Don't quote me on this, but lithium batteries need a higher voltage to charge, thus the need for a DC-DC charger.
There is an isolator for lithium batteries however, I'd looked in to them a few months back when I was piecing my system together but don't recollect what I found.
I ended up going with a charger that can charge either from my engine or solar.
Here's the isolator:
https://www.amazon.com/Battle-Born-Batteries-Lithium-Isolation/dp/B07DY8S815
I want to say there's a non Battle Born version that's a little cheaper.
Here's what I ended up buying:
https://www.amazon.com/gp/product/B07XPG97H7/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
I have yet to use it, I'm dragging butt on my build.