Just How Much of a “Breakthrough” is Tesla’s Tabless Battery Cell?
Tesla's Battery Day came with an important announcement: a tabless lithium-ion battery cell. How does this architecture compare to traditional cells?
At Tesla's recent Battery Day, the company announced what Elon Musk calls a "massive breakthrough" in cylindrical cells. To assess the validity of that claim, it's important to first understand the shortcomings of a traditional cylindrical lithium-ion cell.
A cylindrical lithium-ion cell uses several different layers of chemical compounds to store energy. As shown below, sheet-like anodes, separators, and cathodes are sandwiched, rolled up, and packed into a cylinder-shaped can. Each of the cathode and anode electrodes uses small metallic components called "tabs" to connect to the positive and negative terminals of the battery can.
Internal architecture of a conventional battery cell. Image used courtesy of Peter Miller
Small Li-ion batteries exhibit high power and energy density. With larger batteries that are essential for EV applications, we need larger anode and cathode sheets. This presents several challenges. In this article, we’ll take a look at these challenges and how Tesla’s new battery addresses these issues.
A Single Pair of Tabs May Not Suffice
Several phenomena prevent us from achieving the potential performance that large anode and cathode electrodes can offer. This is due to the fact that with larger anode and cathode electrodes, the electrons need to travel a longer distance as depicted below.
Unwound anode and cathode electrodes and the path that electrons should travel if only a single pair of tabs are employed. Image used courtesy of Wei Zhao et al.
The ohmic resistance of the path leads to power loss and consequently, internal heating of the battery.
Besides, the current chooses the path of least impedance in the electrode plain, and hence, the current distribution is not uniform. Non-uniform current distribution results in non-uniform utilization of the active materials that coat the electrodes.
In other words, with larger battery cells, a larger amount of active materials are used but the average utilization of these materials is less than that in a smaller cell. To address this issue, we can employ multiple pairs of tabs along the electrodes as depicted below.
Depiction of a cell with multiple pairs of tabs. Image used courtesy of Wei Zhao et al.
This divides the electrode area into smaller regions increasing the average utilization of the active materials and shortening the path traveled by the electrons.
Tabs Present Several Challenges
Tabs are metallic components welded onto electrodes. Tab manufacturing is challenging and can affect battery reliability and performance. For example, welding burrs can penetrate the separator layer between the positive and negative electrodes and cause an internal short circuit.
Interestingly, X-ray computed tomography (CT) scans can be used to assess such manufacturing defects. The CT scan of a battery with a welding burr is shown below.
Image of a Burr on a current collecting tab. Image used courtesy of X. Yao et al.
Such manufacturing defects can lead to battery fires and explosions.
Besides, the proper location of tabs is of paramount importance. The charge and discharge current flow through the tabs and significantly increase the temperature of the region around the tab to a level much higher than other parts of the battery.
The generated heat can decompose electrode materials around the tab and affect cell performance, cycle life, and safety. To address this issue, we need to keep the tabs at a certain distance from each other. On the cost side, tabs may not be a desirable option because tab welding adds an extra step in battery production, increasing costs.
Tesla’s Tabless Battery
While a detailed explanation of Tesla’s patent is beyond the scope of this article, the basic idea of the "tabless battery" is to achieve the conventional tab functionality through a conductive portion that runs along the electrode.
Structure of the new tabless battery Li-ion cell. Image used courtesy of Tesmanian and the Limiting Factor
With this tabless design, the maximum distance that electrons should travel is the height of the electrode rather than its length as in the case of a conventional tabbed electrode. The height of an electrode is only 5-20% of its length. Hence, the ohmic resistance that the electrons face and consequently, the heat that is internally generated will reduce significantly.
Additionally, the current distribution will be uniform across the tabless electrode. In this way, local hotspots with large overpotentials that can cause unwanted chemical reactions are avoided and the battery's lifetime is improved.
The Rose-Like Battery Cuts Down on Heat
The new design not only generates a smaller amount of heat due to its lower ohmic resistance, but it also exhibits better heat transfer properties. With conventional designs, the generated current goes through the tab that occupies a small area, and hence, the temperature of the region around the tab increases significantly.
With the tabless design, the whole edge of the electrode is responsible for current (and heat) transfer. Heat transfer occurs through an area as large as the base of the battery cylinder. When the anode and cathode sheets of the new battery are rolled up, it forms a rose-like gathering at the ends.
Depiction of the tabless battery cell. Screenshot used courtesy of Tesla
The advantages discussed above allow Tesla to go from the 2,170 cells (21 mm by 70 mm) used in the Model 3 and Powerwall to much larger 4,680 cells (46 mm by 80 mm) without worrying about performance degradation and heat transfer issues. Tesla hopes that these new batteries will make a $25,000 Tesla vehicle a reality in about three years.