  Operando X-ray absorption spectroscopy (XAS) can probe the
electrode-electrolyte interface with single-digit nanometer depth resolution
and offers a wealth of insight into the evolution and Coulombic efficiency or
degradation of prototype cells, provided that the spectra can be reliably
interpreted in terms of local oxidation state, atomic coordination, and
electronic structure aboutthe excited atoms. To this end, we explore fluorine
K-edge XAS of mono- (Li, Na, K) and di-valent (Mg, Ca, Zn) fluoride salts from
a theoretical standpoint and discover a surprising level of detailed electronic
structure information about these materials, despite the relatively predictable
oxidation state and ionicity of the fluoride anion and the metal cation.
Utilizing a recently developed many-body approach based on the delta-SCF
method, we calculate the XAS using density functional theory and experimental
spectral profiles are well reproduced, despite some experimental discrepancies
in energy alignment within the literature, which we can correct for in our
simulations. We outline a general methodology to explain shifts in the main XAS
peak energies in terms of a simple exciton model and explain line-shape
differences resulting from mixing of core-excited states with metal d character
(for K and Ca specifically). Given ultimate applications to evolving
interfaces, some understanding of the role of surfaces and their terminations
in defining new spectral features is provided to indicate the sensitivity of
such measurements to changes in interfacial chemistry.
