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 center dot cm-1) represents almost a 100-fold increase over that of the parent phase, Li6PS5I (0.0033 mS center dot 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/Brargyrodites Li6+xPS5+x(Cl/Br)1-x (0 < x < 0.75) with a low halide content exhibit surprisingly high ionic conductivities > 1 mS center dot 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.

Automated summary

We report on a new family of halide-deficient and Li-rich argyrodite fast-ion conductors, which demonstrate high ionic conductivities due to the disordered Li ion sublattice and sulfur/halogen anion site disorder. The influence of aliovalent anion substitution in Li6PS5X (X = Cl, Br, I) on Li-ion concentration and disorder is explored. The findings highlight the importance of attaining a disordered Li ion sublattice and sulfur/halogen anion site disorder for enhancing the ionic conductivity in argyrodites.