Tesla 4680 cells and bad journalism

Tesla and Pansonic have introduced the new 4680 battery cell that future battery packs for the Model Y and the Cybertruck will be based on. These larger cells will replace the previous 2170 form factor that current Tesla packs are based on, which in turn replaced the 18650 cells that Tesla inherited from the laptop industry.

Some of the articles about the new cell have talked about the 5 times higher capacity of the cells saying it would address the problem of “range anxiety”:

5 times more energy means less range anxiety and more drive time. It means fewer stops on a road trip and a more enjoyable experience.
(Why The Tesla Tabless Battery Is So Good, torquenews.com, 2021-03-30)

Actually, this claim is embarassingly disingenuous.

Yes, the new cells have higher capacity but that’s because they’re bigger, which means a battery pack of a given capacity will be built from fewer but larger cells. The bottom line of capacity by weight or by volume is largely unchanged.

The new cells are 2.2 times the diameter of their predecessors, meaning they will have a cross section 4.8 times larger, so a given number of square meters of floor plan for a particular vehicle will fit 4.8 times fewer of these larger cells with each storing about five times as much energy as their smaller siblings. If you think this makes for 5 times more range then I have a bridge to sell to you 😉

The cells are also 80 mm long instead of 70 mm, but for energy density it’s basically a wash: The energy density per liter or per kg is unlikely to be vastly different.

Another point of confusion is Tesla’s claim that the cells will have five times the capacity but 6 times the power output. Some articles have interpreted that as 20% more range which is not the case. The truth is that the new cells can be discharged 20% faster without overheating but the total amount of energy released is unaffected by that. It’s like saying a car with 120 HP will have 20% more range than a car with 100 HP because it can drive faster. In reality it will burn fuel more quickly while doing so. This is strictly about peak power (energy by time), not total capacity.

The reason for the higher output is that the new batteries are tabless. All cylindrical Li-ion cells consist of two layers with a separator layer in between, wrapped up as a roll. Think of a double ply roll of toilet paper. When Tesla switched from 18650 to 2170, they made the roll wider (65 mm to 70 mm) but also made made the rolled-up sandwiched layers longer, giving the roll 21 mm instead of 18 mm of diameter.

This increased capacity per cell but it also meant that when energy is released in the ion exchange between the two layers in the innermost part of the cell, the current needs to flow round and round the rolled up layers until it reaches the tabs soldered to the exterior from where the power is transferred to the two opposite end of the cell.

The tabless design does away with that. In it, all the top edges of one layer touch each other and the battery pole at the top while the bottom ends of the other layer touch each other and the bottom pole. That dramatically shortens the path of the conductor through which current needs to flow. Internal resistance and waste heat are greatly reduced.

The bigger diameter means that the exterior steel skin of the cell is lighter relative to the reactive parts inside for some weight savings.

Not directly related to the bigger format, the new cells also break new ground by making do without any cobalt in their anodes which rely on nickel instead. Unlike cobalt which is primarily sourced from the Democratic Republic of Congo (a troubled country with huge corruption and human rights problems), Nickel is available from sources worldwide.

Several online articles have also repeated a claim that the new cells have a capacity of 9,000 mAh vs the approximately 5,000 mAh of the 2170 cells. This is way off the mark and must be based on bad arithmetic. To be consistent with Tesla’s claim of 5 times the capacity per cell, it would have to have about 25,000 mAh of capacity. That is also consistent with the quoted capacity of a 4680 cell quoted by a Chinese supplier of Volkswagen, which is also looking at using this format in the future.

LFP cells and the 4680 form factor

Personally, I think it would be great to also see a LFP (Lithium Iron Phosphate) version of 4680 cells. Panasonic announced that they would not be making it, but some of Tesla’s Chinese suppliers might opt for this format, which would work well for entry level models. LFP is a very safe chemistry and has a long cycle life, even if the energy density is somewhat lower.

In any case, it makes more sense for BEVs not to have the highest battery capacities possible but instead for some of the battery inventory to be used for infrastructure to decouple quick charging from available grid capacity: A certain percentage of annual battery production should be installed in chargers instead of in cars. Actually, recycled batteries from scrapped BEVs make a lot of sense for this, but so do different chemistries such as redox flow batteries including iron batteries.

If for example, most cars travel less than 150 km per day it does not really make much sense that they have a large but heavy battery that gives them 400 km of range but costs a lot of money and whose weight increases electricity use when accelerating. More weight also means more tire wear.

On the few days that cars need to travel further than their limited range, they should be able to quickly recharge from supercharger stations that use on-site battery storage to be able to recharge cars regardless of whether the grid has spare capacity at that moment or not. This is a far more efficient use of scarce resources than giving all BEVs a huge battery and makes for a more robust electricity grid.

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