The Tesla Model Y lithium-ion cell has been one of the most talked about batteries in the industry since the concept was launched by Tesla in May 2020. At launch, the Model Y cell seemed years away from mass production but, as of today, Tesla is selling vehicles on the mass market, powered by their revolutionary technology. The concept of the Model Y cell is likely to have an enormous bearing on the future of energy storage, in and beyond the transport sector. As Musk put it, this is “way more important than it sounds”.
In this blog, we explore the benefits of larger format cells, and why bigger is not always better. We provide insight into the Tesla Model Y cell with About:Energy lab data collected from cells extracted from the Sandy Munro Model Y teardown in 2022.
Key takeaways
The Tesla Model Y cell stores 5× the energy of most 21700 cells, but this does not lead to greater gravimetric energy density at cell level.
The ‘tabless’ design means the Tesla Model Y cell has a low resistance and, consequently, is able to charge and discharge at the rates required for automotive applications.
The ‘tabless’ design is exciting news for thermal management. This will mean a reduction in the design complexity of the thermal management system, more homogeneous operation and longer lasting, safer cells.
The number of Tesla vehicles delivered worldwide each year.
Cylindrical Cell Growth
The Model Y cell represents the next iteration of the cylindrical lithium-ion cell. First, we had 18650s (18 mm diameter, 65 mm in length); then we had 21700s (21 mm diameter, 70 mm in length). Now we are getting a whopping 46 mm diameter and 80 mm length, with the new form factor known as ‘4680’. The move to larger format cells is driven by the need for higher cell energy density and improved pack efficiency. Since Tesla announced larger cylindrical cells, other OEMs such as BMW have announced EV platforms developed with this cell type. Cell manufacturers and material developers such as Samsung, Panasonic, OneD, Echion and many more have adopted the 46XX format.
However, despite the promised benefits of this new cell type, it has yet to make an impact on the industry. Tesla’s own manufacturing scale-up has been mired with delays and sub-optimal performance for early prototypes. Production delays are nothing new in the industry and breaking technology barriers comes with difficulties, Tesla’s engineering teams have been hard at work, recently achieving the milestone for 10,000,000 cells, equivalent to 0.85 GWh!
Cylindrical format lithium-ion cells suffer from inhomogeneous current distribution, and consequently, temperature gradients that hamper accessible energy from the cell; a large chunk of energy stored is lost to waste heat. Thicker cells, as proposed by Tesla, would ordinarily be problematic due to decreased surface area-to-volume ratio. The Tesla cells promise to overcome this via a ‘tabless’ design. Base cooling is being proposed inside these battery packs, which makes a wider cell diameter entirely feasible when combined with the tabless design - in fact, we expect this cell to operate with a very small temperature gradient across its volume, in comparison to other cylindrical cells without the tabless electrodes.
Sizing of the 18650, 21700 and 4680 cylindrical format lithium-ion cells. Source: BatteryDesign.net
Cell Level Benefits
The increase in cell volume means an increase in energy storage capacity. We have found that the Model Y cell is able to store 86.7 Wh of energy, 5× more than Tesla’s most recent 21700 format cell (which we find to store 17.28 Wh). This translates to a reduction in the number of cells required in an electric vehicle battery. If we take 80 kWh as the benchmark battery pack size (e.g. Tesla Model 3), you would need 4630 cells of 21700 format, or just 923 cells of 4680 format. All interesting, but what is the benefit?
Energy storage capacity of 18650, 21700 and 4680 cylindrical cells manufactured by / in collaboration with Panasonic.
At the cell level, the ratio of active material could be increased slightly because you have more active material inside one container (the ‘can’). That said, we do not calculate any outstanding performance numbers for the cell-level gravimetric energy density. In fact, the gravimetric energy-density of the Model Y cell, which sits at 244.0 Wh/kg, is less than that of the Panasonic 21700 cell used in the Tesla Model 3 (253 Wh/kg). The modest decrease is attributed to the larger, heavier auxiliary safety components, and it must also be considered that the tabless design (and thus manufacturing remains at an early stage, compared to the mature Panasonic 21700 technology). We would expect further cell-level energy density enhancements before mass manufacture begins in September 2024.
We have included some examples of gravimetric energy densities from other cell manufacturers below. As always, direct comparison of gravimetric energy densities is simplistic - other considerations must also take place, such as power-capability and temperature dependence (for example, we have found accessible capacity of the LG M50LT drops significantly at low temperatures and it generates enormous amount of heat during charge phases).
Cell-level gravimetric energy density, plotted against gravimetric power density for a large range of current generation cylindrical lithium-ion cells.
So, why the hype around the Model Y cell? The real benefit is hidden inside the cells and stems from the enhanced manufacturing capability.
Typical 21700 cells (including all Tesla cells) have small tabs that connect their electrodes to the terminals of the cell. The images below show our teardown of the LG M50LT cell; the positive and negative tabs can be seen in each. Through these tabs, electrons flow, to transfer energy into the cell (during charge) or out of the cell (during discharge). The size of these tabs has until now limited the size of the cell’s jelly roll because the small tabs have a high resistance. Excessive current flow density leads to a significant loss of energy, in the form of heat. Lots of thorough academic work has been published, highlighting the limitations of this design trait, particularly around potentially dangerous high temperatures that are reached during a fast charge. The consequence, for cell design, is a limitation to jelly-roll volume, and this is why we have not before seen cells larger than the 21700 in automotive or other highly rate-capable applications.
Images taken by the About:Energy lab team during the teardown of the LG M50LT 21700 cell that demonstrates the traditional tab design of a cylindrical cell.
The logical design improvement is to increase the size of the tab, so you can increase the size of the jelly roll. Easily said, but very difficult to achieve. The limiting factor is the welding process, which ensures good electrical contact between the tabs and the cell’s terminals - more on this can be read here.
Tesla has solved this problem by developing and patenting the ‘Cell with a Tabless Electrode’, which in reality is an electrode with one tab that covers its entire end. This is an enormous step in the development of cylindrical lithium-ion cells, evidenced by the vast increase in energy storage capacity referenced above. This tabless electrode will dramatically reduce the internal resistance of the cell, meaning higher-rate charges and discharges, and lower losses (less heat generation). All heat that is generated will also benefit from the tabless design because the tab also acts as an excellent thermal pathway, allowing the thermal management system for an EV to operate effectively.
We expect this cell design to dominate the market, outperforming the incumbent technology. For this reason, we made every effort to get our hands on the Model Y cells, so we could parameterise them, and allow our Voltt subscribers to start designing their systems around what we believe to be the future of battery technology.
Inside the Lab: About:Energy’s Parameterisation of the Tesla Model Y cell
At About:Energy, we scale our parameterisation procedures to suit each cell, based on the capacity and rate capability characteristics of the cell. For example, a 21700 power cell is cycled at much higher rates than an 18650 energy cell. When preparing to cycle the Model Y cell in our lab, we knew that higher currents would be required, compared to our work with 21700s and 18650s, due to the higher capacity of the cell. This pushed us to use higher current cyclers, which are normally reserved for cycling pouch and prismatic cells, alongside high-current rated electrical connections and a version of the About:Energy thermal control system with a vastly increased cooling capacity.
Images taken by the About:Energy lab team during the teardown of the Tesla Model Y cell. Left: removal of the base of the cell to expose the pressure release valve and can-electrode, centre: close up of the can-electrode connection, right: exposed copper tabs which run the entire length of the jelly roll.
We found the Model Y cell to have a very low internal resistance: 5.77 mΩ at 50% SOC and 25 °C (which, given this was the intention of the revolutionary tabless design characteristics, was not unexpected!). As a result, the heat generation rate was modest at low charge and discharge rates. For our 1C cycling, we calculate the cell was generating 3.25 W in waste heat (also at 50% SOC and 25 °C). The figure below shows the voltage response to a 1C discharge (from 100% SOC until the lower voltage limit of 2.5 V is reached) at each of our parameterisation temperatures.
The Tesla Model Y cell’s terminal voltage response to a 1C discharge at different temperatures.
Each test was done in isothermal conditions, where the cooled surface of the cell was maintained to within 0.05 °C using our conductive cooling system. Of course, this does not mean the temperature at the centre of the cell is also controlled. A temperature profile will have developed through the cell’s volume, and this will mean that different parts of the cell will behave differently. You can see evidence of this in the -5 °C dataset. The dramatic reduction in cell voltage, followed by a plateau, as the SOC reduces from 100% down to 90%, is indicative of a temperature profile developing through the cell. The increased temperature would reduce the resistance at the centre of the cell, and consequently the overpotential (which is what has caused the initial sharp drop). It is much easier to identify this phenomenon with truly isothermal conditions at the cell surface.
The enormous capacity of the cell meant that during high rate testing (we took the cells up to 3.5C during our standard Voltt parameterisation procedure), we were dealing with a heat generation rate in excess of 50 W. The poor surface-to-volume ratio makes the Model Y cells particularly difficult to thermally manage during experimentation. Given isothermal cell conditions are essential for accurate parameterisation processes, it was especially important to use our state-of-art conductive cooling apparatus to maintain the conditions during the testing.
Post Processing and Data Analysis
Observations of experimental data helps us to gauge the expected performance of the cell, but the real insights into the underlying electrochemistry comes from post processing. At About:Energy, we use techniques to better understand the composition of the cells on the Voltt, so we can enhance our parameter estimation methods and ultimately provide more detailed insight to our customers. Evaluation of the open circuity voltage (OCV) provides a good example. The figure below shows the OCV as a function of SOC for the Tesla Model Y cell. We’ve also included the OCV curves for three common 21700 cells, for context.
The OCV curves for the Tesla Model Y cell, compared to three common cells with 21700 form factor.
We can identify differences between the OCV curves, for example the reduced potential at low SOC for the Molicel P45B, and we can identify apparent similarities, particularly when comparing the Model Y cell to the LG M50LT. But the raw OCV data is unable to provide the clarity that allows us to comment upon the cell composition. For this, we use normalised differential capacity analysis (also known as incremental capacity analysis) - this is a transformation of the OCV plot that makes some features clearer.
The figure below shows the normalised differential capacity analysis for the same four cells. To understand this plot, you just need to know that the area under the curve within a certain voltage range shows the proportion of the cell’s capacity available in that voltage window. As the plot is normalised, the total area under the curve is the same for every cell. These voltage ranges provide a clear chemical fingerprint as well as information on cell balancing decisions.
Normalised differential capacity analysis for the Tesla Model Y cell, compared to three common cells with 21700 form factor.
The presence of a significant capacity at lower voltage indicates Si content in the negative electrode. The use of SiOx additives boosts overall capacity and current capability, but this extra capacity can rapidly disappear with negative electrode degradation. While the LG M50LT and Tesla Model Y cell contain none at all, the Samsung 50G clearly uses some Si, and the Molicel P45B (an extreme high-power cell) uses much more. The high voltage peak present for all cells is characteristic of Ni-rich positive electrode materials in combination with graphite. We use the mid-voltage region to distinguish between electrode materials, although the chemical fingerprints that are shown can also be influenced by the particular cell balancing - we don’t have time in this blog to explain it all!
Takeaways
The 4680 would be inoperable without the tabless electrodes - this component is essential to the low cell resistance which allows the cell to be charged and discharged at the rates required for automotive applications. The result, as we look at the data, is a performance comparable to the best 21700 cells on the market today, but achieved in a much larger package. Tesla’s marketing has told us how important this is for increasing the gravimetric energy density of the battery pack - fewer auxiliary components around the battery pack and reduced complexity in the thermal management system being key drivers.
The whole cell manufacturing industry will surely aim to follow this trend to enable battery pack design simplification. This is exciting - engineering higher energy density in the battery packs, rather than waiting for new chemistry compositions trickle down from the electrochemists! In one of the industry’s first high-profile examples, Tesla seems to be closing the loop faster than others between designing cells, and building battery packs. In fact, it is imperative that cell design should be in continuous loop with pack design, to not only extract the last drop of energy left in the battery for a given application, but to also optimise design for manufacturing, squeezing the last dollar out of each battery’s production cost. For the Model Y cell, we see this as an important step towards enhanced thermal management at battery pack level. This will result in reduced thermal management design complexity, homogeneous operation and safer, longer lasting battery packs.
About:Energy’s battery database, The Voltt, provides data for 100s of commercial batteries so you can benchmark the state-of-the-art against your own in-house technology. For many of these cells, detailed datasets are available to run drive cycles of battery packs to understand the applications for which your technology can excel. Sign up to The Voltt here, or get in touch today at contact@aboutenergy.co.uk.
