Optimization of Current Collector Design for Operando X-Band-EPR Investigations of Lithium-Ion Batteries Using Numerical Simulations

Monitoring the operation of lithium-ion batteries using electron paramagnetic resonance (EPR) is a powerful tool for gaining a better understanding of the redox processes and degradation mechanisms in anode and cathode materials. For this purpose, a complete operational cell, including current collectors, must be placed into the region of maximum magnetic field and minimum electric field of the specific resonance mode established in the EPR cavity. The homogeneity of the microwave (MW) magnetic field in the measurement region is essential to perform accurate quantitative measurements. Furthermore, the samples, the current collectors, and the dielectric holders should be confined within a region with negligible electrical field, to prevent unwanted power dissipation and to avoid a reduction of the cavity quality factor. Current collectors represent a major problem in the EPR characterization of battery cells, as the introduction of conducting elements into the resonator affects the electromagnetic field distribution, thus altering the EPR spectra, or prevents their acquisition altogether. Furthermore, the collectors should not excessively cover the battery surface, as this would shield the MW field at the position occupied by the active materials in the cell. On the other hand, they should be evenly distributed over the surface of the cell to prevent the establishment of transversal electrical gradients and guarantee a homogenous development of electrochemical processes. In this paper, we discuss the simulation results for various circular current collector geometries with a diameter of 7 mm, suitable for operando X-band-EPR measurements of battery cells in a Bruker Elexsys E540 spectrometer. A commercial 4108 TMHS cylindrical resonator is operated using the TM
₁₁₀
mode in order to excite and monitor the EPR resonances. The designs take into account practical manufacturability considerations: The collectors are specifically engineered to be easily integrated into the assembly, using substrates which are EPR silent and chemically inert against the battery materials.