Enhanced ion conduction by enforcing structural disorder in Li-deficient argyrodites Li6−xPS5−xCl1+x

Solid electrolytes with high ionic conductivity and good stability are advantageous over the current liquid electrolytes in rechargeable Li-ion batteries. Argyrodites, Li6PS5X (X ​= ​Cl, Br, or I), with ionic conductivities on the order of mS/cm have attracted tremendous attention. However, the high potential of argyrodites in fast ion conduction is far from being reached. Significant enhancement in ion conduction relies on the fundamental understanding of the contributing factors for fast ion transport. Here, we have systematically prepared highly conductive Li-deficient Li6−xPS5−xCl1+x and examined the influence of Li-deficiency and Cl substitution of S on ion transport using impedance spectroscopy, solid-state NMR, and first-principles calculations. With increased Cl content, the amount of Cl− at S2− (4d) sites increases, forming a dominant 1S3Cl (4d) configuration. In addition, Li+ redistributes with significantly higher mobility. As a result, the activation energy for Li-ion transport decreases, and the conductivity increases to 17 ​mS/cm at 25 ​°C when x equals 0.7 (Li5.3PS4.3Cl1.7). This work not only reports a record ionic conductivity of Cl-containing argyrodites-type fast Li-ion conductors, but also provides new insights into anion disorder-induced ion transport, which has a wide and universal appeal in the development of fast ion conductors and mixed-anion functional materials.