Richard, the rate of chemical reactions slows with temperature by a factor of two for every 10 degrees C of temperature drop. If the cell loses 50% in say a month, by storing it at 15 C (room temp is around 25 C) should significantly slow the self-discharge. I wouldn't freeze them unless there was a good study that says nothing bad is going to happen. The effects of a phase change might not be easy to predict, and might not be reversible. Storage cells have complex chemistry.