Portrayed as the 'Tabless Battery', the structure of Tesla's new and improved design involves a stunningly simple process that might contradict its popular name. The technique being used solves a tedious problem with the old design but fortunately, a solution has arrived!
As with many of Tesla's events, Battery Day managed to polarize the majority of its audience, yet again
On the surface, it may appear underwhelming to some, but it's kind of like one of those movies you have to watch multiple times to 'get' the meaning behind the outcome because the story is so complex when viewed fresh
But over the next few watches, the story begins to 'click'.. justifications become clear and your appreciation for the work begins to set-in
Before long, you earn a small sense of entitlement as finally, you understand the secrets decorated throughout the storyline and you are sure to honour their importance to everyone!
With Battery Day, the image above is perhaps one of the most significant aspects of the entire presentation
It illustrates Tesla's limitless approach to problem-solving and demonstrates their conviction toward the mission of accelerating the transition to sustainable energy
By no means does the company settle at any one point, regardless of how far their stride to end up where they are, they will re-evaluate and improve, always
Current Battery Structure
Known as the '2170'. This is the 'tabbed' battery design used in Tesla vehicles, supplied by Chinese provider CATL or 'Contemporary Amperex Technology' and the likes
These have served the company well for years, this design was actually a 50% energy increase on the previous 2007 '1865' battery
They do come with their limitations in power, energy and thermal management though
A good way to understand the difference between the 2170 battery and the new 2020 '4680 tabless' battery described below, is understanding their construction followed by their electron flow
So let's start with that!
The one thing in common with these two battery types are the 'layering' of materials, these layers are then wrapped up to create the cylindrical cell
The cylindrical geometry is just a rolled-up 'final version' of this layered design. We can simplify this down to three layers, though others do exist!
This is where the Lithium ions exist. If we think how electrons occupy the Lithium atom we can see that the first shell houses a total of two electrons while the next shell houses just one
As with all atoms, they'd 'rather' have a totally full electron shell or totally empty one - no time for this 'anywhere in between business'!
Since it's got just one electron in its secondary shell (out of a total of 8), it really wants to get rid of it. In its natural state, lithium is very reactive - even placing it in water provides a reaction!
But for now, just remember that the top layer is where the negatively charged electrons and positively charged ions of the Lithium atom sort of 'begin'
The middle layer is what's known as the separator, this does exactly what it says on the tin. It separates the top layer from the bottom layer
The reason for this is that when we attach a load or a 'tesla' or if we want to charge the cell, we want to force the electrons to flow through a defined path before they reach the other side of the battery
So we want the electrons to go from the top layer to the bottom layer by going ' through' something in order to provide electricity and therefore purpose. We do NOT want the electrons to zoom from the top layer to the bottom layer without a resistive path in between
That would be pointless and.. super dangerous - it'll be a short circuit, providing max current and.. lots of unwanted heat
One thing to note about this separator is that thanks to the chemistry nerds responsible, they've been able to make it so that positively charged lithium ions can pass through the separator but negatively charged electrons can not
Meaning, when we do have a load attached, the electrons will travel along a defined path we provide while the Ions will just jump across the separator to the other layer - there's a good reason for this, coming up! ( it's this feature that makes the Lithium-Ion battery rechargeable! )
You may have heard about Graphite being used in these batteries cells, this is where it is
The purpose of the Graphite layer is to house the electrons and ions once they arrive, the electrons will come in from some path we define and the ions will just come across the separator as previously mentioned
This Graphite 'lattice' is effectively a storage medium and when the electrons and ions are stored here you'd be right in calling the battery 'charged' and ready to go!
One important thing to note about this storage medium is that the electrons and ions are unstable here, at least 'more' unstable than the previous medium they were stored in
What this means is that they would really like to get back to their origin, the top layer has what's known as a metal oxide and when lithium is housed there, it's very stable until and is happy to stay put until a new charge potential is bestowed upon them ( like given from a charger device in order to charge the battery by moving electrons and ions to the other layer)
The whole idea of charging is that you're taking electrons and protons and placing them into an unstable state (charged) and then to use them, you provide a path to their stable state while going 'through' a device (powering something)
Rinse and repeat!
So these are the basic constituents of the Lithium-Ion cell, there are additional layers which offer important features but we've avoided those because the tabless battery design doesn't really improve on them 'as much' as they do the electron path and heat dissipation
Talking of the electron path, let's see what we've got with the battery I just described
The Issue with the Electron Path
Let's take the electrons and ions in the furthest region away from the tabs, in both layers
This gives us the 'worst case' for.. a better understanding of the furthest necessary journey
So the Electron path, shown in blue has to travel along the length of the layer which is about 800mm by the way!
That's just the distance to reach the tab, then it has to go through whatever path we define for it through the charger and into the bottom layer. As mentioned, let's say they are housed in the graphite lattice at the furthest distance from the tab, so another 800mm
The ions on the other hand - in blank, hop straight through the separator ( shown in red )
While the ions have an easy life, the electrons have to make their way bashing though the atomic makeup of the material they travel through. The more distance the electron has to travel through, the more collisions it has along its journey. Each 'impact' along its journey is causing friction and that generates heat.. which accumulates.. per impact
To account for this heat build-up, which of course is wasted power! Designers have to incorporate lengthy amounts of coolant within the battery packs, which takes up space and weight within the drivetrain design
Additionally, the tabs are giant thermal bottlenecks in the journey. Imagine all of those hot electrons having to squeeze together through the tab because it's their only exit and entry
In cylindrical cells, heat can build up pretty quickly. With this layered design, some of the internal layers have thermal insulation between them. This means that in the layers that do heat up, they don't radiate heat outward very much but upward or downward
With this being the natural direction of heat to flow in this geometry, the best place to stick a heatsink would be at the top or bottom of the cell (ideally whichever side has the most thermally conductive material, like copper)
Though you want to get the heat out on the top or bottom of the cell, remember.. you've got the tab there, this is the only conductive thing (electrically and thermally)
Therefore you'd have to get the heat of the battery away from the pack using just this tiny strip of conductor...
Unfortunately, that's not doable, certainly not efficient. So what Tesla and may other cylindrical cell users currently do use provide a flow of coolant between the sides of the cells, kind of like the top-down view below:
The coolant flows in between each line of cells, the heat is captured in the thermally conductive coolant and transported away from the batteries
This does cool the batteries, though not as well as it could - what you want to be doing is cooling from the 'caps' on either end, but the tab prevents you from doing that (because that's the only heat conductive part on each layer, it's not enough surface area to dissipate the heat generated from the cell)
So, in conclusion, we get a great battery, sure - but a hot and power-lossy one too
The 'Tabless' Battery
Fortunately, this technology doesn't require anyone to be scientifically literate to understand it - with hope, it should all just make good sense!
Below is the unwrapped design of the Tabless battery, you may notice something and you might even have a question for me
'What are those orange coloured 'tab' looking things on the edges of the upper and lower layers?'
Those are TABS, kind of.. well a lot of them. This is what we mean by the whole idea of naming it a 'Tabless' battery but being contradictory in doing so
So now we have so many tabs shoulder to shoulder, we've basically got a new region along the edge of the upper and lower layers. The unique thing about these regions is that they can 'fold over'
With that, let's consider the electron path.. one more time
Now in this example, there isn't just one exit on each layer ( i.e a tab ) there are many exits along with the layer
So now.. the biggest journey for the electron exists not along the length of the layer but across its width, this is why we've put the electrons and ions at the edge of the length rather than the edge of the width
Here's the interesting bit, imagine rolling up all these layers but this time you fold each of the 'should to shoulder tabs' over each other, effectively connecting the ends of each layer with the layer in front of it.. do you know what you end up with?
You end up with the picture that brought us here in the first place, each of those tabs along the length of the layer has been folded over to connect each layer in front of it, this entire cap now acts as one connection point
The Ions continue to go through their usual short cut separator path but the electrons now don't have to battle through the material, they take the shortest path out, and the shortest path into their new layer
As you can imagine, the electrons don't accumulate nearly as many collisions as before because now the worst case is that they travel 80mm rather than 800mm, that's a magnitudes difference!
What this means is that the power is no longer lost through heat - it can be used
As mentioned with the previous 2170 cell, thermal management isn't optimal
But now, we have a giant copper 'cap' at the end of the cell, this is very thermally conductive and it's easy to attach to a heat sink in order to do away with the heat!
The advantage here is that you don't need all that space and weight allocation for coolant between the cells because the heat is now dissipating and being transported from the bottom, a bit of a win-win!
Why the Jump to 4680
Well since the thermal conditions are better for the general cell structure, we can make it bigger and reap the advantages without baking the thing!
The larger the cell, the more layers giving more capacity to store energy. In the presentation Tesla quote 5 times the energy as the 2170, impressive stuff! - this is based on the new volume of the cell
Additionally, they mention they've achieved 6 times the power
This new power capability comes from the saving of power with the new tabless design, the previously lost power, is now usable!
The Tabless battery is great, and it was just one of the crowning moments of the presentation but in truth, it contained a goldmine of innovations
The new <$25,000 (<20,000 GBP!) car, the saving on manufacturing floor space, the simplifications of manufacturing...to name a few
Overall, a very successful event for the company especially amidst the current COVID climate
So at this point, you might think, 'this is quite simple, why has no one done this tabless thing before?'
Honestly, great question, I suppose no one really thought of it? But it does show the benefit of understanding things 'from the atoms up'. You can observe all steps within a process and maybe once in a blue moon, you uncover a secret passage that's been there all along
A lesson for us all!
Thanks for reading