This study investigates the effect of solid-electrolyte pellet density on the failure of solid-state batteries (SSBs). The research shows that Li-filament growth is suppressed in solid-electrolyte pellets with a relative density above ~95%. Below this threshold, the battery is more prone to short circuit due to faster Li-filament growth within the percolating pores. The microstructural properties of 75% Li₂S-25% P₂S₅ pellets with varying relative densities were 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 in size. The findings improve the understanding of SSB failure modes and provide guidelines for designing dendrite-free SSBs. Solid-state batteries (SSBs) offer higher energy density and improved safety compared to Li-ion batteries. However, failure due to Li-filament penetration of the solid electrolyte remains a critical issue. The study shows that the failure behavior of SSBs is highly dependent on the relative density of the solid electrolyte. The research demonstrates that fully dense LPS SE (relative density >99%) is produced at fabrication pressures above 600 MPa. The ionic conductivity increases linearly during densification of the LPS pellet. However, the failure behavior of SSB cells as a function of densification is more complex: a symmetric cell (Li|LPS|Li) fails much faster as the density of the LPS pellet increases, before reaching a critical relative density (95% for a fabrication pressure of 500 MPa) beyond which cell failure does not occur. The study quantifies the effect of processing parameters such as densification pressure on the micro and macrostructural properties of the SE and on the failure of SSBs due to Li-filament growth. The results show that the pore networks formed during processing play a key role in the failure of SSBs. Modeling results confirm that denser pellets have a much higher Li-filament growth rate, leading to faster cell failure. The study also identifies four mechanisms responsible for Li-filament growth in solid-state cells: percolating pores, chemical reaction, electronic conductivity, and SE fracture. The most prevalent failure mechanism is Li-filament growth in percolating pores within the SE, which is suppressed when the pores become isolated and small in high density pellets above the critical relative density (>95%). The study provides a quantitative guide for relative density optimization of SE pellets to prevent one of the most prevalent failure modes for Li-filament growth.This study investigates the effect of solid-electrolyte pellet density on the failure of solid-state batteries (SSBs). The research shows that Li-filament growth is suppressed in solid-electrolyte pellets with a relative density above ~95%. Below this threshold, the battery is more prone to short circuit due to faster Li-filament growth within the percolating pores. The microstructural properties of 75% Li₂S-25% P₂S₅ pellets with varying relative densities were 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 in size. The findings improve the understanding of SSB failure modes and provide guidelines for designing dendrite-free SSBs. Solid-state batteries (SSBs) offer higher energy density and improved safety compared to Li-ion batteries. However, failure due to Li-filament penetration of the solid electrolyte remains a critical issue. The study shows that the failure behavior of SSBs is highly dependent on the relative density of the solid electrolyte. The research demonstrates that fully dense LPS SE (relative density >99%) is produced at fabrication pressures above 600 MPa. The ionic conductivity increases linearly during densification of the LPS pellet. However, the failure behavior of SSB cells as a function of densification is more complex: a symmetric cell (Li|LPS|Li) fails much faster as the density of the LPS pellet increases, before reaching a critical relative density (95% for a fabrication pressure of 500 MPa) beyond which cell failure does not occur. The study quantifies the effect of processing parameters such as densification pressure on the micro and macrostructural properties of the SE and on the failure of SSBs due to Li-filament growth. The results show that the pore networks formed during processing play a key role in the failure of SSBs. Modeling results confirm that denser pellets have a much higher Li-filament growth rate, leading to faster cell failure. The study also identifies four mechanisms responsible for Li-filament growth in solid-state cells: percolating pores, chemical reaction, electronic conductivity, and SE fracture. The most prevalent failure mechanism is Li-filament growth in percolating pores within the SE, which is suppressed when the pores become isolated and small in high density pellets above the critical relative density (>95%). The study provides a quantitative guide for relative density optimization of SE pellets to prevent one of the most prevalent failure modes for Li-filament growth.