Investigating the role of electrode placement for operando APXPS battery studies

Abstract

To enable the continued development of better batteries, it is crucial to improve the understanding of what happens inside the batteries during operation, especially at the electrode surfaces. For this, operando and in situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) battery studies have been utilized. However, these studies are performed in open beaker cells, where the electrodes are separated by ∼1 cm. Large separations between the electrodes has been found to result in distinct electrochemical behaviour of the cell compared to closely spaced electrodes in a coin cell. In this work, a new sample holder for AP-XPS at the HIPPIE beamline at MAX IV has been designed that places the electrodes side-by-side, only separated by ∼1 mm. This setup has then been compared to the currently used sample holder at HIPPIE, where the electrodes are placed 14 mm apart. The influence of these two electrode placements on electrochemical processes, surface composition, and resistances are investigated through in-house galvanostatic cycling, XPS and electrochemical impedance spectroscopy (EIS) experiments. This has been done by designing and 3D printing in-house sample holders that give the same electrode placement as those designed for the HIPPIE beamline. The electrodes of coin cells were also investigated to compare the performances of the beaker cells to a more more realistic commercial battery. The experiments were carried out on commercial lithium iron phosphate (LFP) and graphite battery electrodes, in a 1M LiPF6 in ethylene carbonate/diethyl carbonate electrolyte. EIS measurement were carried out after the open-current voltage had stabilized, followed by a single galvanostatic charge-discharge cycle at C/10. XPS was then performed on cycled as well as on pristine electrodes. The results show differences in the electrochemical processes in the beaker cells compared to the coin cells, where the discharge capacity was smaller for the beaker cells indicating some irreversible reactions. Furthermore, the result also indicate that the resistances in the cells get larger when the distance between electrodes grows. While the electrode placement has a negligible affect on the surface composition of the LFP electrodes, the composition of the graphite electrodes varies substantially, with the largest difference still being between the coin cell and beaker cells. From this work, it can therefore be concluded that while the newly designed side-by-side electrode placement seems to reduce the overall cell resistances, it can’t be concluded that there are any differences between the electrochemical performance and surface composition of the electrodes between the two beaker cells. Differences can however be seen for the electrochemical performance and surface composition of the graphite for the coin cells in comparison to the beaker cells. This is possibly explained by the large difference in electrolyte volume, rather than electrode placement. This work brings in situ AP-XPS studies a step closer to commercially relevant battery operation with the side-by-side electrode design and shows that there are differences between beaker cells and coin cells which should be considered when AP-XPS is performed.