I am not an expert, but here my thought after after reading some articles, Li ion more compact battery so u could fill up more energy with the same size rather than conventional. While the advantage over aip, battery contain less mechanism, than aip. Less maintenance, and less sound maker
U218 sg (u214 xl) doesnt have li ion battery tech, but has aip, while u216 has li ion, u218 has multipurpose hatch in the front sect. Near torpedo sect. While 216 has multipurpose vertical sect behind the tower.
Good overview about u216 ...