On the battery front, I know Toyota are researching into solid state and lithium air. I read up a little and it seems they were using the SiC semiconductor on the front but not the rear last year.
I don't think Buemi would rather be in F-E over WEC or vice versa. I think he wants to do both and he's upset he cant. Everything from his comments shows this. He doesn't seem mad at Toyota, and if you look on his twitter feed, he mentions Toyota and his drivers title for them then his title in F-E. That might be nothing, but I feel the WEC might be a bit more prestigious