yes D cells for li-ion exist, SAFT makes them for one, and there is a china brand running around. because of the possibility for danger, and the SIZE, they are not easily available.
there is a Protected C now, that AW sells that sufficient juice, its longer than a real C, because of the protection added. people are actually applying this in things.
li-ion of course is a 3.6v type of cell not compatable with your usual consumer stuff.
*li-ion is not to be toyed with, it has specs for operating like anything, a person should know the specs and the caveats of using it before buying anything.