Welcome back everyone!
I’m Jordan Giesige and this is The Limiting Factor.
Samsung recently published a research paper detailing what they referred to as a breakthrough
in solid state batteries.
They claim this battery has high a cycle life of 1000 cycles and high energy density at
greater than 900 watt hours per liter.
In today’s video I’ll explain what the researchers have claimed, walk you through
the design of the battery cell, and give my take on the battery.
Let’s get right into it.
The title of the research paper was High-energy long-cycling all-solid-state lithium metal
batteries enabled by silver-carbon composite anodes.
There was a large team working on the research project, but Samsung called out three of the
key researchers in their press release.
Yuichi Aihara, Dongmin Im, and Yong-Gun Lee.
I’ll paraphrase the abstract into plain English, but first, you’ll need an understanding
of some of the words and concepts they’ve used in the abstract.
An all-solid-state-battery is a battery that contains all solid materials and no liquid.
Typical lithium ion batteries use a liquid electrolyte and solid state batteries replace
that liquid with a solid electrolyte.
This is desirable because liquid electrolytes are typically flammable.
As soon as flammable electrolytes are removed from batteries, we’ll see far fewer battery
explosions and fires.
The electrolytes in a solid state battery also have another advantage, which’ll be
covered shortly.
The anode in a typical lithium ion battery is made of graphite.
Graphite anodes store lithium ions between their layers at an atomic level in a process
called intercalation.
The layered graphite intercalation structure is bulky in comparison to the lithium ions
it stores, which means extra weight and lower energy density.
An anode-less battery cell is a type of battery where the typical graphite anode material
has been removed and the lithium ions bond directly to the current collector.
This means the bulky intercalation structure is eliminated.
We’ll refer to the process of the lithium ions bonding with the current collector and
then layering on top of each other as plating.
In some cases, researchers pre-plate the anode with a small amount of lithium to help improve
the battery performance.
The reason why an anode-less structure isn’t used in batteries, despite its advantages,
is that when the lithium ions arrive at the anode side and begin to plate, they don’t
form a perfectly smooth layer.
They form dendrites.
Dendrites are structures that grow on the surface of the lithium that look like fingers,
needles, and trees that can grow tall enough to touch the cathode and short out the battery,
causing an explosion.
Coulombic efficiency is how efficient a battery is at giving you the energy back that you
put into it.
If this efficiency is low, it means that some of the energy is being used for unwanted reactions
which shorten battery life.
If you’d like to know more about this, check out my video on Jeff Dahn.
Those are the key concepts, and now I’ll paraphrase the abstract:
All-solid-state batteries with lithium plated anodes are a great option to replace the typical
lithium ion batteries on the market today.
However, they have low cycle lives due to low coulombic efficiency and experience dendrite
growth.
In this research paper, we’ve created a solid state battery with a lithium plated
anode that has a long cycle life and is safe.
The anode isn’t pre-plated with pure lithium metal to help the performance.
Instead of the pure lithium metal, we use a layer of silver and carbon.
As the battery charges and discharges, the silver-carbon layer regulates the lithium
ions and helps them form a perfectly smooth layer on the current collector of the anode.
For the cathode, we used a standard Nickel Manganese Cobalt or NMC material and for the
solid electrolyte we used a high sulphur material.
Throughout testing, the cell was compressed and warmed to improve the contact between
the layers.
The end result was a battery cell that had greater than 900 watt hours per liter of energy
density, had a coulombic efficiency of 99.8%, and had useful cycle life of 1000 cycles.
This is a diagram of the battery cell layers provided in the research paper.
Let’s take a look at what’s similar and different about these layers from a typical
lithium ion battery cell.
The Samsung battery uses an Aluminum current collector and an NMC cathode material, both
of which are common to typical lithium ion batteries.
The NMC is shown as the large black spheres that make up the central part of the diagram
on the right.
NMC is a common high energy cathode material that’s well-researched, and for that reason,
it provides an excellent benchmark material for testing high energy battery cells.
For the rest of the video I’ll refer to the NMC cathode material as just cathode material.
Aluminum is shown as the metallic coloured strip that forms the top layer on the diagram.
Aluminum is used as the current collector because it’s electrochemically stable at
the voltage ranges that lithium based batteries operate at.
This is where the similarities end between a typical cell and Samsung’s cell.
Instead of a liquid electrolyte, the Samsung cell uses a solid electrolyte, which is shown
by white spheres that vary in size between small and medium.
Some solid state electrolytes, such as the one chosen for this research, also help prevent
dendrite growth.
This is because the solid electrolyte conducts lithium ions in a way that distributes them
evenly during lithium plating.
The solid electrolyte the researchers chose is a chemical compound high in sulphur, so
it’s classed as a Sulfide Solid State Electrolyte.
Sulfide Solid Electrolytes have high lithium ion conductivity at room temperature, which
is helpful if you want your battery to work at that temperature.
More specifically, they picked a crystal structure called Argyrodite that has slower reaction
speeds than most Sulfide-based solid electrolytes.
This is a drawback because it means slower charge and discharge rates.
However, because it’s less reactive, it’s much more stable when it touches lithium metal.
This makes it a great candidate for a lithium metal battery.
And even though an Argyrodite crystal structure has a slow reaction speed relative to other
sulfides, it’s still a fast enough to create a useful battery chemistry.
For the rest of this video we’ll refer to the Sulfide Solid Electrolyte with an Argyrodite
crystal structure as simply the electrolyte.
If you look closely at the diagram of layers, you’ll notice that the electrolyte is actually
hanging out in two places.
1) There are small white electrolyte spheres mixed in with the large black NMC cathode
material spheres.
The research paper didn’t elaborate on this, but I have an insight as to why they’ve
done this.
In a non-solid state battery cell, the entire battery cell is saturated with liquid electrolyte.
This allows the ions to travel directly from the surface of the cathode materials into
the electrolyte solution.
From there, they can travel on to their destination at the anode, and vice versa.
In solid state batteries, there is no soaking mechanism, so the solid electrolyte is often
manually mixed with cathode materials.
Mixing the solid electrolyte with the cathode material provides the lithium ions with 3
dimensional access to and from the cathode materials, just as they would have with a
liquid electrolyte.
Without this access, the cathode materials buried deep in the cathode wouldn’t be able
to unbury themselves to take part in the charging and discharging of the battery.
2) There’s also a layer of pure electrolyte that’s illustrated with slightly larger
white spheres.
This is similar to the liquid electrolyte layer in a typical battery cell.
It allows the lithium ions to ferry back and forth, to and from the electrodes.
Before we move on to the next layer, there’s a layer mentioned in the research paper that
wasn’t shown in this diagram.
The researchers applied a 5 micron layer of LZO to the cathode material.
LZO is a combination of Lithium and Zirconium Oxides.
The purpose of the LZO layer was to protect the cathode material from unwanted side reactions
that occur when voltages in the battery go above 4 volts.
The LZO layer was necessary because the battery was cycled as high as 4.25 volts.
The next layer is a Silver-Carbon Composite layer.
The tiny black spheres are carbon and the blue spheres are silver.
This is the layer that the researchers claim regulates the lithium ions and helps them
form a smooth layer on the current collector of the anode side.
In this battery the current collector on the anode side is made of stainless steel.
Typically, current collectors on the anode side are made of copper.
Stainless steel is lighter and cheaper than copper but it’s much less conductive.
Both are good anode materials.
The reason for selecting one over the other really depends on your use case.
Samsung didn’t explain why they chose stainless steel.
My immediate thought was corrosion resistance, but pure Nickel is actually more corrosion
resistant in a battery anode than stainless steel.
So that mystery will remain unsolved for now.
Now that we’ve gone through the design, I’ll explain how the battery works.
When the battery is charged, the lithium ions leave the cathode material and travel through
the solid electrolyte.
This is a mind bender, because an ion moving through a solid electrolyte is essentially
a solid moving through a solid.
The ion tunnels its way through, which is another reason why solid state batteries are
such a difficult technology to master - not many materials have this talent.
When the lithium ions arrive at the silver-carbon composite, they pass through the silver-carbon
composite and then form a smooth dendrite free layer on the stainless steel current
collector.
Samsung’s researchers didn’t know what was going on here, but offered an educated
guess on why dendrites were suppressed.
When the lithium ions pass through the Silver-Carbon layer, some of them bump into the silver and
bond with it.
The Lithium-Silver alloy and lithium ions then plate to the stainless steel current
collector.
Then, the magic happens.
The researchers speculate that silver lowers the nucleation energy and improves the conductivity
of the lithium, allowing it to form a pristine lithium plated layer.
Nucleation energy is the amount of energy it takes for a group of atoms to change its
structure.
The best analogy I could come up with for this is making pancakes.
If you add more water to the pancake batter, you’ll get a flatter pancake because the
batter spreads more easily across the pan.
If you don’t add enough water to your pancake batter, you’ll get lumpy pancakes.
That’s what silver is doing for the lithium, it’s like water in pancake batter.
It allows the lithium to plate flatter on the stainless steel current collector, rather
than to plate with lumpy dendrites.
The extra conductivity comes into play the same way gravity would on your wet pancake
batter.
Gravity pulls the pancake batter flat in the pan.
If you made a pancake on a planet with more gravity, the batter would get flatter and
flatten more quickly.
When the battery is discharged, the reverse happens.
The lithium ions leave the silver behind after their tryst.
Over time, this does cause a change the structure of the Silver-Carbon layer.
The silver particles shift towards the current collector and this appears to further help
with the lithium plating.
The researchers aren’t certain if the carbon layer is serving any function here, but they
speculate that the carbon helps conduct lithium ions and provides mechanical support as the
materials shuffle around during charge and discharge.
That covers the basic structure of Samsung’s battery and how it operates.
The researchers then took the structure one step further.
They used this structure to build a bi-cell structure, shown here in figures a through
c.
This bi-cell contained 1 aluminum current collector and 2 of everything else.
The purpose of this was to increase energy density by sharing the aluminum current collector
between what is basically two batteries fused into one battery cell.
Throughout the entire testing process the researchers used warm plates to compress the
battery.
This compression and warmth increased the conductivity between the layers to improve
their results.
This kind of seems like cheating, but it’s often done in testing with solid state batteries.
I would assume if a battery like this ever made it to market one of two things would
need to occur.
Researchers would need to solve the conductivity problem in the solid state chemistry, or,
solve the problem through physical engineering.
For example: Compressing the battery cell into the battery pack.
As we’ve already discussed in the introduction, this bi-cell battery compressed between warm
plates achieved an energy density of greater than 900 watt hours per liter and lasted 1000
cycles.
Time for my review and thoughts.
First, the positives.
My first impression of the research paper was that it had some of the best visuals I’ve
seen in a research paper.
Besides my first impression of the paper, it was a clever solution to some of the problems
plaguing solid state batteries.
The interaction between the silver and lithium to form a smooth layer of lithium was an effective
strategy.
This will probably spawn more research to confirm how the two are interacting to form
a perfect layer of lithium.
If the basic mechanism here can be better understood, the silver could be swapped out
for something cheaper and lighter.
Now on to my thoughts for the actual usefulness of this battery and how it compares to typical
lithium ion batteries.
Graphite and nickel materials make up the bulk of typical lithium ion batteries and
are $10-13 a kilogram.
The Silver and Zirconium used in this battery are $558 and $150 per kilogram respectively.
Even though these materials were used in small amounts, that’s 12-55 times more expensive.
In real world terms, this battery would likely cost as much as an entire Model 3.
What would we get for that price tag?
900wh per Liter is a good number, but not fantastically high.
For comparison, Tesla’s 2170 cells in the Model 3 were probably at around 680 watt hours
per liter in 2017.
I’m basing this off a similar chemistry that has a comparable specific energy density,
seen here.
That’s roughly 30-40% less than Samsung’s solid state.
However, at the 2018 annual shareholder meeting Elon suggested Tesla could have a battery
cell with 30-40% greater energy density in production by 2020.
If that turns out to be true, Samsung’s self-titled breakthrough battery of the future
is Tesla’s battery of today.
I say that not to put down the work of these researchers or to pump up Tesla, but rather
to put this work in perspective after the hype I’ve seen around it.
My passion is the technology, Tesla just happens to have it.
Besides cost and energy density, Samsung’s solid state battery would quickly hit material
bottlenecks.
Silver is a precious metal.
Serious battery research focuses on highly-available, cheap materials like Lithium, Nickel, Graphite,
and Sulphur.
This is the way.
Millions of tons tonnes of material will be required to feed the 2-3 terawatt hours of
battery production we can expect to see by 2030.
This is part of the reason I’m so passionate about battery research from people like Jeff
Dahn, who’s repeatedly said he only wants to work on research if it’s useful.
Or, the Battery 500 consortium that’s working hand in hand with industry partners on pathfinders
to high energy density, low cost, commercially viable technology.
Overall, if I were to give this paper a rating, I’d say it warrants 5 out of 10.
One out of five points for usefulness and four out of five points for interesting.
Over the next few months, besides battery day, my focus will be to break down other
battery chemistries and materials in a similar way.
I’ll highlight their positives, negatives, and when we might expect to see them, or if
we’ll never see them.
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