imagine 3 batteries in series with a 10 ohms load (a light bulb). 2 batteries are fresh charged, the 3rd is almost empty. At first, all 3 will generate voltage, but when the 3rd one has no energy left, it will be just like a resistor. Draw them on paper, and replace the 3rd battery with a 100 ohms resistor. Note that the voltage on the 100 ohm is about the same voltage as the 2 series cells, but with the + where it was the - of the battery.
If the description is not clear, try with 2 alkaline batteries (with 1.5V single use batteries, do NOT try that with Li-Ion cells). Take 2 alkalines, 1 new amd another one discharged, put them in series and connect a light bulb. Wait until the light bulb fades. Do not disconnect the light bulb. Now, measure the voltage on the good battery, and you'll find it can still deliver voltage, and the polarity is correct. Measure the voltage on the discharged battery, and you'll find it reverse polarized.