Safe temperature range for 18650 battery - powerful flashlight
Maker and model specifications for your particular battery will state the range and the cutoff temperature for the thermal fuse which is mandatory in all 18650s sold in the US, that is probably the ultimate limit and I think it's 87-95'C. Today some batteries like Panasonic have very good PTC, you can short them from 4.4 volts and they are messed up but in one piece. 

Battery shops generally specify a discharge temperature of -10 to 60'C and a charge of 0-45'C for 18650's, although charging under 10 degrees does not help lifespan, and will kill a bad battery.

different batteries get too hot at different output currents. Some batteries have temperature protection which switches off the battery above a certain temperature.

Some crazy people test their batteries to see the output performance, and dangerous temperatures:

from https://www.e-cigarette-forum.com/forum/threads/panasonic-ncr18650pf-10a-2680mah-18650-bench-test-results-a-great-battery-beats-mh1-and-mj1.692438/

http://www.diytrade.com/china/pd/12734942/Panasonic_original_18650_NCR18650PF_2900mAh_10A_3_7V_batteries_with_Flat_top.html