Conductivity enhancement of argyrodite Li6SbS5I solid electrolyte via charge modulation around Li diffusion paths through Si substitution

Abstract

Aliovalent cation substitution in Li-argyrodite solid electrolytes for all-solid-state batteries has been reported to increase the conductivity by up to three orders of magnitude, but the mechanism underlying this enhancement remains unclear. This first-principles study examined Si-substituted Li6SbS5I (Li6+xSb1-xSixS5I) to clarify the origin of the dramatic increase in the conductivity of Li-argyrodite achieved via cation substitution. When Si is substituted for Sb in the SbS4 tetrahedra of Li6SbS5I, Si donates more electrons to S, forming stable SiS4 tetrahedra, which greatly stabilizes the Li6+xSb1-xSixS5I system in proportion to the Si content. The electron-rich S ions in SiS4 tetrahedra induce Li ions originally occupying the tetrahedral T5 site to also occupy the trigonal T5a and tetrahedral T2 sites. Importantly, the electron-rich S ions play a key role in reducing the diffusion barrier by stabilizing Li ions diffusing along the diffusion path involving the T5 site through favorable Li-S interactions, resulting in a remarkable increase in conductivity from 4.4 x 10-4 mS cm-1 when x = 0 to 15.4 mS cm-1 when x = 0.75. The Li ion transport in Li6+xSb1-xSixS5I proceeds via a concerted diffusion mechanism, regardless of the Si substitution. Thus, the increase in Li6+xSb1-xSixS5I conductivity with increasing x is due to the Si-induced changes in the charge state of S ions around the Li diffusion path, not the activation of concerted diffusion caused by the Si substitution as previously believed.