Li-Rich and Halide-Deficient Argyrodite Fast Ion Conductors

We report on a new family of halide-deficient and Li-rich argyrodite fast-ion conductors, Li6+xPS5+x(Cl/Br/I)1–x (0 ≤ x ≤ 0.85). Exploration of the influence of aliovalent anion substitution in Li6PS5X (X = Cl, Br, I)─using a combination of high-resolution powder neutron diffraction and electrochemical impedance spectroscopy─reveals that aliovalent anion substitution induces higher Li-ion concentration and Li site disorder, and creates S2–/I– anion site disorder on the 4a site. In the series Li6+xPS5+xI1–x (0 ≤ x ≤ 0.4), the resulting conductivity for Li6.4PS5.4I0.6 (0.13 mS·cm–1) represents almost a 100-fold increase over that of the parent phase, Li6PS5I (0.0033 mS·cm–1), and establishes one of the first fast-ion conducting argyrodite thiophosphate iodides. For Cl-argyrodites, the ionic conductivity decreases a little with lower halide-content but ionic conductivity for the Br-argyrodites is almost unchanged. Overall, all Cl/Br-argyrodites Li6+xPS5+x(Cl/Br)1–x (0 ≤ x ≤ 0.75) with a low halide content exhibit surprisingly high ionic conductivities > 1 mS·cm–1 despite a very low degree of sulfur/halogen anion site disorder. Our findings highlight the importance of attaining a disordered Li-ion sublattice and sulfur/halogen anion site disorder (anionic charge homogeneity) in argyrodites, where Li ions occupy high energy sites and activate concerted ion migration that drives the ionic conductivity.