As I seem to remember, way back when I considered the technology, they were conventional zinc–carbon units with the electrolyte held in a capsule that was fractured when you activated the battery. The main strength of this design is longevity. The main weakness is that the batteries are an obsolete, and fairly weak, chemistry that was further compromised by being made slightly smaller to keep allow for the separate capsule of electrolyte. At their best they had less total electrical capacity than the regular carbon-lead D-cells.

Zinc–carbon cells were largely replaced in the early 70s by 'heavy-duty' zinc–chloride cells which had a greater power store. These were, in turn, made obsolete by alkaline cells in the late 70s. The zinc–chloride cells were marginally better than the zinc-carbon cells whereas the alkaline cells have roughly three times the capacity of the zinc-carbon units.

For a twenty year storage capacity I'm thinking something along the lines of storing uncharged rechargeable cells and some way of charging them. A solar panel and/or human powered generator would provide a long shelf life and good durability in use.

I note that lead acid batteries, large units used in vehicles and boats, are stored dry, without the acid. The sulfuric acid is stored separately and poured in when the battery is initially put into service. The battery, once the acid is added, is charged and ready for use.