In addition to the above, I shoot for the3.5v-3.7v range of charge for long term storage. I believe the theory is that when they are in the nominal charge range they will just kind of sit there (electrically neutral sort of) and is the easiest on them.
I put them in a box with positive facing up and when the 1st layer is full I cover them with packing tape and a piece of cardboard to accept the second layer when that is full packing tape again and if you wish another cardboard cover or close the flaps of the box.
Believe me you cant see it by the picture but all boxes have clear packing tape over the cells
Also don't buy the cheap thin stuff
BTW the boxes need to be pretty tough as they do add up in weight
That my friend is not the best way.The problem is that the poles of the battery touch especially the positive side. you have a very thin yet tough cell wrapper between you and disaster. All that is needed is a sharp spot weld leftover to punch a hole through the positive side of the rapper whichreally is negative and short across to the positive cap.
Best to stand them up and have no contact between the cells terminals.
I put them in 3d printer filament cardboard boxes. They are just wide enough for 1 cell, and closes nicely with a double-lock mechanism. Also makes counting easy as every box fits exactly the same number of cells (125?)
And I write the date on top, so that I can test for self discharge 2+ weeks later.
Perhaps more importantly: I the put the boxes on a metal rack inside a concrete shed detached from the main living unit.
Once I harvest and test the cells I put them in cell holders.
They sit for 2-4 weeks then voltage is checked there are 26 40 cell packs in the photo.
Most of these are 2650+ mAh with many 2750 and a few 2800 , new they were 2600 mAh.
Still have 8 or9 20cell batteries and 100 modem 3 cell batteries to harvest.
+1 on storing them in cell holders. You are going to need cell holders anyway, just order ahead, and store the cells in them. It isolates the positive cap from the other cells and even if you leave some nickel strip on the top or bottom of your cell, there is enough of a gap, that there shouldn't be contact between cells even when you stack them like in floydR's picture.
My thoughts exactly Plus when I handle the cells I try to remove or tap down the remaining nickel strip. 120 + 297 cells left to harvest and test. I could place cardboard on top of the cell holder and stack them to be even safer.