They used Lithium batteries because they are light, small and powerful.
Can Lithium batteries fail? Yes. But keep in mind that flying along with you and the batteries is many tons of flammable fuel, wrapped in aluminum and carbon fiber, which are also flammable, with miles of high-powered electrical lines running through it all. And we know from TWA800 that those are enough, by themselves, to bring down an airliner.
But we also know that that happens very rarely, because engineers can mitigate dangers if they are aware of them.
Can Lithium batteries fail? Yes. But keep in mind that flying along with you and the batteries is many tons of flammable fuel, wrapped in aluminum and carbon fiber, which are also flammable, with miles of high-powered electrical lines running through it all. And we know from TWA800 that those are enough, by themselves, to bring down an airliner.
But we also know that that happens very rarely, because engineers can mitigate dangers if they are aware of them.