Any battery, to my understanding, can suffer thermal runaway, thus causing, at best, the batts to vent. At worst...well, let's just say stick to the protected batts as they can (and have been reported to in the past) to explode....violently.
Now, keep in mind, that's a worst case scenario, and myself personally, I've never had a batt (lith-ion or otherwise) so much as vent on me, let alone explode. Just make sure you're ALWAYS taking adequate precautions, using your meter (get one if you don't have one), and using just plain ol' common sense. Alot of mishaps and malfunctions, IMO, could have been avoided with a bit of the latter. But as they say, sometimes stuff just happens. So one can never really be too careful.
Do your research, learn how to properly handle/charge/use Lith-ion batts, and always use PROTECTED batts in a PV (unprotected simply are not worth the risk, no matter how cheaply you procured them).
I highly suggest reading the following in its entirety, there's alot of useful/necessary info here:
Battery University