If necessary, the sub-cells could be reduced in size and more of them, in order to reduce the potential thermal danger of any single cell going into "meltdown"
and the isolation in a ceramic "jacket" together with fusible connectors would all help to ensure a safe and reliable storage-system
In any event, it is imperative that ALL cells are monitored and balanced.
Thats precisely how the Tesla is designed. It contains 6800 of these (18650 cell) -
List of battery sizes - Wikipedia, the free encyclopedia
isolated from each other.