Do Not Use Lithium Batteries in Speedlights
Canon has issued an Advisory
with respect to using Lithium batteries in their Speedlites. The issue arises with constant repeated use and the batteries themselves getting very hot.
This issues is not just related to Canon equipment, Nikon and other manufacturers have similar concerns.
The Canon Advisory
AA Lithium batteries cannot be used with any Speedlite or Macrolite products and compact battery packs including those listed below because certain AA lithium batteries, in rare cases, may become extremely hot during usage in such products.
Speedlite 270EX II
Speedlite 430EX II
Speedlite 580EX II
Macro Ring Lite MR-14EX
Macro Ring Lite MR-14EX II
Macro Twin Lite MT-24EX
Compact Battery Pack CP-E2
Compact Battery Pack CP-E3
Compact Battery Pack CP-E4
You can find specific Canon EOS advisories here
I have not used Lithium batteries myself but I have spoken with other photographers that have, and they do, indeed, become very hot when repeatedly used in quick succession without any rest, as in a wedding reception situation.