A new method developed by scientists at RWTH Aachen University, Forschungszentrum Jülich, and Technische Universität München provides a unique insight into how the lithium ion batteries’ charging rate can be a factor affecting their safety and limiting their lifetime.
Although advanced lithium ion batteries are powering a revolution in high-end consumer electronics and clean transport, there is still plenty of room for improving charging time. At present, increasing the charging current for reducing charging time compromises safety and battery lifetime.
“The rate at which lithium ions can be reversibly intercalated into the graphite anode, just before lithium plating sets in, limits the charging current,” describes Johannes Wandt, PhD, of Technische Universität München (TUM).
Lithium ion batteries comprise of a negatively charged graphite anode and a positively charged transition metal oxide cathode in liquid electrolyte. Lithium ions move from the cathode (deintercalate) to the anode (intercalate) during charging. However, if the charging rate is very high, lithium ions deposit as a metallic layer on the anode’s surface instead of inserting themselves into the graphite. “This undesired lithium plating side reaction causes rapid cell degradation and poses a safety hazard,” Dr. Wandt added.
Dr. Wandt and Dr. Hubert A. Gasteiger, Chair of Technical Electrochemistry at TUM, together with colleagues from RWTH Aachen University and Forschungzentrum Jülich, started to develop an innovative tool to detect the actual quantity of lithium plating on a graphite anode in real-time. The outcome is a technique the scientists call 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, there are also nondestructive electrochemical techniques that give information on whether lithium plating has occurred during battery charging.
Johannes Wandt, PhD, of Technische Universität München (TUM).
However, neither approach offers more information regarding the onset of lithium metal plating or the quantity of lithium metal present during charging. By contrast, EPR detects the magnetic moment connected with unpaired conduction electrons in metallic lithium with extremely high time resolution and sensitivity 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, 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. Josef Granwehr of Forschungzentrum Jülich and RWTH Aachen University.
Dr. Wandt emphasizes that the quick charging for electric vehicles can be a key application to benefit from additional analysis of the work.
So far, there has been no analytical method available that can directly determine the highest charging rate, which is a function of the state of charge, electrode geometry, temperature, and other factors, before lithium metal plating begins. The innovative technique could offer a much-required experimental validation of frequently employed computational models, and a way of examining the impact of new battery materials and additives on lithium metal plating.
The scientists are now working with other partners in order to benchmark their experimental outcomes against numerical simulations of the plating method 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,” describes Dr. Rüdiger-A. Eichel of Forschungzentrum Jülich and RWTH Aachen University.