In one of my past lives, my work involved EOD (bomb disposal) robots. Came time to modernize and upgrade the power densities, the lead-acid batteries were replaced with Nickle Metal Hydride; Lithium Ion batteries had long been on the market and offered even greater power density, but there was hesitancy to make a potential bomb from a bomb-disposal robot.
I do not yet own an electric bike, but am happy for the Li-Ion cells, their high power density, their range. But given the size of those batteries, I would never charge them on the bike, I would obtain a STEEL (not aluminum) charging box that could totally contain any fire, explosion, and excessive heat, and always charge the battery in the box, with the box sitting on something fireproof, like bricks or a concrete floor. A bit more hassle, but a lot more safe. It would be nice if the battery were internal to the bike if the bike itself could contain a fire, but a frame from aluminum or carbon-reinforced-plastic, no. This is why Boeing had to replace the box holding lithium emergency batteries on the 787, from aluminum, to steel or titanium (I forget which), because aluminum did not adequately contain two separate fires.
My phone and keyboard use lithium batteries. Despite their much smaller size, I never leave anything on a charger unattended.