Li intercalation in graphite: A van der Waals density-functional study

Modeling layered intercalation compounds from first principles poses a problem, as many of their properties are determined by a subtle balance between van der Waals interactions and chemical or Madelung terms, and a good description of van der Waals interactions is often lacking. Using van der Waals density functionals we study the structures, phonons and energetics of the archetype layered intercalation compound Li-graphite. Intercalation of Li in graphite leads to stable systems with calculated intercalation energies of -0.2 to -0.3 eV/Li atom, (referred to bulk graphite and Li metal). The fully loaded stage 1 and stage 2 compounds LiC6 and Li1/2C6 are stable, corresponding to two-dimensional root 3 x root 3 lattices of Li atoms intercalated between two graphene planes. Stage N > 2 structures are unstable compared to dilute stage 2 compounds with the same concentration. At elevated temperatures dilute stage 2 compounds easily become disordered, but the structure of Li3/16C6 is relatively stable, corresponding to a root 7 x root 7 in-plane packing of Li atoms. First- principles calculations, along with a Bethe-Peierls model of finite temperature effects, allow for a microscopic description of the observed voltage profiles.