Welcome back everyone! I’m Jordan Giesige and this is The Limiting Factor.
Images of the current collector plate from the top and bottom of the 4680 battery
cell first began appearing online about a year ago. Since then, there’s been a huge
amount of speculation about what function they serve due to their intricate design.
Today I’m going to walk you through what functions I think they serve. Which, in short,
is that besides their obvious primary function to transfer electrical current,
they also facilitate manufacturing, potentially lower resistance, and serve a thermal function.
However, based on leaks from Cleanerwatt, at least one of the current collectors may
be unnecessary and could be eliminated in future versions of the 4680 battery cell.
Before we begin, a special thanks to my Patreon supporters and YouTube members. This is the
support that gives me the freedom to avoid chasing the algorithm and sponsors. As always,
the links for support are in the description. Additionally, this entire series wouldn’t have
been possible without the battery cell provided by Gali of HyperChange and Corey Coddington who
delivered the battery cell to UC San Diego. At UC San Diego, Shirley Meng organized funding for the
teardown as the Zable Endowed Chair in Energy Technologies in Jacobs School of Engineering.
And finally, Weikang Li spent many hours working on the teardown and answering my questions. So,
a big thanks to everyone involved! To kick things off, why do I refer to the
objects we see on screen as current collectors? I spoke with someone who works at Tesla and other
people who have contacts within Tesla, and all of them referred to this as the current collector.
The name current collector tells us exactly what the primary function of these aluminum and
copper plates is, which is to collect electrical current from the electrode foils and transfer it
to an external terminal like the cell can. But, that doesn’t explain their intricate design.
To understand that, we need to understand the construction of the 4680 battery cell.
On screen is a diagram of a 4680 that focuses on the engineering around the positive terminal.
There’s quite a bit going on here, but we’ll walk through each part and why it’s relevant.
At the top of the 4680 is a coin-shaped aluminum disc which serves as the positive
terminal itself. How do we know it’s the positive terminal? Because it’s connected to the aluminum
current collector and aluminum tabless electrode foils within the battery cell. Aluminum foil
is always used for the cathode, or positive electrode, in lithium ion batteries because
it’s stable against the high voltages that occur in the active material of the cathode.
The cell can is the negative terminal, which we’ll look next at in more detail. With the
negative terminal and positive terminal in such close proximity there needs to be
some electrical insulation, otherwise the cell could short out and explode. The insulation is
provided by the external gasket that sits below the aluminum disc and the internal gasket that
sits above the current collector. You may notice that there are two
things we didn’t talk about here, which is the vertical connection from the aluminum disc to
the current collector. And, the connection from the current collector to the tabless
electrode. For the latter, current collector is welded directly to the tabless electrode.
With that piece of information now in place reason for the intricate design of the current collector,
should now be readily apparent. With the arrowhead sections of the current collector now welded
in place, we can see that the neck of the arrowhead effectively becomes a hinge point.
Although the word hinge may not be the most accurate description of what’s going on here,
it’s the best analogy that came to mind. The structural design around what I’m calling the
hinge point allows the spokes of the current collector to flex, and the point of maximum
travel would be the centre of the spokes. That point at the centre of the spokes is the
perfect spot to rivet or weld a connection because it’s both flexible and because the area behind it,
at the top of the jellyroll, is empty space. And, that’s exactly what we see. The aluminum disc is
connected to the current collector to complete the circuit. It appears there’s some type of rivet
that punches through the top of the cell can, and then the bottom of the rivet is welded to the
centre of the current collector, which completes the vertical connection between the aluminum disc
to the current collector mentioned earlier. Let’s move on to the negative terminal. This
end is far simpler, so it won’t require a diagram. First, we see the copper foil of
the negative electrode, or anode. It looks flaky because the tabless electrode uses a series of
cuts in the electrode instead of a single tab to carry current out of the jellyroll.
Copper is used this time instead of aluminum because copper is stable at the low voltages of
the typical anodes used in lithium ion batteries, whereas aluminum is not. Other chemistries like
sodium ion can actually use an aluminum current collector at the anode. We’ll discuss that more
when I do a technical deep dive of sodium ion. The copper tabless electrode is topped with
a current collector. Once again, the current collector is welded directly to the copper foils
of the tabless electrode. However, notice that the hinge point is reversed. The hinge is now in
the core of the battery cell. This again allows the spokes of the current collector to flex,
but this time point of maximum travel is at the perimeter of the cell rather than the core.
At the perimeter, there’s also a flange with a groove. That groove allows the polymer ring
fitted around the lid of the cell can to seat snugly against the current collector when it’s
crimped on to form the bottom of the battery cell. And, of course, as the lid of the cell
can is crimped on to the body of the cell, the flange of the current collector hinges
downwards. This also forces the flange to form a tight connection with the sides of the cell can
and makes the cell can the negative electrode. Interestingly, the bottom of the cell can is
electrically isolated from the sides of the battery cell. Although it wouldn’t hurt if
the bottom of the cell can was also the negative terminal, a terminal there would serve no purpose.
That’s because Tesla’s making all the electrical connections at the top of the cell. With that in
mind, my view is that the polymer ring fitted to the lid of the cell can is there to seal the can
and just happens to also electrically isolate it. Before we move on, here’s a side by side
comparison of the two current collectors. The hinge points are in green and therefore the
point of maximum travel is in pink, which aligns to where the current collector is
attached to the terminal. The aluminum disc for the positive and the cell can for the negative.
So far, I’ve said that the primary function of the current collectors is to collect current
from the electrode foils. And, that the intricate design was to facilitate
manufacturing. However, I do expect that the current collectors serve two other functions.
First, the shingled spiral of the tabless electrode means that any current flow from point X
on screen would have to traverse layers of contact resistance if it took path B. Current takes the
path of least resistance, and in my view it’s likely that the path of least resistance would
nore often be path A, because the current could flow through a solid sheet of highly conductive
copper. This path of lower resistance would reduce heat generation, but to what extent, I don’t know.
Second, at the anode or negative side of the cell, the copper contact at the perimeter
would provide excellent thermal conductivity to cool the interior of the cell. This is
because the side cooling would work its way from the cell can to the copper collector,
to the tabless electrode, and then to the electrode foils that run through the battery cell.
Moving along, what we see here is the 1st production iteration of the 4680. Would the
current collector design be overkill and is there some way to eliminate it? As I mentioned in past
videos, the design appears to be evolving. Images of a newer design first appeared at
the Giga Austin event, which may indicate the internals of the cell have also been redesigned.
This aligns with what Cleanerwatt is hearing from contacts inside Tesla. According to those
contacts, the first production iteration of the 4680 is referred to as the first generation
and the second iteration we saw at Austin is the second generation, which is still in development.
The second generation cell eliminates the current collector and it’s likely that the
bottom of the cell can is welded directly to the tabless electrode. The spiral pattern
we see at the bottom of the cell in this image is supposedly a weld connecting the
lid directly to the tabless electrode. That is, the current collector may have
proven to be redundant and was removed. The best part is no part. Cleanerwatt also noted that the
generation 2 design is likely part of Tesla’s push to combine notching, winding, and welding
into one machine. If you’d like to know more about that, check out Cleanerwatt’s video on the topic.
It’s worth noting that Cleanerwatt focused on the negative terminal of the battery cell
with the copper current collector. There was no mention of whether the aluminum current collector
was also removed at the positive terminal of the cell. My view for now is that the
top of the cell is complex for a reason. The reason being that, for pack assembly,
the positive and negative terminals are adjacent, which means there needs to be
a method of electrical isolation between them, which results in all the engineering
that went into the positive terminal. The negative terminal is a different
story because there’s no positive terminal to navigate and therefore a lower risk of
the cell shorting. Therefore, connecting the copper current collector directly to
the lid through a weld faces fewer challenges. How would that work? Here’s my assumption. As
we covered a moment ago, the reason why the intricate current collector
was needed on the negative terminal of the cell was to help deal with the compression
stress of crimping the lid to seal the cell. With the second generation of the cell, as we
see here, it doesn’t look like the lid was crimped on, for two reasons. First, there’s no gasket like
we saw with the first generation cell. Second, there’s no crimp line around the rim of the cell.
Here it looks more like the lid was pressed against tabless electrode,
followed up by welding at the rim of the lid and the spiral at the centre. I say this because the
metal appears to be blackened and charred at the rim and the centre spiral. Welding the rim rather
than crimping would reduce mechanical stress during assembly and negate the need for a flexible
current collector to deal with crimping stresses. But what about electrical current flows? Although
steel doesn’t have the electrical conductivity of copper, the distance travelled from the weld to
the side of the cell can is relatively small. Much smaller in fact than the distance the current has
to travel from bottom of the cell can to the top, which is where the electrical connections are.
As for thermal conductivity, if there’s no electrically isolated gasket at the bottom
of the cell, it would have a direct thermal connection to the side of the cell. That is, the
thermal pathway would simply be the bottom of the cell can and wouldn’t use the copper
current collector as an intermediary. Copper does have higher thermal conductivity,
but as we saw in the last video, a cell with a steel bottom provides plenty of cooling capacity.
That is, the bottom of the second generation cell appears to be easier to manufacture while serving
all the electrical and thermal requirements of the copper current collector. As a side note,
the plug in the centre would likely be for filling the electrolyte and might
also serve a pressure relief function in the event of thermal runaway.
In summary, the intricate design of the current collectors doesn’t appear to be
for accommodating the expansion of a Silicon anode, dampening vibration,
or protecting the tabless electrode as myself and others were speculating. Rather, the most likely
reason for the intricate design is to facilitate cell assembly.
The other functions of the current collector such as collecting and transferring current,
reducing resistance, and providing a thermal pathway for could have all been done with
a design that was less intricate. And, at least on the negative terminal of the cell,
it looks like Tesla’s pursuing that opportunity. They teased a newer version of the 4680 at Austin
which Cleanerwatt has confirmed uses a simplified design with fewer parts.
The lower part count will make generation 2 of the 4680 cheaper to manufacture and
may help facilitate rolling several of the manufacturing steps into one.
Furthermore, the current collector and the polymer gasket are inactive material in the battery cell
that don’t store energy. By eliminating them, it makes room for more active energy storing
material, which may help improve energy density. Troy Teslike has leaked information suggesting
that Tesla has 3 generations of the 4680 already on the drawing board. We’ll have to wait and see
how accurate the energy density figures are. I hope to provide an accurate energy density figure
soon for the first generation of the cell, which will provide us with a good benchmark.
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