Influence of M/A substitution on material properties of intermetallic compounds MSn$_2$ (M = Fe, Co; A = Li, Na): A first-principles study

TL;DR

This study uses first-principles calculations to analyze how alkali metal substitution affects the structural, mechanical, vibrational, electronic, and defect properties of FeSn2 and CoSn2 intermetallic compounds, relevant for battery anodes.

Contribution

It provides a systematic comparison of alkali metal substitution effects on FeSn2 and CoSn2, revealing changes in lattice, elasticity, and electronic properties crucial for battery applications.

Findings

Na substitution causes more lattice expansion than Li.

FeSn2 becomes brittle with Na substitution, while CoSn2 remains ductile.

Imaginary phonon modes appear only in FeSn2 and FeNaSn2.

Abstract

Iron and cobalt distannides \ce{MSn2} (M = Fe, 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, Na) substitutional effect on the structural, mechanical, lattice vibrational, electronic and defect properties of these distannides. Special attention is paid to systematic comparison between \ce{FeSn2} and \ce{CoSn2}. Our calculations reveal that M/A substitution induces a lattice expansion and decrease of elastic constants, which is more announced with Na substitution than Li, and moreover changes the elastic property of \ce{FeSn2} from ductile to brittle whereas preserves the ductility of \ce{CoSn2}. An imaginary phonon frequency mode appears only for \ce{FeSn2} and \ce{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 difference of electronic structures between \ce{FeSn2} and \ce{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.