Early lithium batteries contained metallic lithium, which is highly flammable, however the lithium ion batteries in use today do not. You may remember when some laptop computers caught fire due to their lithium batteries, but this is no longer a problem. So, I wouldn't worry about the lawn mower battery catching fire. However, they must not be overcharged or they can become extremely hot, which could result in a fire, which is why they all use smart chargers that will sense battery heat and stop charging if they get too hot.