A simple question, but I did the answer while googling. I would like to say "The proposed method aims at delineating cells populations".
I think it should be singular in this case, as in cell populations. As a native speaker, I'd avoid an "s" here unless it was possessive (and therefore also had an associated apostrophe). I don't think your usage is possessive.
For proof, see this ngrams graph showing that "cell populations" is much more common than "cell populations".
For explanation, it's a noun modifier, and these are apparently traditionally singular (although there is some change recently and they can be plural, sometimes).