Volvo XC60 in snow

Lithium-ion battery parts crack under extreme cold

Lithium-ion batteries are notorious for their sluggishness in the cold, with consequences for some of their most important applications. Researchers at the US Department of Energy’s SLAC National Accelerator Laboratory have shed light on how extreme cold can crack battery materials, degrading performance.

Although lithium batteries can withstand the cold better than some others, extreme temperatures hamper their ability to store and release energy. These concerns can, for instance, affect the willingness of a consumer to buy an EV rather than a car with an internal combustion engine.

When a lithium battery is being discharged, lithium ions travel from the graphite anode to the lithium cobalt oxide cathode via the movement of electrons through an external circuit. This process is reversed during charging. This electrochemical reaction slows down at low temperatures.

While this issue is frequently mentioned and has received much attention from researchers and manufacturers, the significant irrevocable capacity loss in cold weather is not well understood. This cannot be resolved by measures such as the battery preheating approach populating in EVs.

Now, SLAC scientist Yijin Liu and postdoctoral researcher Jizhou Li have elucidated this poorly understood problem: storing these batteries at below-freezing temperatures can crack parts of the battery and separate them from the surrounding materials, reducing their capacity to store energy.

They discovered this problem when investigating the cold-weather performance of the cathode; previous studies had shown that storing cathodes at sub-zero temperatures cause batteries to lose up to five per cent more of their capacity after 100 charges than batteries stored at warmer temperatures. To understand why, they combined machine learning with X-ray analysis at SLAC’s Stanford Synchrotron Radiation Lightsource. This allowed them to identify individual particles in the cathode, allowing them to study thousands at once, rather than the few they could identify by eye.

Liu explained that cold temperatures were causing the “meatball-like” particles within the cathode and cracking them in the process, or making existing cracks worse. As materials differ in the way they expand and contract in response to temperature changes, extreme cold was causing the cathodes to detach from surrounding materials.

Their results point to some possible fixes to improve lithium battery capacity, Liu said. Scientists could address the detachment issue by searching for battery materials better matched to the lithium cobalt oxide cathode in terms of temperature response. By engineering different particle structures inside a battery – notably, building them from smoother particles which are “less meatball like” – this could help prevent cracking.

This has the potential to improve other types of batteries, too, as all batteries expand and contract to a certain extent with temperature changes.

“Beside developing electrolytes that have stable performance, designing batteries for use in a wide temperature range also calls for the development of electrode components that are structurally and morphologically robust when the cell is switched off between different temperatures,” the researchers wrote.

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