Research at the SLAC-Stanford Battery Center found that high-current charging of lithium-ion batteries just before they leave the factory is 30 times faster and can extend battery life battery life by 50%.
The first charge of a lithium-ion battery is more important than it seems. It determines how well and how long the battery will work from that point on – specifically, how many charge and discharge cycles it can withstand before it breaks down.
In a study published in Joule, researchers SLAC-Stanford Battery Center report that charging batteries for the first time with extremely high currents increased their average life by 50% and reduced the initial charge time from 10 hours to 20 minutes.
Equally important, the researchers were able to use scientific machine learning to pinpoint the specific changes in the battery electrodes that account for this increase in life and performance. This is invaluable insight for battery manufacturers looking to optimize their processes and improve their products.
The study was conducted by a SLAC/Stanford University team led by Professor Will Chue in collaboration with researchers from the Toyota Research Institute (TRI), the Massachusetts Institute of Technology and the University of Washington. This is part of SLAC's sustainability research and broader efforts to reimagine our energy future using the lab's unique tools and expertise, as well as partnerships with industry.
"This is a great example of how SLAC is doing manufacturing science, to make critical technologies for the energy transition more accessible», — Chue said. "We are solving a real problem facing the industry; It is important that we cooperate with the industry from the very beginning.
It was the latest in a series of studies funded by TRI under a collaborative research agreement with the Department of Energy's SLAC National Accelerator Laboratory.
The results have practical implications not only for the production of lithium-ion batteries for electric vehicles and electric grids, but also for other technologies, said Stephen Torrisi, a senior scientist at TRI who participated in the study.
"This study is very exciting for us", — he said. "The production of batteries requires large capital investments, energy and time. It takes a long time to get a new battery into production, and it's very difficult to optimize the manufacturing process because there are so many factors involved.
Torrisi said the results of this study "demonstrate a generalized approach to understanding and optimizing this important step in battery production. In addition, we may be able to transfer what we have learned to new processes, facilities, equipment and battery chemistries in the future.
"Which layer" is key to battery performance
To understand what happens during the initial battery cycle, Chueh's team creates packet cells in which the positive and negative electrodes are surrounded by an electrolyte solution where ions lithium move freely.
As the battery charges, lithium ions flow to the negative electrode for storage. When the battery discharges, they flow back and move towards the positive electrode; this triggers a flow of electrons to power devices, from electric cars to the grid.
The new battery's positive electrode is 100 percent filled with lithium, said Xiao Cui, a lead researcher in the battery informatics group in Chue's lab. Every time a battery goes through a charge-discharge cycle, some of the lithium is deactivated. Minimizing these losses extends battery life.
Ironically, one way to minimize the overall loss of lithium — is to intentionally lose a large percentage of the initial lithium supply when the battery is first charged, Tsui said. It's like making a small investment that will bring good profits later.
This loss of lithium in the first cycle is not in vain. The lost lithium becomes part of a soft layer called the solid electrolyte interface, or SEI, that forms on the surface of the negative electrode during the first charge. Instead, SEI protects the negative electrode from side reactions that can accelerate lithium loss and drain the battery more quickly over time. Correct SEI is so important that the first charge is known as the forming charge.
«Forming is the final step in the manufacturing process, — said Tsui, — therefore, if it fails, all the value and effort invested in the battery up to this point will be wasted».
High charging current increases battery performance
Manufacturers usually give new batteries their first charge with low currents, believing that this will create the most reliable SEI layer. But there is also a drawback: charging at low currents takes a long time and is expensive, and does not necessarily give optimal results. So when recent research showed that faster charging with higher currents didn't hurt battery performance, it was exciting news.
But researchers wanted to dig deeper. The charging current is only one of dozens of factors involved in SEI formation during the first charge. Testing all possible combinations in the lab to see which one works best is a daunting task.
To reduce the problem to a manageable size, the research team used scientific machine learning to determine which factors were most important for achieving good results. To their surprise, only two of them – temperature and amperage with which the battery is charged – stood out among all the others.
Experiments have shown that high current charging has a huge impact, increasing the life of the average test battery by 50%. It also deactivated a much higher percentage of the lithium at the beginning – about 30%, compared to 9% in previous methods – but it turned out to be a positive effect.
Removing more lithium ions from the front is like scooping water from a full bucket before carrying it, Tsui said. The extra space in the bucket reduces the amount of water splashed on the road. Similarly, the deactivation of more lithium ions during SEI formation frees up space in the positive electrode and allows the electrode to operate more efficiently, improving subsequent performance.
"Brute force optimization by trial and error is commonplace in manufacturing. How should we perform the first charge and what combination of factors is the winning one?» — Chue said. "Here, we didn't just want to determine the best recipe for making a quality 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.