Researchers discover surprising method to increase battery performance

The very first charge of a lithium-ion battery is more significant than it sounds. It determines how well and how long the battery will perform from then on – in particular, how many charge and discharge cycles it can withstand before it wears out.

In a study published today in JouleResearchers at SLAC-Stanford Battery Center report that by giving batteries unusually high currents for the first time, they increased their average battery life by 50%, while also reducing the initial charging time from 10 hours to just 20 minutes.

Just as importantly, using scientific machine learning, the researchers were able to identify specific changes in the battery electrodes that are responsible for this longer lifespan and higher performance. These insights are invaluable for battery manufacturers looking to optimize 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, 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 critical technologies for the energy transition more affordable,” said Chueh. “We are solving a real challenge facing industry. It is critical that we partner with industry from the start.”

The results have practical implications not only for the production of lithium-ion batteries for electric vehicles and the power grid, but also for other technologies, said Steven Torrisi, a senior scientist at TRI who worked on the research.

“This study is very exciting for us,” he said. “Battery manufacturing is extremely capital, energy and time intensive. It takes a long time to ramp up production of a new battery and it’s really difficult to optimize the manufacturing process because so many factors come into play.”

Torrisi said the results of this research “demonstrate a generalizable approach to understanding and optimizing this critical step in battery manufacturing. In addition, we may be able to transfer what we learn to new processes, equipment, devices and battery chemistries in the future.”

A “soft layer” that is crucial for battery performance

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

When a battery is charged, lithium ions flow into the negative electrode for storage. When a battery is discharged, they flow back out and migrate to the positive electrode. This triggers a flow of electrons that powers devices, from electric cars to the power grid.

The positive electrode of a newly manufactured battery is 100 percent filled with lithium, says Xiao Cui, the lead researcher of the battery informatics team in Chueh’s lab. Each time the battery is charged and discharged, some of the lithium is deactivated. Minimizing these losses extends the battery’s lifespan.

One way to minimize the overall loss of lithium, oddly enough, is to intentionally lose a large percentage of the original lithium supply when the battery is first charged, Cui said. It’s like a small investment that will yield good returns later.

This loss of lithium in the first cycle is not for nothing. The lost lithium becomes part of a spongy layer called the solid electrolyte interphase (SEI) that forms on the surface of the negative electrode during the first charge. In turn, the SEI protects the negative electrode from side reactions that would accelerate lithium loss and wear out the battery faster over time. Getting the SEI right is so important that the first charge is called the formation charge.

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

High charging current increases battery performance

Manufacturers generally charge new batteries at low currents the first time, believing that this will create the most robust SEI layer. But there is a downside to this: charging at low currents is time-consuming and expensive, and does not necessarily produce optimal results. So recent studies suggesting that faster charging at higher currents does not affect battery performance were exciting news.

But the researchers wanted to dig deeper. Charging current is just one of dozens of factors that play a role in the formation of SEI during the initial charge. Testing all possible combinations of them in the lab to find out which one works best is an overwhelming task.

To reduce the problem to a manageable level, the research team used scientific machine learning to figure out which factors were most important for achieving good results. To their surprise, only two of them – the temperature and the current used to charge the battery – stood out from all the others.

Experiments confirmed that high-current charging had a huge impact, extending the life of the average test battery by 50%. It also deactivated a much higher percentage of lithium beforehand – about 30% compared to 9% with previous methods – but this proved to be a positive effect.

Removing more lithium ions up front is a bit like scooping water out of a full bucket before carrying it, Cui said. The extra headroom in the bucket reduces the amount of water that splashes out along the way. Similarly, deactivating more lithium ions during formation frees up headroom in the positive electrode and allows the electrode to cycle more efficiently, improving subsequent performance.

“Brute-force optimization through trial and error is routine in manufacturing – how should we do the first charge and what combination of factors is best?” said Chueh. “Here, we didn’t just want to find 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.”