Welcome back everyone!
I’m Jordan Giesige and this is The Limiting Factor.
This is part six of the Lithium Mine to Battery Line Series to break down and understand what
was unveiled at Tesla Battery Day.
The best kept secret of Battery Day was that Tesla Silicon opens the door to a faster charge
rate.
Today we’re going to look at what limits battery charge rates, how Tesla Silicon changes
the equation, and what charge rate we might expect from Tesla vehicles in the future.
Before we begin, a special thanks to Bradford Ferguson of Halter Ferguson Financial, the
Patreon supporters listed in the credits as well as my YouTube members.
This is the support that keeps me grinding away on niche topics.
For this video, I’d also like to thank Battery Bulletin for fact checking me and working
me through the technical details of how voltage works in a battery, which is covered later
in the video.
Back to Fast Charge.
In the Q3 earnings calls, Drew Baglino said that the fundamental limiting factor of a
high charge rate is lithium plating which can be addressed by the correct electrode
design and material choice.
Tesla Silicon is the correct material choice, and it opens the door to the correct electrode
design.
In the Cell Design video, I said that the limiting factor for charge rate varies with
context, but heat is what Tesla focused on in the Battery Day presentation, so that’s
what I focused on in that video.
Heat is important to the charge rate of a battery cell because heat causes batteries
to degrade more quickly.
Without the tabless electrode, Tesla’s large diameter 4680 cell would have quickly heated
during charging, which would have had a brutal effect on battery cycle life.
In my Big F’n Battery Cell video, and other past videos, I suggested that Maxwell Dry
Battery Electrode, or DBE, would allow for faster charge and discharge rates by allowing
ions greater access to the active energy storing materials of the battery cell.
DBE was my first best hope for a faster charge rate.
That is, until Tesla confirmed on Battery Day that they’ll be using higher loadings
of Silicon, which should mean faster charge rates.
However, even after Tesla made the announcements, I didn’t want to Osbourne Tesla by promoting
the idea of much faster charge rates enabled by Tesla Silicon.
Then, a friend of the channel, Zachary Pernikliyski sent me a link to this video by the Stanford
Energy YouTube channel, which is a panel discussion that included Celina Mikolajczak, Panasonic’s
Vice President of Battery Technology and Engineering for Panasonic Energy North America.
Have a listen.
Many people are already aware that Giga Nevada is in the process of converting their lines
to a cell that has 5% greater energy density and Panasonic is working towards 20% greater
energy density.
What most people don’t know is that 5-20% energy density improvement is most likely
being provided in large part by a higher Silicon loading.
Is Giga Nevada using Tesla Silicon or another form of Silicon?
We’ll discuss this more in an upcoming video on Giga Nevada.
This gradual increase in Silicon in next 5 years will result in a gradual increase in
fast charging capability across Tesla’s entire vehicle line up.
This squashed any concerns I had about the Osbourne effect.
This is already going into vehicles today, and much the same as the driving range of
Tesla’s vehicles is gradually increasing, so too will the charge rate increase over
time.
As I suggested in the last video, Tesla may also choose to implement Tesla Silicon into
the 4680 cell gradually.
Panasonics gradual ramp in the silicon loading of their anodes lends some weight to this
suggestion.
Silicon has a lot of potential, but it’s a beast that Tesla and Panasonic may tame
slowly.
This is because testing for safety and cycle life can take time, not to mention the time
it takes to ramp production of a new battery material like Tesla Silicon.
The focus of this video, of course, is on what can be achieved with the Silicon Tesla
announced at Battery Day.
In the last video, I suggested that the 20% range increase from Tesla Silicon would require
a Silicon loading of 20-30% Silicon, with the rest being graphite.
Let’s assume a 30% Silicon loading and work from there.
What can we expect in terms of charge rate?
This is a slide from a presentation by Jim Cushing of Applied Materials.
Applied materials is developing a battery with a 30% Silicon anode which provides a
4C charge rate.
A 4C charge rate is 0-100% charge in 15 minutes.
As all of us know who have anything battery powered, the fastest charge rate occurs when
the battery is low.
A battery capable of a 4C rate could do a 5-minute top up from 0-50% or 30-80% of charge
IF there was a supercharger out there powerful enough to supply that much energy that quickly.
In other words, the supercharger would become the bottleneck rather than the battery.
For example, a 50% charge in 5 minutes on a 100kwh battery pack like the Model S would
require a 600-kilowatt charger.
For reference, the v3 Supercharger has an output of 250 kilowatts.
It doesn’t sound like Tesla has plans for a charger like this in the near future.
In the Q3 earnings call, Jerome Guillen stated that “the 350-kilowatt or so that we might
be looking for cars is not going to be enough for Semi.”
Based on that comment, the 5-minute 50% top up is still many years away, even if the batteries
are capable.
If Tesla is targeting 350 kilowatts with the v4 Supercharger and it’s still in development,
we’d need to wait for a v5 or v6 Supercharger to hit 600 kilowatts - which probably won’t
happen until later in the decade.
This gets me to thinking.
If the Cybertruck has for example, a 200-kilowatt hour battery pack, it would take 1.2 megawatts
to achieve a 50% charge in 5 minutes.
It’s probably unlikely that the Cybertruck would come with a megacharger port, but it
would be pretty cool party trick to top up at a Semi Charger in a few minutes.
I’m sure many of you are asking yourselves the question: Wouldn’t batteries with a
faster charge rate capability be able to take advantage of the maximum v3 charge rate for
longer instead of tapering?
Indeed, they would.
There would still be a charge rate taper, but the taper would be reduced.
The 0-50% charge rate would only be a few minutes quicker, but the 0-80% charge rate
might drop from 27 minutes to something like 17 minutes.
We’ll discuss the charge rate taper a bit more later in the video.
There is a chance that Tesla doesn’t achieve these charge rates with Tesla Silicon.
We don’t know the exact implementation Tesla is using.
For example, a 4C charge rate might be idealistic for Tesla.
The path that Tesla takes for their high loaded Silicon anode might require a different electrolyte
solution that slows ion diffusion in the electrolyte solution.
Or the polymer coating on Tesla Silicon might reduce ionic conductivity with the surface
of the Silicon particles.
I haven’t researched this that deeply yet.
Neither of these might be the case, or both might be the case.
We simply don’t know enough about Tesla’s implementation.
With that said, reading the tea leaves gives us a strong sense that the cells both Tesla
and Panasonic currently produce or will produce have faster charge rates and those charge
rates will increase over time.
This backed up by the comments from Drew, Celina, Jerome, and generally well supported
in the research I could find.
How will Tesla Silicon result in a faster charger rate?
The answer gets deep into electrochemistry.
I’ll be providing an executive summary here.
For those very technically minded and curious, this will leave some open questions.
However, the full explanation would take a few hours.
In the future, I’ll make some basic educational videos on electrochemistry.
Let’s start by looking at a graphite particle at an atomic level.
Graphite is made of graphene sheets.
Lithium ions enter the sheets through the edges of the sheets and are stored between
those sheets in a process called intercalation.
Since lithium ions can only enter the sheets through the edges, it limits the speed of
intercalation.
Let’s zoom out from the atomic level to the particle level.
On screen is simplified visual of what occurs during a battery charge cycle in a normal
graphite anode.
The Cathode releases a thick cloud of ions that diffuses and migrates towards the anode.
In this case, the battery is still operating normally.
The lithium ions have enough time to find their way into the graphite.
As a side note, the cloud of lithium ions is technically known as a concentration gradient.
The image on screen shows the voltage of several materials in comparison to lithium metal.
The only thing you need to know from this image is that the voltage of graphite is about
.1 to .2 volts greater than lithium metal and Silicon is .3-.6 volts greater than lithium
metal.
In other words, on the anode side, lithium metal is set as the benchmark at zero.
This is because at zero, so much voltage is applied by the charger that the lithium ions
starting bonding directly with electrons to form lithium metal.
However, storing lithium as lithium metal is dangerous because it tends to form dendrites,
which are a topic for another video.
The next best thing, voltage-wise, is graphite.
Graphite stores lithium neatly and more safely in its structure through intercalation.
Graphite’s voltage potential of .1 to .2 volts is great for storing energy.
However, it’s not great when a battery is charged hard.
Charging involves pushing just enough voltage over a battery’s natural internal voltage
that it disrupts the equilibrium between the cathode and anode.
This is called overpotential.
Overpotential causes electrons and ions to break free on the cathode side and transfer
to the anode side.
The stronger the overpotential, the quicker the battery charges.
Graphite gives a slim margin of error for overpotential because it’s voltage potential
is just a fraction above pure lithium metal.
This means the battery must be charged delicately.
Just enough voltage for the lithium to intercalate into the graphite, but not so much voltage
that the voltage on the anode side drops to zero volts.
This is, of course, because lithium metal forms at zero.
When a battery is charged hard, the cathode releases more lithium ions more quickly.
On the anode side, the graphite soon hits the limits of its ability to absorb ions and
the lithium ions begin to form solid metal lithium over the top of the graphite particles
in a process called plating.
The lithium plates on the anode for two reasons: First, because graphite absorbs lithium ions
at just a fraction above the voltage required to form lithium metal.
As the battery is charged harder, the voltage on the anode side approaches zero, where lithium
metal forms instead of intercalating into the graphite.
Second, the lithium plates because there is a thick cloud of lithium ions readily available
to form pure lithium metal.
It’s a perfect storm of voltage and lithium ions.
Silicon is the correct material to solve the plating problem, and does so in four ways:
First, the Silicon particles are smaller than the surrounding graphite material.
Smaller particles have a higher surface area to volume ratio than larger particles.
This allows the lithium ions greater access to the active energy storing volume of the
particles through the greater surface area.
Second, Silicon stores lithium ions by forming a chemical bond with them, instead of intercalation.
The bonding of silicon to lithium is called alloying and can happen anywhere on the surface
of the Silicon particle.
This means that with the alloying process, the lithium ions have 3D access to the active
material instead of 2D access to the active material like we see with graphite.
Third, Silicon has a voltage potential of roughly .3-.6 volts.
This means Silicon takes less voltage to charge and allows the Silicon overpotential to be
much higher before lithium plating kicks in.
Yes, this does mean a battery using Silicon has a voltage curve that’s slightly lower,
however, it’s more than made up for by the fact that Silicon stores 9 times more energy
Interestingly, the combination of graphite and silicon in the same anode creates a cascade
effect for lithiation.
It takes less voltage from the charger for the Silicon to alloy with Lithium ions, and
so the lithium ions alloy to the silicon first.
As charging progress, the Lithium ions next intercalate into the graphite.
It’s only when the battery is pushed extremely hard that the lithium ions would begin to
form lithium metal at the anode.
Fourth, and finally, Silicon helps with fast charging because it allows for an anode that’s
much thinner and more energy dense than a graphite anode.
A thinner and more energy dense anode has a greater capacity to rapidly absorb lithium
ions from the cloud of ions created by the concentration gradient.
The reason Silicon allows for a thinner anode is that Silicon particles can store 9 times
more energy than graphite, meaning less material is required, meaning a thinner anode.
A thinner anode was likely one aspect of what Drew was referring to in the Q3 earnings call
when he said that lithium plating can be addressed by electrode design.
Let’s recap the silicon charging process.
The battery begins charging, releasing a cloud of lithium ions that first seek out the Silicon
particles.
The thick cloud of ions doesn’t plate to the anode because the overpotential from the
charger isn’t high enough to cause the cloud of lithium ions to plate.
When the Silicon is full, the lithium ions then intercalate into the graphite.
This brings us to a final point about high loaded silicon anodes and how they charge.
There will still be a taper in charge rate.
A 30% loaded silicon anode would be about 70% of the energy storing capacity of the
anode.
This means the Silicon would be charging for the first 70% of the charge cycle.
After this, the graphite would begin charging rapidly, but slow down past about 80%.
In summary, Silicon solves the lithium plating problem and opens the door to fast charging.
Panasonic has already started gradually increasing the silicon loading in their battery cells
from Giga Nevada.
This should mean Tesla’s entire vehicle line up is slated for gradual improvements
to fast charge capability and energy capacity.
We don’t know yet whether Tesla will jump straight to a 20-30% Silicon loading with
their 4680 production lines or gradually increase Silicon over time.
Tesla may be able to jump to a higher Silicon loading more quickly than Panasonic due to
the increased strength and power density of the Dry Battery Electrode, which was called
out in previous videos.
Battery cells with a high silicon loading of up to 30% are capable of charge rates at
4C or more, which is a full 0-100% charge in 15 minutes.
Even more impressively, these batteries can do burst charges of 50% in 5 minutes.
With cells capable of these charge rates, the bottleneck now clearly becomes supercharger
capability.
I don’t have any idea what Tesla’s timeline is for their v4 supercharger, but I’d like
to see it roll out before or with the massive battery packs we’ll see in the Cybertruck.
A faster charge rate with larger battery packs have such extreme power demands that if the
long-range versions of the Cybertruck are capable of a 50% top up in 5 minutes, they
might require a mega charger.
Realistically, in the next three years, taking account the chemistry changes and supercharger
infrastructure, we may see a gradual increase to roughly double the maximum charge speed
of Tesla vehicles along with the 20% range increase and 5% cost decrease that Tesla has
already promised from Tesla Silicon.
Why didn’t Tesla discuss this at Battery Day?
I can think of several reasons.
First, there was already enough Osbourne inducing hype at battery day.
Second, as we’ve seen in this video series, what Tesla unveiled at battery day is more
complex than their presentation suggested.
Any discussion of charge rate would have also required a broader discussion about what customer
and investors should expect, which would have involved a conversation about how this is
rolled out to vehicles and future versions of the Tesla Supercharger.
It would make sense to leave this for a dedicated presentation on fast charging or to slowly
provide more information in earnings calls, on Twitter, or gradual improvements via software
updates.
In the next video, we’ll cover the implications of Tesla Silicon and I’ll release some additional
footage of my conversation with Shirley Meng where we discussed Silicon Graphite Composites
vs a polymer coated silicon particle.
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