Li-ion electrochemical material has an intrinsic voltage of 3.7-4.2 V, while "regular" batteries, including Ni-metal rechargables have an intrinsic voltage of about 1.5V, and they are also not able to handle the high current drain that PVs demand. None the less, there have been a few example of people making PVs from regular batteries, although I don't know how they ended up. My guess is they drain too fast, even if you have stacked enough of them to make the voltage desired. The other type of battery that is used is the 3.0V LiPO4. The main points are how to get up to 7V, and have the battery last long enough to enjoy some good
vaping, and in general Li-ions fill the bill. As for safety, one should always use protected batteries in mods, especially if they are being stacked, and as an added safety, use a recharger that has its own cutoff when the battery is fully charged. The vast majority of them do now.