Two other electrolytes are used less commonly in automotive batteries. One is the nickel metal hydride (NiMH), which packs a relatively higher energy within a unit of space. This is also referred to as high energy density. NiMH batteries can be easily recycled, since they do not contain toxic metals. Lithium-ion (Li-ion) batteries also has a high energy density. They also self discharge the least over a period of time.