In operando investigations of electrochemical systems by EPR spectroscopy

Dissertation, RWTH Aachen University, 2018; Aachen 1 Online-Ressource (xiii, 138 Seiten) : Illustrationen, Diagramme (2018). = Dissertation, RWTH Aachen University, 2018
The understanding and development of electrochemical systems, i.e. batteries, electrolysis and fuel cells, is a key link for electromobility. Since most systems substantially suffer from several problems, the magnetic properties of battery and fuel cell materials were targeted by electron paramagnetic resonance (EPR) spectroscopy. In this work, the in operando mode of investigation of electrochemical systems with EPR spectroscopy was developed for high temperature polymer electrolyte fuel cells and lithium-ion batteries. Furthermore, influences of direct currents to in operando EPR measurements were analyzed and EPR imaging was developed for metallic lithium. Battery and fuel cell setups compatible with the EPR microwave were developed since commercial cells do not fulfill requirements. A first approach was done with a 3D printed fuel cell showing the capabilities and possibilities of rapid prototyping techniques and moreover, clarifying the requirements for electrochemical systems in EPR spectroscopy. In a next step, a reusable battery cell setup with high reproducibility, electrochemical functionality and minimum interactions with the EPR microwave was developed from quartz glass. With this cell setup, spinel as well as layered oxide cathode materials for lithium-ion batteries were analyzed by in operando EPR spectroscopy, i.e. during battery operation. A LiNi0.5Mn1.5O4 (LNMO) spinel cathode showed the impact of Mn3+ on the Li+ motion inside the spinel. Moreover, state of charge dependent linewidth variations confirm the formation of a solid solution for slow cycling, which is taken over by mixed models of solid solution and two-phase formation for fast cycling due to kinetic restrictions and overpotentials. Long-term measurements for 480 h showed the stability of the investigated LNMO, but also small amounts of cathode degradation products became visible. The results point out how local, exchange mediated magnetic interactions in cathode materials are linked with battery performance and can be used for material characterization. Iron substituted spinel structure LiFe0.4Mn1.6O4 (LFMO) was analyzed showing a manganese oxidation in the low voltage regime while the high voltage process was identified as Fe3+ / Fe4+ redox couple. The long-term change of magnetic properties exhibits an electrochemical inactive iron species after 523 h of battery cycling giving hints towards the local magnetic structure, i.e. a change from an exchange coupled system to a dipolar broadened system. After electrolyte soaking, lithium cobalt oxide (LCO) battery cathode material showed a considerable amount of paramagnetic Co2+ that was oxidized during the formation cycle. The absence of an EPR signal during subsequent battery cycles, irrespective of the state-of-charge, shows that no EPR active Co4+ is formed. This suggests the presence of an oxygen redox mechanism in the active material while the cobalt oxidation state remains constant at Co3+. LiNi0.18Co0.1Mn0.59O2 (NCM) cathode material showed a nickel oxidation in combination with oxygen oxidation. On a long run, battery degradation showed an altered EPR linewidth and intensity depending on the state-of-charge that give information about the capacity and voltage fading. Both are effects of an increasing amount of inactive Ni2+ and a slightly growing amount of nickel that cannot be oxidized to Ni2+ anymore. As in operando measurements come along with currents inside the EPR resonator, the EPR signal for metallic lithium has been investigated with direct currents applied to the sample. Currents up to 900 mA change the signal intensity and signal shape. Experimental as well as theoretical considerations show that the observations are an effect of interactions between DC induced magnetic fields, the static magnetic field B_0 and the interaction field B_1. Especially in the case of microwave shielding due to the skin effect in conductors, DC currents enable to mirror the whole sample properties by EPR spectroscopy. Finally, conduction EPR imaging (CEPRI) was developed for metallic lithium. It was found to be a sensitive characterization method to image the microstructure of lithium deposits in lithium-ion battery components. The versatility of the method is demonstrated for both, imaging surface-patterns of thick lithium metal anodes, as well as obtaining high-resolution images of lithium dendrites formed inside a separator with several micrometre pixel size. The determined spatial distribution of dendrites may then serve as an indicator of the current density distribution inside battery cells.
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