Abstract
We report on a new family of argyrodite lithium superionic conductors, as solid solutions Li6+xMxSb1–xS5I (M = Si, Ge, Sn), that exhibit superionic conductivity. These represent the first antimony argyrodites to date. Exploration of the series using a combination of single crystal X-ray and synchrotron/neutron powder diffraction, combined with impedance spectroscopy, reveals that an optimal degree of substitution (x), and substituent induces slight S2–/I– anion site disorder—but more importantly drives Li+ cation site disorder. The additional, delocalized Li-ion density is located in new high energy lattice sites that provide intermediate interstitial positions (local minima) for Li+ diffusion and activate concerted ion migration, leading to a low activation energy of 0.25 eV. Excellent room temperature ionic conductivity of 14.8 mS·cm–1 is exhibited for cold-pressed pellets—up to 24 mS·cm–1 for sintered pellets—among the highest values reported to date. This enables all-solid-state battery prototypes that exhibit promising properties. Furthermore, even at −78 °C, suitable bulk ionic conductivity of the electrolyte is retained (0.25 mS·cm–1). Selected thioantimonate iodides demonstrate good compatibility with Li metal, sustaining over 1000 h of Li stripping/plating at current densities up to 0.6 mA·cm–2. The significantly enhanced Li ion conduction and lowered activation energy barrier with increasing site disorder reveals an important strategy toward the development of superionic conductors.
