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

TL;DR

This study uses van der Waals density functionals to accurately model the structure, stability, and energetics of lithium intercalation in graphite, revealing stable configurations and temperature effects relevant for battery applications.

Contribution

It provides a first-principles analysis of Li-graphite intercalation, incorporating van der Waals interactions to predict stable phases and temperature-dependent disordering.

Findings

Li intercalation energies are -0.2 to -0.3 eV per Li atom.

Stable stage 1 and stage 2 compounds with specific lattice structures.

Dilute stage 2 compounds become disordered at higher temperatures.

Abstract

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 LiC$_6$ and Li$_{1/2}$C$_6$ are stable, corresponding to two-dimensional $\sqrt3\times\sqrt3$ 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 Li$_{3/16}$C$_6$ is relatively stable, corresponding to a $\sqrt7\times\sqrt7$ 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.