Researchers Discover Surprising Method to Improve Battery Performance

The first charge of a lithium-ion battery is more important than it seems. It determines the quality and operating life of the battery, specifically the number of charge and discharge cycles it can withstand before deteriorating.

In a study published today in JouleResearchers at the SLAC-Stanford Battery Center report that giving the batteries this first charge at unusually high currents increased their average lifespan by 50 percent while reducing the initial charge time from 10 hours to just 20 minutes.

Equally important, the researchers were able to use scientific machine learning to identify specific changes in the battery electrodes that account for this increase in lifespan and performance – valuable information for battery manufacturers looking to streamline their processes and improve their products.

The study was conducted by a SLAC/Stanford team led by Professor Will Chueh in collaboration with researchers from the Toyota Research Institute (TRI), the Massachusetts Institute of Technology and the University of Washington. It is part of SLAC’s sustainability research and a broader effort to reimagine our energy future by leveraging the lab’s unique tools and expertise and partnerships with industry.

“This is a great example of how SLAC is using manufacturing science to make technologies critical to the energy transition more affordable,” Chueh said. “We are solving a real challenge that industry is facing; it is critical that we partner with industry from the start.”

The findings have practical implications for making not only lithium-ion batteries for electric vehicles and the power grid, but also for other technologies, said Steven Torrisi, a principal investigator at TRI who collaborated on the research.

“This study is very exciting for us,” he said. “Battery manufacturing is extremely expensive in terms of capital, energy and time. It takes a long time to get a new battery up and running, and it’s very difficult to optimize the manufacturing process because there are so many factors involved.”

Torrisi said the results of this research “demonstrate a generalizable approach to understanding and optimizing this crucial step in battery manufacturing. Furthermore, we may be able to transfer what we have learned to new processes, facilities, equipment and battery chemistries in the future.”

A “soft layer” essential to battery performance

To understand what happens during the initial battery cycle, Chueh’s team builds pouch cells in which the positive and negative electrodes are surrounded by an electrolyte solution in which lithium ions move freely.

When a battery charges, lithium ions flow into the negative electrode for storage. When a battery discharges, they flow back out and toward the positive electrode, triggering a flow of electrons to power devices from electric cars to the electrical grid.

The positive electrode of a newly manufactured battery is filled with 100 percent lithium, said Xiao Cui, a principal investigator in the battery computing team at Chueh’s lab. Each time the battery goes through a charge-discharge cycle, some of the lithium is deactivated. Reducing these losses extends the battery’s life.

Interestingly, one way to minimize overall lithium loss is to deliberately lose a large percentage of the initial lithium reserve when the battery is first charged, Cui explained. It’s like making a small investment that pays off in the long run.

This first-cycle lithium loss is not in vain. The lost lithium is part of a soft layer called the solid electrolyte interphase, or SEI, that forms on the surface of the negative electrode during the first charge. In turn, the solid electrolyte interphase protects the negative electrode from side reactions that would accelerate lithium loss and degrade the battery more quickly over time. Getting the SEI right is so important that the first charge is known as a formation charge.

“Forming is the final step in the manufacturing process,” Cui said, “so if it fails, all the value and effort invested in the battery up to that point is wasted.”

High charging current improves battery performance

Manufacturers typically charge new batteries with low currents for the first time, assuming that this will create the most robust SEI layer. But there’s a downside: Low-current charging is time-consuming and expensive, and doesn’t necessarily yield optimal results. So when recent studies suggested that faster charging with higher currents doesn’t degrade battery performance, it was exciting news.

But the researchers wanted to go further. Charging current is just one of dozens of factors that come into play in the formation of the SEI during the first charge. Testing every possible combination of these factors in the lab to see which one works best is an overwhelming task.

To reduce the problem to a manageable size, the research team used scientific machine learning to identify the most important factors for achieving good results. To their surprise, only two of them – temperature and battery charging current – ​​stood out above all others.

The experiments confirmed that high-current charging has a significant impact, increasing the average test battery life by 50%. It also disabled a much higher percentage of upstream lithium (about 30%, compared to 9% with previous methods), but this turned out to have a positive effect.

Removing more lithium ions upstream is a bit like scooping water from a full bucket before transporting it, Cui explains. The extra free space in the bucket reduces the amount of water that splashes around along the way. Similarly, deactivating more lithium ions during formation frees up free space in the positive electrode and allows the electrode to operate more efficiently, improving subsequent performance.

“Brute-force optimization through trial and error is a common practice in manufacturing: How should we do the first charge and what is the winning combination of factors?” Chueh said. “Here, we didn’t just want to identify the best recipe for making a good battery; we wanted to understand how and why it works. This understanding is critical to finding the best balance between battery performance and manufacturing efficiency.”