Interlocked Chalcogenide Lattice Showing High Sodium-Ion Conductivity and Facile Electrochemistry

Abstract

In this work, a new class of alkali ion containing quaternary chalcogenides with compositions Na3 MGaQ 4 (M = Zn, Fe, Mn; Q = S, Se) have been synthesized using building block approach. The crystal structure of these compounds is built up of three-dimensional (3-D) network of corner-shared supertetrahedral (T2) units, where two such 3-D networks are interlocked. The d-block metal and the main group metal, Ga, occupy same crystallographic site with 1:1 ratio making it a rare form of building unit. Na3ZnGaQ 4 (Q = S, Se) possess ionic conductivity of 3.74 × 10–4 and 0.12 mS/cm with activation energies of 0.38 and 0.42 eV, respectively, for sulfide and selenide analogues at 30 °C. To determine the possible Na-ion diffusion pathways, Nudged Elastic Band (NEB) calculation based on the DFT is performed. The defect formation energies of the two chemically inequivalent Na atoms is calculated. NEB calculations support the experimental findings that the sodium is more mobile in the selenide host than the sulfide. Na-ion electrochemistry of Na3FeGaQ 4 (Q= S, Se) as a cathode material in Na-ion batteries is evaluated. Preliminary results suggest that a modest capacity from 0.5 e/Fe redox reaction at a potential of 2.08 V (Na3FeGaS4) and 1.98V (Na3FeGaSe4) vs Na+/Na for Fe2+/3+ redox couple can be achieved with a very low polarization. Moreover, an excellent cycle life with more than 95% capacity retention for 100 cycles could be achieved at a moderately high rate of C/5, which can be attributed to the open framework structure with facile Na diffusion path.