Influence of M/A substitution on material properties of intermetallic compounds MSn2 (M = Fe, and Co; A = Li, and Na): a first-principles study

Iron and cobalt distannides MSn2 (M = Fe, and Co) are regarded as a promising conversion-type anode material for lithium- and sodium-ion batteries, but their properties are not well understood. In this work, we report a first-principles study of alkali metal (A = Li, and Na) substitutional effect on the structural, mechanical, lattice vibrational, electronic and defect properties of these distannides. Special attention is paid to systematic comparison between FeSn2 and CoSn2. Our calculations reveal that M/A substitution induces a lattice expansion and decrease of elastic constants, which is more pronounced with Na substitution than with Li, and moreover changes the elastic properties of FeSn2 from ductile to brittle, but preserves the ductility of CoSn2. An imaginary phonon frequency mode appears only for FeSn2 and FeNaSn2, and M/A substitution provokes a definite gap between high and low frequency regions. We perform a careful analysis of electronic density of states, band structures and Fermi surface, providing an insight into differences of electronic structures between FeSn2 and CoSn2. With further calculation of defect formation energies and alkali ion diffusion barriers, we believe this work can be useful to design conversion-type anode materials for alkali-ion batteries.