Lithium batteries are more prone to self-immolation/thermal runaway; heavy fuel oils aren't prone to spontaneous ignition at typical temperatures & pressures.

I would think that preventative measures would be much easier to realize on big ships with lower density demands

Furthermore, oil fires can be extinguished by water and/or CO2 - Lithium battery fires can't be smothered and require exponentially more water to bring under control (producing copious amounts of heavily contaminated water too). And once put out can easily re-ignite.

Use lithium ferrophosphate instead- no fire