Battery chemistry and hoped-for improvements get a lot of attention, and legitimately so. Even modest improvements in that chemistry could translate to increased runtime or vehicle range, better performance versus temperature, and a host of other benefits. It seems that not a day goes by without someone in academic research or start-up mode announcing a breakthrough with respect to a better chemistry.
Of ours, the path from lab improvement to pilot run to full production in a commercial environment is a very long and difficult climb, and many apparently promising improvements have failed along the way. That’s the reality of the situation: the big breakthroughs are hard to come by, and progress is often the result are the very small advances that add up over time.
But there’s another critical aspect to improved battery performance that’s separate from the attention-getting chemistry, especially when used in large packs such as in electric vehicles, Figure 1. The physical construction of individual cells and the tradeoffs made in their design has a major impact on pack and system-level performance.
Figure 1 The electrical, thermal, mechanical, and other design challenges of using a large number of tightly packed batteries such as in the 2017 Chevy Bolt EV increases dramatically with the number of batteries. Source: Chevy/GM via Car & Driver
Further, the ways the cells are packed and electrically connected as well as the “tabbing” used as cell contacts plays a large role. It’s especially complicated since the electrical and thermal behavior of a single cell standing by itself or even a small cluster is far different than it is for a densely packed conglomeration of cells.
I’ve been following research and development in cell design, manufacturing, and assembly for two reasons: first, there’s been significant but often unacknowledged progress there, and often just as meaningful as advances in the lithium-ion chemistry advances; also, on a personal note, the advanced chemistry seems to me as if it is some sort of alchemist’s magic. (I’m not at all saying it is, it’s just that’s how it feels to someone who never “got it” beyond the very basic principles in chemistry class.)
Fabrication and packaging aspects of the cells generally focuses on standard sizes such as the widely used 18650 cell (65 mm high and 18 mm radius, Figure 2) as well as larger ones such as the 4680 cell (80 mm high, 46 mm radius) which is being phased in. There are likely opportunities for changes to their physical design and arrangement leading to overall improvement, and at little or no cost.
Figure 2 The most common lithium-based cells have the 18650 designation with the first two digits referring to the nominal diameter, the next two to the length, and the last digit signifying round shape. Source: AliExpress
Two closely related papers (References 1 and 2) looked in detail at possible new structures for fabricating the battery layers and rolling the cell into a battery core (called a “jelly roll), and for creating the tabs which carry the current into and out of the battery.
What I found interesting was that the papers used extensive multiphysics modeling with COMSOL (a very widely used finite element analysis solver and simulation software package) to simultaneously examine the linked issues related to mechanical design, current flow paths within the cell, and thermal issues. Unlike some well-intentioned academic papers which tend to gloss over a model’s limitations (not all do, but many must do so to get the project done), these papers were fairly clear and direct about the model assumptions made, the simplifications that were used, and the nature of some of the tradeoffs they chose.
For example, the author of one paper notes: “it is tricky to draw objects in a spiral geometry, such as adding multiple tabs in the interior of the jelly roll. In addition, it is difficult to visualize the results inside the spiral layers, for instance plotting the current density through the separators at different positions in the roll.” So, he went to a flattened geometry which enables allows for easy visualization of the cross-separator current density, Figure 3.
Figure 3 The current distribution (A/m2) in the through-plane direction of one of the separators is modeled. Source: COMSOL
The article also looked at possible benefits of so-called “tabless” designs where the additional metal connecting tabs are removed. Instead, they are replaced by extending the foils of the battery jelly roll to reach outside the electrode areas. To minimize ohmic losses, multiples of these foil-based extended tabs are used in parallel.
Figure 4 The comparison of the electric potential in the negative current collector using integrated a single-tab cell (left) and 20 tabs (right) shows a substantial difference. Source: COMSOL
The multiphysics simulation looked at the potential distribution in the negative current-collector foil and found a significant difference. The cell with the single traditional had a 30 mV higher loss than the one with twenty tabs, which is considerable. Further, the current distributions in the foil layer within the cell was much more uniform with multiple integrated compared to the traditional single tabbing which leads to better overall performance along with more uniform thermal conditions.
Battery chemistry is obviously important and improvements there are needed welcome. At the same time, there’s much more to a battery than just the chemistry which gets so much attention. Is the multiple-tab approach viable in production? Do its benefits outweigh negative aspects, of any? Is it reliable for the long term or have any downsides?
These are all legitimate questions which need further investigation and are not easily answered. All we know for sure is that when it comes to most electronic components (and perhaps especially to batteries), there’s a long and challenging path from concept to lab, and from pilot run to full-scale production of reliable, cost-effective units. Unlike the mindset that guides the IC world, electrochemical devices such as batteries don’t scale easily in production volume and are not subject to Moore’s “law”.
- COMSOL, “Improving Tabbing Design in Cylindrical Batteries”
- Tech Briefs, “Fundamental EV Battery Models Explain New Tab Design”