Researchers from Technische Universitӓt München, Forschungszentrum Jülich and RWTH Aachen University have developed a new technique that provides a unique insight into how the charging rate of lithium-ion batteries could be limiting their lifetime and safety.
Lithium-ion batteries are behind a revolution in clean transport and high-end consumer electronics. There is still plenty of improving to do as far as their charging time. Currently, reducing charging time is done by increasing the charging current, which in turn compromises battery lifetime and safety.
"The rate at which lithium ions can be reversibly intercalated into the graphite anode, just before lithium plating sets in, limits the charging current," explains Johannes Wandt, Ph.D., of Technische Universität München (TUM).
Lithium-ion batteries have a positively charged transition metal oxide cathode and a negatively charged graphite anode in a liquid electrolyte. During charging, lithium ions move from the cathode to the anode. But if the charging rate is too high, lithium ions deposit as a metallic layer on the surface of the anode, rather than being inserted into the graphite.
"This undesired lithium plating side reaction causes rapid cell degradation and poses a safety hazard," Dr. Wandt added.
The team wanted to develop a tool that detects the amount of lithium plating on a graphite anode in real-time. This research resulted in a technique called operando electron paramagnetic resonance (EPR).
"The easiest way to observe lithium metal plating is by opening a cell at the end of its lifetime and checking visually by eye or microscope," said Dr. Wandt. "There are also nondestructive electrochemical techniques that give information on whether lithium plating has occurred during battery charging."
Neither of these approaches provides much information about the onset of lithium metal plating or the amount of lithium metal present during charging. EPR detects the magnetic moment associated with unpaired conduction electrons in metallic lithium with high sensitivity and time resolution on the order of a few minutes, or even seconds.
"In its present form, this technique is mainly limited to laboratory-scale cells, but there are a number of possible applications," explains Dr. Josef Granwehr of Forschungzentrum Jülich and RWTH Aachen University. "So far, the development of advanced fast charging procedures has been based mainly on simulations but an analytical technique to experimentally validate these results has been missing. The technique will also be very interesting for testing battery materials and their influence on lithium metal plating. In particular, electrolyte additives that could suppress or reduce lithium metal plating."
Dr. Wandt says that fast charging for electric vehicles could be a key to the application that would benefit from further analysis of the work.
But until now, there has not been an analytical technique that can directly determine the maximum charging rate. This is a function of the state of charge, temperature, electrode geometry and other factors before the lithium metal plating even starts. The EPR technique could provide a needed experimental validation of frequently used computational models, as well as investigating the effect of new battery materials and additives on lithium metal plating.
The researchers are now working with other teams to benchmark their experimental results against numerical simulations of the plating process in simple model systems.
"Our goal is to develop a toolset that facilitates a practical understanding of lithium metal plating for different battery designs and cycling protocols," explains Dr. Rüdiger-A. Eichel from Forschungzentrum Jülich and RWTH Aachen University.
The paper on this research was published in ScienceDirect.