From FB TSLA Shareholders group discussion about why the big 4680 cells need their design to be tabless for fast charging.
Jeffrey Levine
From what I’ve found, the reason why the 18650 cells are capable of handling the highest current is that the ‘jellyroll’ is shorter than 2170 or larger cells. A single tab handles all current so, for example, if a cell’s tab can handle 1 amp current, the small area of the electrodes get more current per unit area. In the case of 2170, the larger area of the electrodes means that the tab’s 1 amp has to be spread over the larger area, thus it takes longer to charge. To counter this the tab’s current is increased. But the tab’s resistance has not decreased; it’s the same. And in addition the jellyroll is longer and has a higher resistance. So the increased current also increases the power dissipated and the heat in the cell.
So we go from the 2170 to the 4680 *with* a tab. The jellyroll is even longer and the big cell takes even longer to charge at the higher tab current. So generally, the bigger the cell’s diameter, the more current is needed to charge the cell quickly and the more heat will be generated in the cell, both in the tab and in the much longer jellyroll.
This cell scaling limit problem became apparent to cell and battery pack designers and was what drove the cell designers to go to a tabless design. Apparently the tabless design at first had a problem with low yield — only 20% of the 4680 cells produced passed the full tests. That meant 80% were rejects and that was unacceptable for a production line. I haven’t found any information on if the high failure rate was being caused in part by the tabless design. So they have been working for more than a year to lower the number of rejected cells. Now it is down to 20 to 30%.