Researchers discover a surprising way to improve battery performance
The very first charge of a lithium-ion battery is more important than it sounds. It determines how well and how long the battery will work from then on – specifically, how many cycles of charge and discharge the battery can handle before deteriorating.
In a study published in Joule, researchers at the SLAC-Stanford Battery Center report that giving batteries the first charge with unusually high currents extends their average lifespan by 50%, while reducing initial charge time from 10 hours to just 20 minutes .
Just as importantly, the researchers were able to use scientific machine learning to identify specific changes in the battery electrodes responsible for this increase in lifespan and performance – invaluable insights 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 reshape our energy future, leveraging the laboratory’s unique tools and expertise and partnerships with industry.
“This is an excellent example of how SLAC is doing manufacturing science to make critical technologies for the energy transition more affordable,” said Chueh. “We are solving a real challenge facing the industry; crucially we work with the industry from the start.”
This was the latest in a series of studies funded by TRI under a research agreement with the Department of Energy’s SLAC National Accelerator Laboratory.
The results have practical implications for the production of not only lithium-ion batteries for electric vehicles and the electric grid, but also for other technologies, said Steven Torrisi, a senior research scientist at TRI who collaborated on the study.
“This study is very exciting for us,” he said. “Battery production is extremely capital, energy and time intensive. It takes a long time to get production of a new battery up and running, and it is very difficult to optimize the production process because there are so many factors involved.”
Torrisi said the results of this study “demonstrate a generalizable approach to understanding and optimizing this critical 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” that is essential for battery performance
To understand what happens during the battery’s first cycle, Chueh’s team builds pouch cells in which the positive and negative electrodes are surrounded by an electrolyte solution in which lithium ions can move freely.
When a battery is charged, lithium ions flow to the negative electrode for storage. When a battery runs out, they flow back out and travel to the positive electrode; this creates a flow of electrons to power devices from electric cars to the electricity grid.
The positive electrode of a new battery is 100% filled with lithium, says Xiao Cui, the principal investigator of the battery informatics team in Chueh’s lab. Each time the battery goes through a charge-discharge cycle, some of the lithium is deactivated. Minimizing these losses extends battery life.
Curiously, one way to minimize overall lithium loss is to deliberately lose a large percentage of the initial lithium supply during the battery’s first charge, Cui said. It’s like making a small investment that will yield a good return over time.
This lithium loss in the first cycle is not in vain. The lost lithium becomes part of a soft layer called the solid electrolyte interphase or SEI, which forms on the surface of the negative electrode during the initial charge. In return, the SEI protects the negative electrode from side reactions that would accelerate lithium loss and degrade the battery more quickly over time. Getting the SEI just right is so important that the first charge is known as the formation charge.
“Formation 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.”
A high charging current improves battery performance
Manufacturers typically charge new batteries at low current for the first time, thinking this will create the most robust SEI layer. But there is a downside: charging at low current is time-consuming and expensive and does not necessarily produce optimal results. So when recent studies suggested that faster charging with higher currents doesn’t degrade battery performance, it was exciting news.
But 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 laboratory to see which one worked best is an overwhelming task.
To reduce the problem to a manageable size, the research team used scientific machine learning to identify which factors are most important for achieving good results. To their surprise, only two of these stood out from the rest: the temperature and the current at which the battery is charged.
Experiments confirmed that charging at high currents has 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 that turned out to have a positive effect.
Removing more lithium ions beforehand is a bit like scooping water from a full bucket before wearing it, Cui said. The extra free space in the bucket reduces the amount of water splashing out along the way. Similarly, deactivating more lithium ions during SEI formation frees up space 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 load 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 insight is crucial for finding the best balance between battery performance and production efficiency.”
This research was funded by the Toyota Research Institute through its Accelerated Materials Design and Discovery program.
Research report:Data-driven analysis of battery formation reveals the role of electrode use in extending cycle life