Electronic Structure, Chemical Bonding and Electrochemical Characterization of Li₂CuSn₂ and Li₂AgSn₂

Abstract

Polycrystalline samples of the stannides Li2CuSn2 and Li2AgSn2 were obtained by high-frequency melting of the elements in sealed niobium ampoules in a water-cooled sample chamber. Both stannides crystallize with the tetragonal Li2AuSn2 type, space group I41/amd. They are characterized by three-dimensional [CuSn2]δ-, respectively [AgSn2]δ- networks which leave large channels for the lithium ions. Electronic structure calculations show extensive filling of the transition metal d bands and residual DOS at the Fermi energy, compatible with metallic character. Calculated Bader charges and the course of the crystal orbital overlap population curves fully support the bonding picture of cationic lithium and a covalently bonded polyanionic network with considerable charge transfer to both, transition metal and tin atoms. Electrochemical investigations have indicated that a reversible insertion and extraction of lithium into the stannides is taking place in the voltage range between 0 and 2:5 V vs. Li=Li+. From CV measurements, the diffusion coefficents of Li2CuSn2 and Li2AgSn2 were estimated to be in the order of 10-14 cm2 s-1