Oh, I fudging LOVE big ol' D-cells.
The Li-ion ones have a circuit that limits the output. They do create interference, though, so you can't use them with radios. Which might suck in an emergency if you're salvaging D-cells from a flashlight (which doesn't care about the interference) to the radio.
For emergencies, D-cells are GREAT, so I LOVE NiMH for their versatility. They hold their charge for quite a long time, too.
I did consider using rechargeable D-cells in my Mag-Lite 3D lights, previously used as emergency lights. I discarded the thought as I read alkalines are around 15000 mAh, whereas the rechargeables (at least the ones I would consider reliable brands) are at best only 3000 mAh (1,5 1,2 V NiMH-version).
My reasoning was I would be replacing the cells anyway (I don't know, every 5 years) way before a cell as big as a D-cell would leak.
EDIT:
Sorry, typo on the 1,5 V - I meant 1,2 V.
Powerex/Maha makes true 10,000 mAh NiMH cells, but they are pricey (~$30 for two cells).
The lower mAh ones are actually like AA cells in a giant spacer. They are a LOT lighter, but that's why the capacity isn't very good.
At that point, the Eneloop spacer sleeve is about as good. The Powerex 10 amp hour cells are HEAVY BOYS, like an alkaline D-cell.
FWIW,
@Lumencraft (Matt) uses Amazon Basics NiMH D-cells. They're not as good as the Powerex ones, but they tend to be in the 8-9,000 mAh range from what I've seen. And they are cheap (a set of 4 is ~$25). I think it's safe enough to say, if they're good enough for Lumencraft, they're good enough for us, haha.
The harder part these days is finding a charger for D-cells that charges more than two at a time, haha.
I have a pair of Powerex cells in my 2D Maglite, to run Malkoff M31 modules.
I am going to eventually get some of the Scamazon cells to run in a 4D setup, to run M61 modules. I can take the 15% less capacity for 50% less of a price.