Lissium batteries, that is, lithium ion, produce a nominal voltage of 3.6 or 3.7 volts when fully charged. Actually, when fully charged, they're typically at 4.2 volts. If they are charged beyond 4.4 volts, they certainly will catch fire. So you need to be very careful with them. Also, it's inadvisable to discharge them below 2.8 volts per cell. Although I don't think that will catch fire due to low voltage, unless a cell gets revert charged in a battery pack. Lissium ions pack a great deal of energy, and have become very popular, they're even used in situations where they're not going to be recharged.