Stability, composition and properties of Li2FeSiO4 surfaces studied by DFT

Surface structures and energies of the Pmn2(1) polymorph of the electrode material Li2FeSiO4 are studied by first-principles calculations using density functional theory. In total, 29 surface terminations of stoichiometric polar and nonpolar slabs were studied. These surfaces were preselected by energy estimation via a model that accounts for bond cutting and additionally for polarity compensation in the case of polar surfaces. The model provides a way to quantify the most important contributions to the surface energy. Furthermore, we analyze the relaxation of surface atoms statistically. This clearly shows that SiO4 tetrahedra are rather rigid whereas the local environment of Fe and Li can change strongly under relaxation near the surface. We furthermore compare results obtained by generalized gradient approximation (GGA) and GGA+U exchange correlation functionals. Thus, we estimate the thermodynamic equilibrium shape of Li2FeSiO4 by the Wulff construction scheme for stoichiometric surfaces obtaining crystallites that are terminated by {110}, {010}, and {001} surfaces.