Storage Recommendations for Lithium Ion Batteries:

Panasonic:
The batteries should be stored at room temperature, charged to about 30 to 50% of capacity.
We recommend that the batteries be charged about once per year to prevent overdischarge.

Moli Energy:
Storage temperature range is -20 to 60° C.
Recommended storage voltage range is 4.1 to 2.0 volts per cell.
For prolonged storage periods, store discharged [i.e. 2.0 to 3.0 volts per cell] and at -20° to 25° C.