Flipping the electron spin

Lithium plating, the depositing of metallic lithium at the anodes of lithium-ion batteries, is one of the primary factors that limits charging current. The performance of batteries suffers significantly from these metallic deposits. In extreme cases this can result in short circuits and even batteries going up in flames.

When charging batteries, the positively charged lithium ions move through the liquid electrolytes and are deposited in the porous graphite anodes. However, the larger the current and the lower the temperature, the greater the probability that the lithium ions will not be deposited within the electrodes, as desired, but rather as a solid metallic layer on the outer surface.

Indirect evidence does not serve the goal

Even though this phenomenon is basically well-known, many aspects remain shrouded in mystery. Until now, it was not possible to directly observe how and under which circumstances lithium plating takes place. "Using traditional methods of microscopy, we can only observe a battery after the fact, because it needs to be cut open," explains Dr. Josef Granwehr at the Jülich Institute of Energy and Climate Research. "In the process, further reactions that distort the results become inevitable."

Even highly developed processes like neutron scattering allow for only indirect analyses. Compounding the problem is the fact that available slots for measurements at research reactors and large particle accelerators are scarce. This makes these tools more suitable for fundamental investigations than for tedious, practical test series.

Electrons show the way

The electron paramagnetic resonance (EPR) spectroscopy process presented in the scientific journal Materials Today, on the other hand, can be readily integrated into laboratory procedures -- with only moderate investment. The method is akin to the better-known nuclear magnetic resonance (NMR) spectroscopy, albeit focusses on electron spins rather than atomic nuclei.

"Electrons are placed in an externally applied, static magnetic field," explains Granwehr. Unpaired electrons in the sample are "sounded out" using microwaves. In the magnetic field, these stimulate the electrons to flip, which can be measured via the associated drop in microwave radiation intensity. EPR can differentiate between metallic lithium plating and lithium embedded in the graphite anodes.

The test cell is the key

"The key to detecting lithium plating using EPR was the construction of a test cell compatible with the requirements of EPR spectroscopy while at the same time exhibiting good electrochemical properties," explains lead author Dr. Johannes Wandt. "The geometry is also important. Precise measurement results are contingent on the sample being exposed to the magnetic field but not the inevitably present electric field."

To ensure this, Wandt developed a rod-shaped cell while he was a doctoral candidate in the group of Prof. Hubert A. Gasteiger, Chair of Technical Electrochemistry at TUM, that allows the formation of metallic lithium to be detected directly and with quantitative precision.

The right strategy for quick charging

"Using this process, it is now for the first time possible to investigate lithium plating and the associated processes in a differentiated manner that is relevant to a whole array of applications," says Rüdiger-A. Eichel, a director at the Jülich Institute of Energy and Climate Research.

"One example is the development of safe and at the same time fast charging protocols," explains Rüdiger-A. Eichel. Our process make determining the maximum charging current before lithium plating sets in possible, as well as ascertaining other boundary conditions like temperature and the influence of electrode geometry."

Beyond this, the methodology is well suited as a test procedure for a variety of battery materials, for example the development of new admixtures that suppress lithium plating.

The research was funded by the German Federal Ministry of Education and Research (BMBF) in the context of the ExZellTUM II project and the Bavarian State Ministry of Economic Affairs and Media, Energy and Technology in the context of the EEBatt project. The EEBatt project is part of the "TUM.Energy" priority program coordinated by the Munich School of Engineering (MSE).