The 787 battery is also multi-cell (there are 8 cells per battery).
On the original design, a cell failure could propagate to other cells (known as "thermal runaway") and cause very high temperatures / risk of fire.
On the redesigned batteries, the cells are now spaced and lined with ceramic insulation to prevent thermal runaway. The battery charging system was also modified. And then the firebox was added as an additional layer of defense.
We still don't know why the cells failed. It's hard to redesign something when you don't know what may be wrong with the previous design. And for all we know the root cause might have been manufacturing defects, not design defects.