All batteries can be dangerous. It's not hard to see why...
Take a 5v, 2000mAh cell (a very, very, ordinary, small battery like you'd have in your mobile). Power = Volts x Amps, but we have mA... no problem... 2000mA = 2A. So that's 5v x 2 A = 10W. But the cell is 2000mA HOURS, so this simple battery can (theoretically) provide 10Wh of power... or ten watts (enough to light an electric lightbulb) for up to an hour.
A short circuit of any kind on this battery will provide all of that power direct from the chemical reaction in under a second. Under a second! That's one 3600th of the time. Now it doesn't scale exactly (you don't get 3600 x 10 watts out of it) but you are putting a hell of a lot of energy (generated by a chemical process with some not-very-nice chemicals) into a tiny space very, very quickly. The chemistry of the battery (NiCd, lead-acid, NiMH, Li-Ion, Li-Polymer) doesn't matter at all in those circumstances... it's simple physics. Rechargeables tend to be worse because they are normally much higher power and have a much lower cell resistance (so a short circuit is *really* a short circuit). To counter this, charging circuits as well as the batteries themselves tend to have control circuitry to detect short-circuit/overcharging and cut off the power as soon as possible.
Even the old NiCd's used to do this. An ordinary AA NiCd from when they first came out could easily heat up to melting point in a matter of seconds. I know... I did it once when playing with a toy science kit when I was about 9. I melted the end of the battery by creating an inadvertent short-circuit and couldn't pick up the battery for about five minutes it was so hot. And that was a primitive, well-used, low capacity cheap NiCd.
I've had ordinary AA rechargeable batteries explode so badly (inside a school I worked for) that they had to replace the entire carpet and scrub the walls. Luckily, nobody was in the room at the time. Why do you think that every battery ever made has "Do not short-circuit, do not throw in fire" written on it?
My dad worked as a lorry engineer. They used to break all sorts of Health & Safety rules (well, they didn't, because they didn't exist back then) during their lunch hours. They once deliberately (from an enormously long way away) dropped a spanner across the terminals of a lorry battery (24v, standard lead-acid). The spanner GLOWED RED, then GLOWED WHITE (this is inch-thick steel we're talking about), then shattered and exploded across the floor. The battery also exploded, it blew a fork truck backwards and sprayed acid onto every wall of a large warehouse (think about the size of warehouse you need to store fifty lorries and you'll understand the power). He tells me that car batteries are just the same - most garages keep a large wooden baton handy in case someone is working on a battery and accidentally touches terminals with their hands - because it's DC, you can't let go of the power and you grip tighter - the baton is for your colleagues to beat your hands off it to save your life. A simple battery that everyone has in their cars (and, arguably, the most dangerous and powerful type of battery you'll come across in your average day) can turn into an explosive device, or provide enough power to your body that you can't let go and just die with clenched teeth/muscles.
Lead acids are inherently dangerous because they release hydrogen when charging (which is explosive - never smoke around a charging car battery or when jump-starting), they are filled with... well, acid. Proper undiluted sulphuric acid, not some vaguely acidic substance. They usually contain on the order of 12V, 400Ah, so that's about 4800Wh - 4.8KW for an hour! Now release that in under a few seconds with NO safety measures at all (because, say, you got your jump leads the wrong way round).
Batteries are dangerous, if mishandled, it's as simple as that. The chemistry of a particular battery is really fine detail - just looking at the numbers should be enough to scare you.
Oh, BTW: when investigating the "do mobile phones set petrol stations on fire" myth, it's been proved conclusively by just about every country's safety inspection team that the mobile phone signals provide zero hazard. However, a poorly fitted or ill-produced battery can easily create enough fuss that a petrol pump can catch on fire, particular if the phone is dropped in the forecourt. In all the recorded accidents in the UK/US where a petrol station has caught fire, a large proportion are actually due to static sparks from re-entering a car and fake/faulty mobile phone batteries.
