The recommended way of dealing with lithium battery fires is cooling with water in or on a non-flammable surface, not least because the typical failure mode is that cells are made into a battery pack and then one fails (due to damage, poor construction, etc) and overheats, heating the others which then also fail, etc. Cooling the pack slows the release of energy from the failed cell and prevents the others from also overheating and failing, minimising energy release.
Using a Halon(-type) extinguisher will cool the pack exterior briefly, but not remove much energy overall, nor slow the chemical reactions in the pack for long. So it would seem appropriate if the battery pack has set something else on fire, but not for a pack on its own where plenty of water will be more effective.