This study investigates the effect of solid-electrolyte pellet density on the failure mechanisms of solid-state batteries (SSBs). The research highlights that Li-filament growth is suppressed in pellets with a relative density exceeding ~95%. Below this threshold, battery short circuits occur more frequently due to faster Li-filament growth in the percolating pores of the pellet. The microstructural properties of 75%Li2S – 25%PSx (LPS) pellets at various relative densities are quantified using focused ion beam-scanning electron microscopy tomography and permeability tests. Modeling results provide insights into Li-filament growth within pores ranging from 0.2 to 2 μm. The findings improve understanding of SSB failure modes and provide guidelines for designing dendrite-free SSBs. The study emphasizes the importance of optimizing pellet density to prevent Li-filament growth and enhance battery safety and performance.This study investigates the effect of solid-electrolyte pellet density on the failure mechanisms of solid-state batteries (SSBs). The research highlights that Li-filament growth is suppressed in pellets with a relative density exceeding ~95%. Below this threshold, battery short circuits occur more frequently due to faster Li-filament growth in the percolating pores of the pellet. The microstructural properties of 75%Li2S – 25%PSx (LPS) pellets at various relative densities are quantified using focused ion beam-scanning electron microscopy tomography and permeability tests. Modeling results provide insights into Li-filament growth within pores ranging from 0.2 to 2 μm. The findings improve understanding of SSB failure modes and provide guidelines for designing dendrite-free SSBs. The study emphasizes the importance of optimizing pellet density to prevent Li-filament growth and enhance battery safety and performance.