https://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery
The tl;dr is slightly lower energy density (~14%) in exchange for non-toxicity, extremely low self-discharge rates, not prone to runaway thermalling, and great cycle life.
It also has the advantage of being able to be essentially a drop-in replacement for many lead-acid uses.
What are the effects of too much lithium though?
(To answer myself, from Wikipedia..) >Common side effects include increased urination, shakiness of the hands, and increased thirst.[2] Serious side effects include hypothyroidism, diabetes insipidus, and lithium toxicity.[2] Blood level monitoring is recommended to decrease the risk of potential toxicity.[2] If levels become too high, diarrhea, vomiting, poor coordination, sleepiness, and ringing in the ears may occur.[2]