Comparing the Solid Electrolyte Interphases on Graphite Electrodes in K and Li Half Cells

In both Li-ion and K-ion batteries, graphite can be used as the negative electrode material. When potassium ions are stored electrochemically in the graphite host, the electrode capacities fade faster than in the lithium ion counterpart. This could be due to the high reactivity of the potassium metal counter electrode (CE) in half cells or a less stable solid electrolyte interphase (SEI) in the potassium case. Previous surface studies on graphite electrodes cycled in K half cells have focused on the SEI characteristics of different electrolyte formulations or different states of charge. In this study, we exploit the fact that graphite can store both lithium and potassium ions. Cell and component parameters have been largely maintained the same, with the only differences between Li and K half cells being the cation of the electrolyte salt and the alkali metal at the CE. The SEI layers formed under these conditions in either setup are studied using X-ray photoelectron spectroscopy with the aim to draw a direct comparison between the surface layers in both charged and discharged states. The results show a considerable crosstalk under OCV conditions between K-metal and the working electrode. Furthermore, the relative SEI layer composition after cycling varies considerably between Li and K half cells. Different dominant SEI species are present depending on the alkali metal used. The strong capacity fade observed in graphite–K half cells is likely linked to much smaller concentrations of inorganic compounds, such as KF, and increased amounts of organic compounds in the SEI.