Good point. The Dreamliner has such problems in its Li-ion batteries to the point that the weight savings due to composite construction was subsequently nullified by metal retrofits to contain battery fires (https://en.wikipedia.org/wiki/Boeing_787_Dreamliner_battery_...). Additionally, IATA has put strict limits on Li-ion cargo due to Lithium being too reactive, so any plane with Lithium batteries must consider that.
That could be more of a problem. Regardless of the height, dumping a battery means a projectile falling, which could injure someone or cause property damage.
If fuel is dumped high enough, it pretty much evaporates and doesn't fall to the ground. If the plane is low when it dumps fuel, it can hit the ground and bystanders; but, it's not going do much, if any damage. (Assuming it doesn't hit someones barbecue or a smoker.)
Batteries could be used as part of the structure itself (similar to the transition we made from separate fuel tanks to simply putting fuel in the wings). That saves a lot of weight.