Electrolyte Pre-Degradation Monitored by In-situ Electron Paramagnetic Resonance
Radostina Stoyanova, Rositsa Kukeva, Mariya Kalapsazova, Hristo Rasheev, Georgi Vassilev, Alia Tadjer
Abstract: The unveiling of the electrolyte degradation pathways is a challenging research task lately
with a pivotal significance for the design of long-cycling and safe batteries. That is why, during the past years there is a strong scientific competition to set up a unified and affordable methodology. Thus, different experimental methods including in-situ and operando spectroscopies (FTIR, Raman, NMR, etc.), chromatography and mass spectrometry, pressure techniques, as well as radiolysis, have been put forward. Among the experimental tools, electron paramagnetic resonance spectroscopy (EPR) is the best suited method for simultaneous detection, speciation and quantification of paramagnetic species both in solid and liquid state. In spite of these fruitful features, EPR has only been applied to probe the local structural transformations in electrodes, the electrode dissolution after cell cycling and the dendrite formation of lithium metal anode, while the reactions proceeding in liquid electrolytes remain out of sight.
Herein we present an innovative in-situ EPR spectroscopy approach complemented with computational modeling as a methodology for assessing the intermediate products of the nonaqueous electrolytes decomposition. To prove the universality of the in-situ EPR methodology, we
start with standard battery-grade lithium electrolyte (LP30® Merck) with less than 15 ppm H2O content [1]. Through in-situ EPR, long-lived radicals were detected in a broad potential window (more than 2.0 V) prior to the electrolyte oxidation or reduction. Based on DFT calculations, we demonstrate that EC has a strong affinity to capture any free electron and the resulting anionradical, EC•-, is robustly stabilized in the polar environment. The pathways of radical formation are discussed in terms of the imperfection in the electron flow across the electrolyte-electrode interface and of the strong affinity of EC to electron trapping. The proposed methodology for monitoring of lithium electrolyte degradation is transferred to access the decomposition of carbonate-based sodium electrolytes. The decomposition products in sodium electrolyte were identified depending on the kind of the electrolyte salt and solvent. Although this is a model study, the appearance of long-lived anion radicals in the electrolyte could help to understand the controversial experimental results on the degradation mechanism of different electrolyte components at given potentials.
References
[1] R. Kukeva, M. Kalapsazova, H. Rasheev, G. Vassilev, A. Tadjer, R. Stoyanova, In Situ Electron Paramagnetic Resonance Monitoring of Predegradation Radical Generation in a Lithium Electrolyte, J. Phys. Chem. Lett., 14 (2023) 9633.
ACKNOWLEDGMENT. The authors acknowledge the financial support of the Bulgarian National Science Fund, contract CARiM (NSP Vihren, КП-06-ДВ-6)
The authors from CARiM’s Research Team are bolded.
VIHREN-CARiM 2019 