Hot Pressed, Fiber-Reinforced (Li₂S)₇₀(P₂S₅)₃₀ Solid-State Electrolyte Separators for Li Metal Batteries

Herein, we detail a hot pressing process that was used to fabricate dense, aramid-fiber-reinforced (Li 2 S) 70 (P 2 S 5 ) 30 glass–ceramic solid-state electrolyte (SSE) composites. Green SSE composites were hot pressed at 240 °C and 200 MPa so that the softened sulfide glass flowed around the rigid aramid fibers. The SSE composite with 10 wt % aramid fiber loading had a relative density of 99.8% and an ionic conductivity of 2.4 mS/cm. The composites were mechanically robust and flexible and showed no signs of macrocracking even after uniaxial compression to 3 MPa. Furthermore, the fabrication of freestanding films with 100 μm thickness was demonstrated. Scanning electron microscopy (SEM) revealed that the improved strength of the SSE composites was attributable to aramid fibers impeding the propagation of microcracks in the sulfide glass phase. Critical current density tests were performed on green and hot pressed SSE composite separators. A green sample shorted at 200 μA/cm 2, a green sample made with preannealed SSE powder shorted at 400 μA/cm 2, and a hot pressed sample shorted at 400 μA/cm 2 . Failure of green samples was attributable to interparticle deposition of Li deposits, while failure of hot pressed samples was attributable to the deposition of Li through microcracks.