Binding Energies of Molecules Grafted onto Calcium Surfaces: A Periodic DFT Investigation

Understanding the spectroscopic signatures of solid-electrolyte interphase (SEI) components is crucial for advancing calcium-based batteries, as the SEI composition directly impacts the performance. X-ray photoelectron spectroscopy (XPS) is a key technique for probing surface chemistry, yet fundamental studies on binding energies of SEI-grafted molecules remain limited. Additionally, accurately computing XPS chemical shifts for surfaces and adsorbates while balancing precision and cost is a challenge. This work employs density functional theory (DFT) with periodic boundary conditions to investigate the adsorption of tetrahydrofuran (THF) and its degradation products on calcium surfaces, including metallic Ca, CaO, and CaH2. A fully hydroxylated CaO surface is also considered. Two primary interaction mechanisms are identified: (i) adsorption via unsaturated carbon atoms and (ii) proton transfer from alcohol groups to CaO. After evaluating various computational approaches, the Slater–Janak scheme is adopted to compute the XPS spectra for O 1s, C 1s, and Ca 2s. Significant shifts in oxygen and carbon binding energies are observed, while these of calcium remain largely unchanged. These findings provide valuable insights into SEI formation and offer useful trends for interpreting XPS spectra, with good agreement observed between simulations and available experimental data.