Understanding Battery Safety
June 17, 2024
As lithium-ion batteries have become ubiquitous, their safety risks have increasingly been a focus of public concern.
EV Battery Cell Formats for Lithium Metal
January 16, 2024
EV Battery Cell Formats for Lithium Metal
January 16, 2024
You can find lithium-ion batteries in everything from electric vehicles to mobile phones. But, different applications have different requirements when it comes to the characteristics of the battery format, and EVs are a particularly challenging use case. There are several ways to package EV battery cells – cylindrical, prismatic and pouch. Each offers unique tradeoffs, and there is currently no clear winner among the three alternatives.
Each of these distinct designs has its benefits, but also particular drawbacks, especially when it comes to batteries using next-generation solid-state lithium-metal technology. The most crucial difference between a lithium-metal cell and a conventional lithium-ion battery is that the cell expands as lithium plates directly on the separator of a lithium-metal cell. As such, the overall cell is thicker when fully charged. This expansion is particularly pronounced in an anode-free design like QuantumScape’s, and since it is greater than what conventional lithium-ion cells experience, it presents a challenge in selecting an appropriate cell format.
In this blog, we’ll review the benefits and drawbacks of today’s three major EV battery cell designs and examine the suitability of each for solid-state lithium-metal technology.
Battery Format Comparison1
Cylindrical
Prismatic
Pouch
Design
Rolled layer
Stacked/folded layer(s)
Stacked layers
Pros
- Highly manufacturable
- Good energy density
- Fits well in an EV pack
- Thermal management
- Low-cost material
- Good energy density
Cons
Wasted space in pack
- More challenging to manufacture
- Poor energy density
- Fragile; safety risks
- Requires module
Lithium metal?
No
Yes, with drawbacks
Yes, with drawbacks
Cylindrical
The cylindrical battery format is probably the most familiar and is found in everything from laptops to some of the most popular EVs on the market. They’re typically referred to by their physical dimensions: for example, the 2170 cell currently used in many top-selling EVs is 21 millimeters wide and 70 millimeters long. The cell consists of an anode layer, plastic separator, and cathode layer, manufactured as long sheets rolled up and inserted into a metal tube, which is then filled with a liquid electrolyte.[1] Although there are many subtleties to the design and construction of cylindrical cells, the basic concept is pretty straightforward.
Sony first popularized the design in the 1990s when it was launched in handheld camcorders.[2] As such, the design is not a natural fit for electric vehicles. When placed in an EV pack, the cylindrical shape means there will always be space between the individual cells, reducing the energy density of the pack and the range of the vehicle. Some of this empty space can be used for cooling in modern EVs, but there is still wasted space.
Despite these drawbacks, the cylindrical cell is still prevalent due to some of its unique advantages. For example, as lithium-ion batteries age, they produce gas that causes pressure to build up inside the cell. A cylindrical shape is more mechanically robust than a box shape and can withstand this internal pressure using thinner metal walls, giving cylindrical cell designs a slight boost in energy density compared to other shapes.
Is it possible to make cylindrical cells with lithium-metal anodes? We don’t believe so. As lithium plates on the anode, the anode layer expands significantly; if the layers are rolled up, this expansion will cause them to try to unroll, putting stress on the battery’s internal structure and damaging it. Ultimately, we don’t believe cylindrical formats are suitable for lithium-metal battery cells.
Prismatic
Despite what the name might suggest, the prismatic battery cell is essentially a rectangular metal box. The individual layers of the battery are either stacked like a deck of cards or wound up and then pressed flat to fit into the cell casing. Prismatic cells are particularly useful in EVs because they can be packed tightly next to each other without wasting space, giving them excellent packing efficiency.
For prismatic cells, cooling is often done through the bottom of the pack, with the metal casing serving as a heat sink, absorbing the heat and carrying it down toward the cooling element. However, tightly packed cells make it inherently more challenging to keep the batteries cool while fast charging, especially if they are large – for example, prismatic cells with energy storage capacities above 50 amp-hours (Ah) versus 4.8 Ah in a standard cylindrical cell.4 The metal case serves as a heat sink, allowing the heat to drain away from the cell.
Prismatic cells have other challenges, too. For example, because the box shape is less rigid than a cylinder, the walls of the cells must be thicker, which adds weight and cost to the cell. The top of a prismatic cell (known as the header) is also relatively complex to assemble, adding cost and time to manufacturing.
When it comes to lithium-metal battery cells, the prismatic format is suitable but with caveats. The layers of a lithium-metal cell can be stacked flat in a prismatic format. However, as the anode layers expand, something else inside the cell, such as a spring or foam, would have to be present to accommodate this expansion. Since this mechanism would sit between the layers and the metal casing, it would partially block heat transfer from the interior, limiting fast-charging performance, and taking up extra space, hurting energy density.
Pouch
The last type of cell may be the simplest. A pouch cell is a series of anode, separator and cathode layers stacked on top of one another and encased in a flexible laminate material (often called a pouch) made of plastic and aluminum.5 Pouch cells have several advantages. First, because there is no hard casing surrounding the cells, they can offer good cell-level energy density and keep costs low. The pouch material can stretch to accommodate the swelling that legacy lithium-ion batteries experience over their lifetime. In addition, they’re made of inexpensive materials and don’t contain many complex parts, like prismatic cells, which means they can be manufactured very rapidly.
However, they also have drawbacks. The pouches themselves are not rigid, and the material is easily damaged, which presents a safety risk in conventional lithium-ion cells. In an EV pack, this means they must be protected by a more robust (and heavier) module structure, thus offsetting the energy density advantage of the cells. The module must also serve the purpose of pulling heat out of the cells and containing the swelling of the cells over time. The pouch simplifies cell design by offloading some of the mechanical requirements from the cell level to the module level but at the expense of a heavier and more complicated module design. Contrast this with prismatic or cylindrical cells, which are more self-sufficient and, in some cases, even enable cell-to-pack integration that can eliminate the module entirely.
For a lithium-metal system, pouch cells have similar drawbacks to prismatic cells. Any mechanism at the module level designed to expand and contract as the pouch cells charge and discharge would be a less-than-ideal heat sink, increase cost and complexity, add weight and take up precious space. Thermal management is especially important for batteries to offer repeated 15-minute fast charging, as the heat generated can be significantly greater than at lower charge rates used with current batteries. We don’t believe a pouch cell is the best choice for lithium-metal EV batteries.
The QuantumScape solution
QuantumScape’s lithium-metal battery technology platform has the potential to significantly improve the performance of battery electric vehicles and beyond. However, packing this next-generation technology into real-world vehicles presents some unique challenges that the three existing formats don’t address.
So, we developed a proprietary cell format, FlexFrame. It features a frame that wraps around the edge of the cell stack and a flexible polymer outer layer and is designed to accommodate the uniaxial expansion and contraction that lithium-metal batteries experience during charge and discharge. The cell is designed to dissipate excess heat during fast charging and deliver good packaging efficiency, enabling our technology to achieve our cell-level energy density targets.
When fully discharged, the cell stack is in its most contracted position, with the face of the cell sitting about one millimeter below the frame. As it charges and the anodes of each layer are plated with pure lithium metal, they push the faces of the cell out, along with the flexible packaging material. When fully charged, the face of the cell is designed to be almost flush with the frame.
We believe this purpose-built approach can unlock the greatest potential to truly deliver on the promise of a high-performance lithium-metal battery cell. To learn more on this innovative design, read our article Introducing FlexFrame.
1For cell energy density and pack-level efficiency, ratings are derived from World Electr. Veh. J. 2020, 11(4), 77; https://doi.org/10.3390/wevj11040077. For manufacturability, the lower rating of the prismatic cell is due to the complex header that connects the electrode tabs to the cell terminals. For thermal management, the extra space between cylindrical cells in a battery pack provides an advantage in thermal management. For all cell formats, the exact characteristics depend heavily on factors such as the particular battery chemistry, cell design decisions (i.e., energy versus power cells), and overall pack construction. The description here reflects a simplification and is inherently subjective. It is intended for educational purposes only.
2Song, Y., Lu, B., Ji, X., & Zhang, J. 2012. Diffusion Induced Stresses in Cylindrical Lithium-Ion Batteries: Analytical Solutions and Design Insights. https://doi.org/10.1149/2.079212jes
3George E. Blomgren. 2017. The Development and Future of Lithium Ion Batteries. J. Electrochem. Soc. 164 A5019. https://iopscience.iop.org/article/10.1149/2.0251701jes
4Jason B. Quinn et al. 2018. Energy Density of Cylindrical Li-Ion Cells: A Comparison of Commercial 18650 to the 21700 Cells. J. Electrochem. Soc. 165 A3284. https://iopscience.iop.org/article/10.1149/2.0281814jes
5Yoo S, Hong C, Chong KT, Seul N. 2019. Analysis of Pouch Performance to Ensure Impact Safety of Lithium-Ion Battery. Energies. 12(15):2865. https://doi.org/10.3390/en12152865
Forward-Looking Statements
This article contains forward-looking statements within the meaning of the federal securities laws and information based on management’s current expectations as of the date of this current report. All statements other than statements of historical fact contained in this article, including statements regarding the future development of QuantumScape’s battery technology, the anticipated benefits of QuantumScape’s technologies and the performance of its batteries, and plans and objectives for future operations, are forward-looking statements. When used in this current report, the words “may,” “will,” “estimate,” “pro forma,” “expect,” “plan,” “believe,” “potential,” “predict,” “target,” “should,” “would,” “could,” “continue,” “believe,” “project,” “intend,” “anticipates” the negative of such terms and other similar expressions are intended to identify forward-looking statements, although not all forward-looking statements contain such identifying words.
These forward-looking statements are based on management’s current expectations, assumptions, hopes, beliefs, intentions, and strategies regarding future events and are based on currently available information as to the outcome and timing of future events. These forward-looking statements involve significant risks and uncertainties that could cause the actual results to differ materially from the expected results. Many of these factors are outside QuantumScape’s control and are difficult to predict. QuantumScape cautions readers not to place undue reliance upon any forward-looking statements, which speak only as of the date made. Except as otherwise required by applicable law, QuantumScape disclaims any duty to update any forward-looking statements. Should underlying assumptions prove incorrect, actual results and projections could differ materially from those expressed in any forward-looking statements. Additional information concerning these and other factors that could materially affect QuantumScape’s actual results can be found in QuantumScape’s periodic filings with the SEC. QuantumScape’s SEC filings are available publicly on the SEC’s website at www.sec.gov.
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. In this blog article, we’ll look at battery safety testing and do a deep dive into a couple of key test results from energy-dense 24-layer QuantumScape prototype cells based on our Alpha-2 design.
Understanding Battery Safety
June 17, 2024
As lithium-ion batteries have become ubiquitous, their safety risks have increasingly been a focus of public concern.
Interpreting QuantumScape’s Safety Test Results
June 17, 2024
. In this blog article, we’ll look at battery safety testing and do a deep dive into a couple of key test results from energy-dense 24-layer QuantumScape prototype cells based on our Alpha-2 design.
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