My 18650s are a different type as yours, right?
Safety
Main article: Lithium-ion battery § Safety
Apple iPhone 3GS's Lithium-ion battery, which has expanded due to a short circuit failure.
LiPo cells are affected by the same problems as other lithium-ion cells. This means that overcharge, over-discharge, over-temperature, short circuit, crush and nail penetration may all result in a catastrophic failure, including the pouch rupturing, the electrolyte leaking, and fire.[18]
All Li-ion cells expand at high levels of state of charge (SOC) or over-charge, due to slight vaporisation of the electrolyte. This may result in delamination, and thus bad contact of the internal layers of the cell, which in turn brings diminished reliability and overall cycle life of the cell.[7] This is very noticeable for LiPos, which can visibly inflate due to lack of a hard case to contain their expansion.
For a comparison with LFP cells on this subject, please see LiFe cell safety
Weren't those banned on planes at one time? A little research.
Q2. What kinds of batteries does the FAA allow in checked baggage (including gate-checked bags)?
A2 . Except for spare (uninstalled) lithium metal and lithium-ion batteries, all the batteries allowed in carry-on baggage are also
allowed in checked baggage. The batteries must be protected from damage and short circuit or installed in a device. Battery-powered
devices—particularly those with moving parts or those that could heat up—must be protected from accidental activation. Spare
lithium metal and lithium ion/polymer batteries are prohibited in checked baggage—this includes external battery packs. Electronic
cigarettes and vaporizers are also prohibited in checked baggage. “Checked baggage” includes bags checked at the gate or planeside.