Extend Cell Life with Customized Compression Properties

Like muscle fibers powering human movement, lithium-ion cells in modern EV batteries are the engines that propel electric vehicles. And just like muscle fibers, lithium-ion cells undergo expansion and contraction cycles, the so-called breathing of the cells. This is a reversible dimensional change during a charge/discharge cycle caused by lithium ions moving back and forth between anode and cathode.

When muscle contraction works against resistors, the muscle tends to increase volume over time. While this mechanism is often wanted as an adaptation to training, engineers try to avoid or minimize a similar mechanism for battery cells. Unfortunately for engineers, the expansion of cells is not fully reversible — after many charging/discharging cycles, the cell volume and thickness increase irreversibly, the so-called swelling. Several factors can contribute to swelling including solid electrolyte interface (SEI) growth, lithium plating, gas formation, etc. This cell swelling is a concern for the cycle life, safety and performance of today’s lithium-ion batteries as it leads to cell aging and capacity degradation over time.

LIB cell breathing and swelling cannot be fully avoided today. However, by applying pressure on the cells, not only can the cell expansion and swelling be reduced, but also their performance can be maximized which results in extended cell lifespan. Not enough pressure has limited effects and too high pressure can damage cells, reduce their performance and cause safety risks. Thus, finding the ideal compression force for specific battery designs is important.

Weilin, what is meant by customized compression properties of compression pad and how do they differ from standard properties?

To find the ideal pressure on LIB cells, cell chemistry and the specific design of battery module/battery pack need to be considered. There is no “one-size-fits-all" solution. After analyzing the specific application requirements, we can start optimizing material properties in different ways. By changing material formulations, e.g., adjusting resin formulation, varying crosslinking density, or using different filler types and loading, the material can be tuned and tailored to certain extents.

Together with innovative material design such as the combination of different materials, varying geometries and material of corrugated or patterned structure, microstructure of polymer foam (e.g. open vs closed cell, pore size, wall thickness, etc.), and other parameters, standard properties can be modified to achieve significant advantages through customized solutions.

What are the benefits of customized compression properties?

The most important benefits of customized compression solutions are that these materials fulfill the requirements of the application best and allow for maximized performance of the cells, reduced cell swelling through exactly defined pressure and therefore extended lifetime of the battery pack with high power output even towards the end of life of the battery.

Customization does not necessarily mean developing the most expensive materials. It is about finding the right materials which can also help avoiding over-engineered solutions.

How does Tape Solutions fulfill these customization requests?

Of course, I cannot go into the details here but through material characterization testing related to EV battery application, we identify which materials from our broad portfolio would fit best as a start. With accurate mechanical models, a digital twin can be developed.

Sophisticated digital tools help to recommend “ideal” material and thickness based on a specific battery module design to achieve the target compression properties.

What is a digital twin and which added value does that add for customers?

A digital twin allows for more efficient material screening by emulating the application in digital simulations. This helps to explore large design space such as vary material type and thickness, recommend optimal material and thickness without the need for physical tests at this stage.

These tools help to identify suitable options faster that can then be further processes for final designs.

This offers significant advantages as it saves time and costs for testing in the laboratories and quick turnaround in RFI/RFQ processes. By verifying simulated results with testing actual materials in our state-of-the-art research and development centers in-house, we ensure customers benefit from our EV know-how and fast solutions.

Can you provide examples of applications where customized compression properties are essential for extending cell life?

Of course. For pouch and prismatic cells with liquid electrolyte, customized compression is important as repeated expansion and contraction of the electrode material will cause separation among their stacking layers if no pressure is applied. Tailored compression can also minimize the risk of high local current density and accelerated side reactions in the cell. On the other hand, if the pressure is too strong, the crystal structure of electrode materials can be distorted, SEI layers could be damaged and ion transport paths could be hindered.

Therefore, optimal pressure helps to ensure good contact between battery components (anode, cathode, separator) and to prevent excessive swelling and thus avoid potential delamination, while it does not lead to battery component failure.

In lithium metal anode-based batteries, customized pressure can suppress the growth of lithium dendrites (needle-like structures) that could cause internal short circuits and safety hazards. Customized cell pressure helps mitigate this risk.

In solid-state-battery (SSB) applications, customized pressure helps to ensure adequate contact between solid-state electrolyte (SSE) and electrode to establish low-impedance interfacial connection for better Coulombic efficiency. However too strong compression forces can lead to detrimental effects such as SSE structural fractures and premature short circuits.

Thank you very much Weilin for helping to understand how customized compression materials can be designed to resist permanent deformation (compression set) and continue providing tailored support throughout the battery cell's life which can ensure battery cells to function optimally over time.