  We study the spectral function and quasiparticle scattering in Li-decorated
graphene (Li@graphene) with an atomistic $T$-matrix formalism and uncover
adatom-induced spectral effects which shed light on experimentally observed
angle-resolved photoemission spectroscopy (ARPES) features. From transport
studies, alkali adatoms are known to introduce charged-impurity scattering
limiting the carrier mobility. Here, we demonstrate that Li adatoms furthermore
give rise to a low-energy impurity band centered at the $\Gamma$ point which
originates from the hybridization between the atomic 2s state of the Li adatoms
and graphene "surface" states. We show that the impurity band is strongly
dependent on the concentration $c_\mathrm{Li}$ of Li adatoms, and aligns with
the Li-induced Fermi level on the Dirac cone at $c_\mathrm{Li}\sim 8\,\%$
($E_F\approx 1.1\,\mathrm{eV}$). Finally, we show that adatom-induced
quasiparticle scattering increases dramatically at energies above $\sim
1\,\mathrm{eV}$ close to the van Hove singularity in the graphene density of
states (DOS), giving rise to a large linewidth broadening on the Dirac cone
with a concomitant downshift and a characteristic kink in the conduction band.
Our findings are highly relevant for future studies of ARPES, transport, and
superconductivity in adatom-doped graphene.
