Tuning of Li-argyrodites ionic conductivity through silicon substitution (Li6+xP1-xSixS5Cl0.5Br0.5) and their electrochemical performance in lithium solid state batteries

Highlights: • Silicon substituted Li-argyrodite was successfully prepared using dry ball milling process. • Li_6.2P_0.8Si_0.2S₅Cl_0.5Br_0.5 electrolyte exhibited the high ionic conductivity of 5.12 mS cm⁻¹. • The si substitution increased the volume of the unit cell thus facilitates to fast Li-ion transport. • The prepared solid electrolyte has good electrochemical stability and compatibility with lithium. • Our fabricated solid state battery showed high reversible capacity of 148 mAh g⁻¹ at 0.1 c. -- Abstract: Lithium solid state batteries are one of the state of the art energy storage systems due to their high safety. However, ionic conductivity in solid electrolytes is a concern, because at present it does not match the ionic conductivity of non-aqueous Li-ion batteries, thus resulting in sluggish electrochemical kinetics. In this report, we enhance the ionic conductivity of Li-argyrodites (Li₆PS₅Cl_0.5Br_0.5) through Si substitution at the P-site using a dry ball milling process. Among the silicon substitutions, Li_6.2Si_0.2P_0.8S₅Cl_0.5Br_0.5 exhibited the high ionic conductivity of 5.12 mS cm⁻¹ compared to pristine Li₆PS₅Cl_0.5Br_0.5 at 4.02 mS cm⁻¹. The Rietveld refinement analysis revealed that after silicon substitution, volume of the unit cell gets increased that allows the lithium at T2-site, that promotes the fast Li-ion transport. Moreover, the optimized solid electrolyte was utilized in a solid state battery system, and demonstrated a high initial capacity of 148.1 mAh g⁻¹ at 0.1 C rate compared to pristine argyrodite (135.1 mAh g⁻¹). Further, we demonstrated the interface phenomena between electrode and solid electrolyte using ex-situ XPS analysis. This confirmed the formation of interface products such as LiCl, Li₂S, lithium polysulfides and P₂S_x, which influence the cycling stability of the ASSLBs.