Ohorn,
I do not use a lithium battery for cranking yet… but plan to when it needs replacement.. I have Ionic Lithiums for my trolling motor, and the offer cranking batteries as well.. good customer service… I would call them and see what they have to say… I know a lot of guys have them for cranking now…contact them at Lithium Hub.com…
Mercury has released a service bulletin for their 4 stroke motors stating that certain lithium batteries are acceptable. Check with Mercury to find out which ones. I know that Ionic has at least one on their list.