[KaeseEs]

> I mean when you get a rate that high, it seems like it must be just a relatively heavy element, and a metal, like Gold, and relatively unreactive (just carrying ions and such).

This is the opposite of the truth. Lithium is not dense at all (~.53g/cm^3), especially for a metal, and as a Group 1 metal is extroardinarily reactive - if you cut lithium it will oxidize as you watch, and if you expose it to water is will explode into flaming chunks.

> I imagine they don't get to 100% because they just don't bother to heat it high enough to melt things in there with a ridiculously high melting point and risk creative copper fumes and whatnot (from some metals evaporating). i.e. it's still easy, they just don't want to.

No. Lithium has a very low melting point (roughly 180C/355F).

Furthermore, your contention that a process with an efficiency of 93% should be easy to bring to 100% is so off base I don't even know where to begin. Squeezing out the last few percentage points is the hardest part!