Synthetic optimization and application of Li‐argyrodite Li 6 PS 5 I in solid‐state battery at different temperatures

Lithium argyrodite electrolytes (Li 6 PS 5 X (X = Cl, Br, I)) have received tremendous attention due to their low cost and high conductivity among sulfide electrolytes. However, the synthesis details and application of Li 6 PS 5 I in solid‐state batteries have not been fully investigated yet. Here, we unravel the synthetic process for the Li 6 PS 5 I phase with the mechanical milling route, in which the argyrodite phase appears after 500 r·min −1 for 12 h. The pure Li 6 PS 5 I phase with the highest ionic conductivity (2.1 × 10 −4 S·cm −1 ) is obtained after 20‐h milling, and a subsequent annealing process causes a decrease in the conductivity. The Li 6 PS 5 I is applied with both the pristine and LiNbO 3 ‐coated LiNi 0.7 Mn 0.2 Co 0.1 O 2 cathodes in solid‐state batteries. The coated LiNi 0.7 Mn 0.2 Co 0.1 O 2 material delivers higher discharge capacities (211.4 vs. 140.7 mAh·g −1 at 0.05C for the 1st cycle, and 144.0 vs. 66.5 mAh·g −1 at 0.50C for the 2nd cycle) and higher coulombic efficiencies. Moreover, the coated LiNi 0.7 Mn 0.2 Co 0.1 O 2 electrode exhibits potential operation at − 20 °C. In situ/ex situ electrochemical impedance spectrum (EIS) are applied to unravel the resistance evaluation of solid‐state batteries (SSBs) at different temperatures, which show that the low electrolyte conductivity and the slow lithium‐ion mobility across the interface are the major bottlenecks for good electrochemical performance.