Graphite
intercalation compounds continue to be central to technologies
for electrochemical energy storage from anodes in established Li-ion
batteries to cathodes in beyond Li-ion concepts paired with multivalent
anodes. When used as a cathode, graphite intercalates a variety of
anions with PF6
– being among the most
common. Paired with Li intercalation at the anode, the corresponding
dual carbon battery yields high energy and power densities. Given
the available choice of anions as intercalants, it is important to
elucidate how the graphite structure accommodates them in order to
tailor the molecular species to maximize charge and reversibility.
However, the changes in electronic structure of the host graphite
lattice upon anion intercalation are poorly understood compared to
cations, which represent a fundamentally different reaction. In this
work, PF6-intercalated graphite has been studied using
techniques sensitive to electronic structure, namely, X-ray Raman
spectroscopy (XRS), X-ray absorption near-edge spectroscopy (XANES),
and X-ray emission spectroscopy (XES). Complementary full-potential,
all-electron density functional theory calculations yielded excellent
agreement with the spectra, thus providing insight into charge compensation
in the graphite lattice. In particular, a pre-π* feature emerged
in XRS/XANES, which is direct evidence of the removal of charge from
the host lattice to compensate the intercalated anions, leading to
an overall lowering of the Fermi energy level. This is expected to
be characteristic of many intercalants in anion-intercalated graphite.
The unambiguous identification of the origin of the pre-π* spectral
feature, which is frequently seen in graphitic systems, is of broad
interest to the spectroscopy of graphitic systems beyond the practical
implications of anion-induced changes in the electronic properties
for real devices.