This invention discloses a method of fabricating a reticulated solid electrolyte/separator (RSES) which is suitable both as electrolyte and separator in a solid state battery. The reticulated composite is produced by casting and drying of a slurry which exhibits a high yield stress (greater than 50 dyne/cm2) and comprised of a high MW resin dissolved in a solvent (having solution viscosity of higher than 100 cp at 5% in NMP at room temperature) and dispersed nanoparticles of solid electrolyte of high specific surface areas (i.e. greater than 1 m2/g, preferable greater than 10 m2/g) including but not limited to LLZO, LSP, or LIPON or derivatives thereof. This reticulated solid electrolyte/separator exhibits superior cycling properties and high ionic conductivity, resists lithium dendrite penetration, and maintains a high dimensional stability (less than 10% shrinking) at elevated temperatures (up to 140° C.). In addition, the present disclosure relates to electrochemical cells comprising such a reticulated film composite to act as both electrolyte and separator.