The point is, we cannot get water to put a fire on the lower cargo hold, so there is not much use.
Lithium Ion batteries seem to be not as harmful as the Lithium Metal ones. It is my understading that the Li-Ion (recharchable) are transported without charge and Halon might have some effect fighting it. Lithium metal (CAO) are more dangerous (some details on the Boeing Aero magazine
http://www.boeing.com/commercial/aer...ERO_2013q4.pdf) and it seems that halon has no effect on it.
I suggest watching this video
Extinguishing In-Flight Laptop Computer Fires - Lithium Battery Thermal Runway - YouTube, specially at 6:13 to see what the halon extinguisher does. And that is the "least harmful" one...
Unfortunately those batteries are like flying bombs and on the flight deck of freighters, pilots are the "weakest link".