Ever since Tesla announced the use of the new 4680 battery cell in the future at the Tesla Battery Days three years ago, there has been a real buzz around this new cylindrical battery cell. Although the 46 mm diameter and 80 mm long cell has only been used so far in the Tesla Model Y, there has been an increasing amount of interest and announcements, especially from the automotive industry. In addition to Tesla as a pioneer, OEMs such as BMW and General Motors have also announced their intention to use the cell in the future.
At the same time, there is an expanding field of battery manufacturers that are building production capabilities, or at least aiming to build manufacturing capabilities. LG Energy Solution hopes to set up a sample production line in Ochang, South Korea this year, while Panasonic hopes to set up a large-scale production line in Wakayama, Japan. Samsung SDI and BAK are also conducting trial production, and CATL and EVE Energy have signed contracts to build Gigafactories as qualified suppliers for BMW. In the automotive industry, expectations for batteries are high: high power density and fast charging capabilities, more efficient integration into the battery pack or vehicle chassis, and cost advantages due to the simpler manufacturing process of cylindrical cells.
Although the size of this cell is large compared to other cylindrical cells such as 18650 and 21700, its size and battery capacity are still much smaller than the latest generation of prismatic cells. For example, the internal volume of a 46 mm cell with a height of 80 mm is about 120 ml. The volume of the “blade” type battery produced by BYD is slightly less than 1.2 liters.
Therefore, for OEMs, the use of new cylindrical battery packs may still require a high level of integration. But at the same time, there is flexibility in the mechanical and electrical design of the battery pack: For example, NIO and BMW have announced their intention to use 800 V structures in the future. Due to the small number of individual cells in large-format batteries, it is not possible to achieve such high system voltages when connected in series. With the new 4680 cylindrical cells, a typical battery pack can achieve high system voltages even if 3 to 5 cells are connected in parallel.
Advantages and disadvantages of new battery designs
In addition to the 4680 cell initially announced by Tesla, other cell formats are currently being discussed, with heights ranging from 40 to 120 mm.
In principle, different combinations of materials and electrode designs are possible. According to the initial teardown report, Tesla used a combination of NMC811 (lithium-nickel-cobalt-aluminum-oxide with 80% nickel content) and graphite in the first generation of cells, a material system that can be said to be the most advanced today. However, the new feature of the battery design may be the particularly thick electrodes.
Tesla also uses dry coating technology for the negative electrode, but the extent to which this has been achieved is not yet known. The electrode combination we assume includes a 100-micron-thick graphite negative electrode and an 85-micron-thick NMC811 positive electrode, so the electrode length is about 3.5 meters, and the negative electrode or outermost copper foil may be about 20 cm long. The average voltage of the cell is slightly below 3.7 V (which is typical for this type of material), and the cell capacity is about 25Ah, so the energy density is slightly below 700Wh/l, or 250Wh/kg. The cell is thus indeed able to reach the energy range of the known 21700 cell, but is still slightly below it. This is mainly due to the weight of the active materials in the negative and positive electrodes. The achievable capacity of the active materials is lower due to the absence of silicon oxide in the negative electrode and the high specific capacity NCA material (lithium-nickel-cobalt-aluminum oxide) previously used in the 21700 cell. It is not clear to what extent the use of a dry coating process is responsible for the absence of the silicon additive. Cost factors or the advantages of balancing the negative and positive electrodes compared to NCA may favor the use of NMC811.
The new battery cell design scores points in terms of the weight of the current collector and separator, which is reduced due to the reduced thickness of the electrodes. The contribution of weight relative to the storage capacity between 21700 and 4680 remains almost unchanged, despite the increase in the wall thickness of the cell steel case from 300 to 600 microns.
The peculiarities of the cell design are particularly evident when comparing the cell heights discussed. For this purpose, we analyzed cells with heights ranging from 40 mm to 120 mm. With the described materials and electrode configurations, the achievable capacity increases from about 11 Ah (40 mm) to more than 38 Ah (120 mm). The reason for this nonlinear capacity behavior is the space requirement in the lower and upper areas of the cell. A height of 5 to 10 mm is required inside the cell for contacts, safety functions and sealing, which cannot be used for the electrode winding.
For example, in the configuration we calculated, the winding height in the 4680 cell is only about 70 mm. This value varies depending on whether additional gaskets are used in the “lug-less design” to contact the folded current collectors. The disadvantages of a cell height of only 40 mm also emerge accordingly. On the other hand, particularly long cells (such as the 120 mm format) may also require some compromises if heat dissipation of the cell or the wetting properties during electrolyte filling require a reduced electrode layer thickness.
It is clear from industry reports that the configuration we analyze can only be “first generation”: Tesla already expressed its intention to use silicon-based anodes in new battery formats at the “Battery Day 2020”. StoreDot has also recently disclosed its development activities for the use of fast-charging silicon technology in 4680 cells. This approach combines material-level performance characteristics with the high thermal and electrical conductivity of a tab-less design.
In principle, various silicon-based anode concepts are conceivable: for example, silicon/graphite composites or pure silicon anodes. For composites, the use of silicon nanoparticles (SiNPs) is mainly discussed, while silicon microparticles (SiMPs) and other morphologies are also being investigated for pure silicon concepts.
In our model calculations, we only assume minor adjustments to the design of future battery concepts in the 4680 format on the cathode side. With the same film thickness and slightly reduced porosity, NMCA-type nickel-rich materials (Li(Mn,Co,Ni,X)O2 X=Al, Mg) could be available in the next few years and could provide reversible capacities of more than 200mAh/g. For a silicon/graphite composite with about 20 wt% SiNPs (assuming a capacity of 780 mAh/g), the coating thickness of the anode is only 80 µm, which is much smaller compared to the “Generation 1” cell design. If a pure silicon anode is used instead (assuming a capacity of 2500 mAh/g), the coating thickness is reduced again to just over 60 µm. However, the anode layer must have an extremely high porosity of 75% to absorb the high volume changes of the silicon.
As an intermediate step, a silicon anode is conceivable, which does not utilize the complete alloying of silicon and lithium (Li15Si4) and is therefore exposed to lower volume changes, potentially allowing the use of SiMPs. Due to the thinner anode, the electrode length in the silicon-rich concept increases to more than 4 meters.
In addition to these high-energy concepts, the use of lithium iron phosphate (LFP) or the further developed lithium manganese iron phosphate (LMFP) with manganese substitution is also a possible alternative. So far, BYD has developed 4680 cylindrical cells based on LFP. The energy density of the 4680 cell can exceed 450Wh/L, and if further developed LMFP materials are used, the energy density can even exceed 500 Wh/L.
Production of 46xx series cylindrical cells
From a production perspective, the main advantage of the new cell is clearly reflected in its internal structure: the electrodes are wound and do not need to be stacked in a complex process. At the same time, the size of the cell also significantly reduces the individual steps required to produce a finished battery pack. In terms of details, there may be further changes compared to the previous cylindrical cell design: for example, the tab-free design reduces the number of discontinuities required for electrode coating, which were originally required for contact points with the small tabs of the current collector. In addition, the welding work of these tabs is also saved.
On the other hand, electrode cutting and winding process may increase the complexity of the process, because the edges of the current collector foil must be cut relatively tightly and folded during the winding process. Comparing the different cell types from 4640 to 46120, it can be assumed that the overall manufacturing workload is quite similar. Even the production of longer cells with a height of 120 mm does not significantly shorten the assembly cycle compared to short cells.
Only the electrolyte wetting process is likely to increase significantly with increasing cell length. In summary, the 4680 offers higher energy per cell compared to the 46120, but saves up to 20% in assembly costs. However, this saving is worth considering from the perspective of the total battery cost, since assembly costs may only account for 5% to 7% of the total cost due to the current very high material prices.
Potential of a new generation of cylindrical cells
Not only the performance characteristics are promising, but also a large number of announcements from producers and OEMs indicate that activity around the new 46 mm cell format will be high in the coming years. However, it will take several years for this battery to be widely used in electric vehicles. It may take one to two years to establish new production lines on a large scale.
In addition, we can also assume that technical innovations such as tab-free design or the use of particularly thick electrodes will bring one or two challenges to the production ramp-up. Tesla’s production may switch more to the new battery this year. BMW does not expect to adopt the new battery in its new generation of cars until 2025. In the long term, however, applications beyond electric vehicles, such as stationary energy storage, will also benefit from the new batteries, but as is often the case, they will first have to wait in line behind their main customers in the automotive industry.
