How do lithium-ion batteries age
Why do lithium-ion batteries age?
When it comes to mobile energy supply, nothing works without lithium-ion batteries. Despite numerous attempts, the batteries age more or less quickly. Scientists suspect that this could be due to the loss of lithium during charging cycles.
Examination of lithium-ion batteries in the laboratory.
Photo: David Baillot / UC San Diego Jacobs School of Engineering
Lithium-ion batteries have established their place in electromobility and portable electronic systems. However, their capacity deteriorates both in storage without use and in charging and discharging. It was unclear why that was so far. The assumption is that irreversible chemical reactions occur.
A research team from the University of California (UC) in San Diego has found out why batteries deteriorate in efficiency over time. With its research, it questions established theories and hopes for better batteries for various applications in the medium term.
Electrode failure theory is challenged
The main problem with lithium-ion batteries is known to be their low coulomb efficiency: they go through a limited number of cycles before they no longer function. This is because the supplies of active lithium and electrolyte are used up over the course of the battery cycle.
Battery researchers have so far explained this with a growing solid electrolyte interphase (SEI) layer between the anode and the electrolyte. "Although experts have developed various methods to control and stabilize the SEI layer, they still have not solved the problems with lithium metal batteries," says Ying Shirley Meng. She is a professor of nanotechnology at UC San Diego. Meng: “The cells are still failing because a lot of inactive lithium is being formed in batteries. So there is another important aspect that is being overlooked. "
New measuring method exposes processes on electrodes
Your knowledge is based on new attempts. Meng found that lithium metal deposits break off the anode when discharged and then become entangled in the SEI layer. There they lose their electrical connection to the anode and become inactive lithium. The metal is no longer available for electrochemical processes. “This lithium is mainly responsible for lowering the coulomb efficiency of the cell,” says Meng. With her work she questions the leading SEI hypothesis.
Meng's team developed a simple but precise method to measure how much unreacted lithium metal is deposited as inactive lithium. The researchers put normal water and a sample of inactive lithium from older lithium-ion batteries in a sealed flask. The chemical reaction produces hydrogen. By measuring gas production, researchers can calculate the amount of lithium metal trapped.
Inactive lithium as a key factor in aging
These experiments revealed that unreacted lithium metal is the main component of inactive lithium. The more of it is formed, the lower the Coulomb efficiency. Inactive lithium also consists of another component: lithium ions, which can be found as building blocks in the SEI layer. Their number can be calculated simply by subtracting the amount of unreacted lithium metal from the total amount of inactive lithium. There were no differences in the lithium ions; the metal turned out to be decisive. Such results were observed with 8 different electrolytes: a criterion for transferability into practice.
"This is an important finding as it shows that the primary failure product of lithium metal batteries is unreacted metallic lithium instead of the SEI," said Meng. "With our method, we were able to quantify both components with the greatest possible accuracy, which was not possible before."
Electrodes under the microscope
By examining the micro and nanostructures of lithium deposits in various electrolytes, the researchers answer another question: Why do some electrolytes significantly improve Coulomb efficiency, but others do not? The answer has to do with how lithium deposits on the anode when the cell is being charged. Some electrolytes cause lithium to create micro and nanostructures that support further processes.
For example, in an electrolyte developed by General Motors, the metal forms dense, columnar microstructures. This led to minimal losses of lithium during electrode processes - and to a Coulomb efficiency of 96% in the first charge cycle. “This excellent performance is due to the microstructure that has formed on the surface, which greatly improves the structural bond,” says Mei Cai of General Motors.
In contrast, when using a commercially available carbonate electrolyte, lithium separated out as fine, hair-like structures. During the charge and discharge cycles, more lithium was trapped in the SEI - and the coulombic efficiency decreased to 85%.
Lessons for Industry
What do Meng's findings bring in practice? The researcher suggests developing strategies to better control the deposition and loss of lithium in electrochemical cells. This includes applying pressure to electrodes, but also creating even layers of SEI. “The control of micro and nanostructures is a key in the entire process,” explains Meng. "We hope that our findings will provide impetus to take rechargeable lithium metal batteries to the next level."
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A contribution by:
Michael van den Heuvel
Michael van den Heuvel studied chemistry. Among other things, he works for Medscape, DocCheck, for the University of Munich and for specialist pharmaceutical magazines. He has been a freelance journalist and shareholder of Content Qualitäten since 2017. His topics: chemistry / physical chemistry, energy, environment, AI, medicine / medical technology.
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