In this presentation, we will provide insight into the most prevalent failure mechanisms of solid-state batteries and correlates it with the relative density of the solid electrolyte pellet. We show that Li dendrites can be suppressed only when the relative density of the solid electrolyte pellet is beyond a threshold value of ~95%. Before this threshold value, the battery shorts easer if the pellet is denser due to faster Li dendrites growth within the percolating pores in the pellet. Detailed characterizations with FIB-SEM tomography and permeability test are employed to quantify the microstructural properties (such as pore size, connectivity, porosity and tortuosity) of the LPS pellet made at different relative densities. Our modeling results reveal the growth details of the Li filament inside pores with size ranging from 0.2 μm to 2 μm. The findings in this paper will greatly help understand the failure modes of solid-state batteries and advise the design strategies for the dendrite-free solid-state battery system.