While I love hating on the TSA, DHS et al, there's a shred of logic here, just obfuscated in the familiar ways. Lithium metal batteries have bulk lithium metal inside (obviously) while lithium ion cells contain active low work function lithium in a carbon matrix. While both lithium metal and lithiated carbon are fire hazards, lithium metal is far more explosive, and it just takes a little humidity to set it off. I'd rather the feds stay on the safe side of the stuff. Note that lithium ion cells are generally OK. Alkaline cells use a zinc anode, (rechargeables generally have a NiOH anode) are a-ok (though zinc in powder form, as I learned at an airport a few years ago is a no no).

Lithium metal cells (mostly relegated to coin format watch batteries and generally time keeping) have less than 2 grams of lithium, and packaged are generally 5 grams. So watches, etc, are fine.

On the border are the AA/AAA form factor lithium cells. Energizer sells the l91 AA and the l92 AAA cell. The AA cells weigh in at 15 g but have on the order of 3000 mAh each. Lithium metal has a theoretical energy density of 3900 mAh per gram, or therebout, and account for discharge losses/safety factor/ assembly losses, that number can ben anywhere from 1000 to 3000 mAh in practice. Thus, if the good engineers at Energizer squeeze the high value at, we're well under 2 grams, but if they're at the lower bound, no bunnies on a plane for us. Given the growing popularity of these cells, I wouldn't be surprised if the two gram limit takes these cells into account and are OK. Of course, on the otherhand, given the arbitrary nature of the TSA, maybe those of use who like to use these cells are SOL.

At the bottom of that energizer PDF is a small clause

Transportation: For complete details, please reference:

Global (except US): Special Provision A45 of the International Air Transport Association Dangerous Goods Regulations

United States: 49 CFR 173.185

That last little googles up at this document which shows that this 2 gram limit is actually above previously stated 1.5 g limits imposed after a lithium battery fire at LAX in 1999.

Again:

The big question mark in my mind is pacemakers, which I though had 3 or 4 grams of lithium in 'em, but I could be wrong on that.