Batteries contain their own oxidiser and fuel, so the only way of extinguishing a better fire is to remove the heat (fire triangle: fuel, oxidiser, heat).

The best lay-person's method for extinguishing battery fires is flooding with cold water, which will be hard on an airplane at the best of times.

The article goes into more details on that. Basically the fire extinguishers are not powerful enough against lithium-ion battery fires.