49 CFR 173.185
(iv) For transportation by highway or rail only, the lithium content of the cell and battery may be increased to 5 g for a lithium metal cell or 25 g for a lithium metal battery and 60 Wh for a lithium ion cell or 300 Wh for a lithium ion battery, provided the outer package is marked: "LITHIUM BATTERIES—FORBIDDEN FOR TRANSPORT ABOARD ...