Theoretical and experimental study of reversible intercalation of Li ions in the Jarosite NaFe3(SO4)2(OH)6 structure

Abstract

Jarosite NaFe3(SO4)2(OH)6 (NFSOH) with a trigonal structure (R-3 m space group) was prepared by the hydrothermal synthesis at 150 °C and then ball milled with carbon black. Crystal and local structure, morphology, thermal stability, conductivity and electrochemical properties of the as-prepared composite NaFe3(SO4)2(OH)6/C (NFSOH/C) were investigated. For the first time, the Na/Li electrochemical ion-exchange in NFSOH/C in a hybrid Na/Li cell was studied in detail by theoretical and experimental methods. According to DFT calculations, mixed structurally-stable Na1-xLixFe3(SO4)2(OH)6 Jarosite phases can exist up to complete Na/Li substitution, however, it was not achieved in full experimentally. Upon cycling in a Li cell, the crystalline-amorphous transformation occurs similar to that observed in a Na cell, followed by subsequent decomposition of the mixed Na1-xLixFe3(SO4)2(OH)6. The diffusion paths for both Na+ and Li+ ions in NFSOH and LiFe3(SO4)2(OH)6 structures were determined by the Voronoi-Dirichlet partition (VDP) of crystal space. It was found that the value of the average diffusion coefficient in NFSOH increases by two orders of magnitude after the first cycle both in Na and Li cells.