"The idea here is simple: smart watches are round-ish. Batteries today are usually rectangular. So by building a hexagonal battery (not a symmetrical hexagon, sadly) it becomes possible to pack extra power into a wearable."
I'm wearing a watch, it's not "smart". Unfortunately I have no idea what shape the battery (cell) is because I've never seen it. The watch is over 10 years old and I've never had to do more than be exposed to sunlight occasionally to charge it. It tells the time pretty well.
Cells in watches today are usually circular in one elevation and being three dimensional might be described as "cylindrical". The vanishingly small market of "smart" watches compared to "working" watches might like to dally with odd shaped cells (in 2D if that is their thing) but that is their business.