Just about anything can be exploded if you pile on too much energy. Lots of youngsters have been applying this concepts to electrolytes, except that most newer electrolytes have a safety-valve feature to just vent.

NiCd and NiMH takes serious amounts of abuse, and you don't accidentally happens to build a NiCd charger with charging currents several times stronger than intended. Except when people take a quick-charger for large cells and try to bridge the distances so they can charge smaller cells. A 30-min charger for 2700mA AA cells would be quite rough on a AAA cell - but thankfully a commercial charger would normally also have temp sensors.

I have seen high-speed chargers fail (30min or 60min chargers) - not using any timer, not using temp sensor (???) and who failed to detect the dV bump. End result? A sound you would recognize from a frying pan. The poor batteries internally boiling. They normally do not explode, since the can is designed to split and let pressure out. For some strange reason, the company who sold the chargers didn't stop selling them when I noticed them to the fact the chargers where still pumping full power after a full night... Being microcontroller-operated, I have to assume that the chargers did have temp sensors, but no hw protection from a microcontroller lockup.

An important factor with NiMH is that they are not as capable of withstanding heat as NiCd is, so a charger should either reduce the current or switch to maintainance mode if the cells gets too hot. Or a fast charger should have a serious fan to cool the batteries. Charging them at high temperatures makes them age very fast, quickly loosing capacity. This is a reason why NiCd with sintered poles have been a favourite for RC people - you can get in a huge charge in 5-10 minutes and then drain that charge in a couple of minutes without destroying the cells.

Lead-acid batteries would normally also just boil if overcharged - too much current from using a large charger with a battery of low Ah rating, or continuing to charge to too high voltages.

The interesting thing with Li-Ion technology on the other hand, is that you don't need overcurrents to get a fire or explosion. It's enough to use the normal current, but continue past the safe voltage. Or to try to charge a too empty battery. That is the reason why many chargers slows down significantly in the end, so the last 10% can take as long as the first 90%. At least some Sony products quick-charges to "100%" and then support an extra 8-10% to be gained if the user continues to charge the batteries. So they get fast charge times in the manuals, while describing the last percentages as a bonus charge.

Anyone who really wants to see an explosion should play around with lithium-thionyl batteries. The smaller button cells don't have any safety valves mandatory for larger cells. We are talking about explosions giving permanent damages to hearing and serious risk from flying debris.