Variation in structure and Li +-ion migration in argyrodite-type Li 6PS 5X (X = Cl, Br, I) solid electrolytes

Abstract

All-solid-state rechargeable lithium-ion batteries (AS-LIBs) are attractive power sources for electrochemical applications due to their potentiality in improving safety and stability over conventional batteries with liquid electrolytes. Finding a solid electrolyte with high ionic conductivity and compatibility with other battery components is a key factor in raising the performance of AS-LIBs. In this work, we prepare argyrodite-type Li 6PS 5X (X = Cl, Br, I) using mechanical milling followed by annealing. X-ray diffraction characterization reveals the formation and growth of crystalline Li 6PS 5X in all cases. Ionic conductivity of the order of 7×10 -4 S cm -1 in Li 6PS 5Cl and Li 6PS 5Br renders these phases suitable for AS-LIBs. Joint structure refinements using high-resolution neutron and laboratory X-ray diffraction provide insight into the influence of disorder on the fast ionic conductivity. Besides the disorder in the lithium distribution, it is the disorder in the S 2-/Cl - or S 2-/ Br - distribution that we find to promote ion mobility, whereas the large I- cannot be exchanged for S 2- and the resulting more ordered Li 6PS 5I exhibits only a moderate conductivity. Li + ion migration pathways in the crystalline compounds are modelled using the bond valence approach to interpret the differences between argyrodites containing different halide ions.