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Photoelectron spectroscopy (PES) studies of solid electrolyte interphases (SEI) of cycled battery electrodes are mostly performed in half cell configurations (i.e., against metallic counter electrodes). In contrast to less reactive Li metal, problems arise in post‐Li systems, like K‐ion cells, where crosstalk phenomena strongly interfere with the surface layer formation process. This raises the question of whether surface analysis data from half cell experiments are still representative and transferable to corresponding full cells in post‐Li systems. Here the major differences between SEI layers formed on graphite electrodes are outlined in K‐ion half and full cells derived from an in‐depth surface analysis approach combining in‐house and (synchrotron‐based) hard X‐ray PES. This results highlight significant changes in SEI characteristics, both in terms of SEI layer thickness and gradual compositional changes across the interphase, when K‐metal (half cell) is replaced by a Prussian white positive electrode (full cell). Furthermore, the initial SEI layers formed on the first cycle are found to evolve and age differently upon further cycling, depending on the two cell configurations. This study stresses the additional complexity of studying post‐Li cells and the need to carefully design surface analysis experiments for meaningful material and electrode interphase characterization. [ABSTRACT FROM AUTHOR]