Most Li-ion batteries also contain Cobalt, which is a somewhat toxic heavy metal. Layers of solid cobalt oxide used in the batteries could dissolve and leach out into the soil and ground water if large amounts of Li-ion batteries are disposed of in landfills.

I do not have a source for this, but given the relative abundance of lithium and ease of purifying salt deposits, it will be a long time before recycling lithium from spent Li-ion batteries would be cost effective. Lead isn't rare at all, but large lead acid batteries are easy to recycle. The lead mostly stays on the battery plates and isn't mixed in with other metals. Besides, there are laws that make disposing of lead more expensive than recycling it.