All solid-state batteries based on thiosulfate solid electrolyte hold the promise of safer and more energetic batteries, especially once coupled to Li metal anode and high voltage cathodes. Unfortunately, it was demonstrated in the literature that their electrochemical stability window is far from optimal being very narrow (less than 1V). This is causing severe chemical degradation upon oxidation/reduction of the solid electrolyte, showing that the solid electrolyte is a very active player in the solid-state batteries. The decomposition products generated during oxidation/reduction are causing drastic increase in cell resistance as well as some structural/chemical changes both hindering the long-term cycling of solid-state batteries. On top of that, the mechanical stability of the solid-state batteries is also questioned, as during cycling, the electrode breathing will lead to volume changes generating stress i) in the composite electrode materials, ii) at the interfaces, and iii) in the electrolyte. The stress propagation will be soon translated into fractures at all pre-cited levels, affecting the lithium transport mechanism within the cell causing premature cell failure. As described in the literature, this chemo-mechanical degradation looks unavoidable. However, the literature is relying on the investigation of half-cells/full-cells to explain the chemo-mechanical degradation, but the solid electrolyte alone and especially its sintering/shaping could also be the main responsible of most of the pre-cited issues. Poor sintering, as an example, promotes chemical degradation and voids propagation, thus, getting a deep understanding of the sintering process prior to any electrochemical cycling is of utmost importance. We propose a multiscale approach based in operando characterizations at the laboratory scale and at large scale facilitate (synchrotron and neutrons) to fully understand the sintering process of the electrolyte and in particular to establish the relationship between structural/chemical/morphological/electrochemical parameters as a function of the pressure/temperature. To apply this methodology, novel special electrochemical cells will be developed to perform advanced operando-based techniques. Neutron diffraction will be used to follow the structural evolution during sintering process as a function of temperature/pressure, whereas X-ray tomography and FIB-SEM will be employed to follow the evolution of the porosity as a function of the pressure/temperature. Quasi elastic neutron scattering (QENS) owing to the contrast of lithium isotopes will provide information about Li ion transport whereas chemical decomposition will be investigated by X-ray absorption spectroscopy, etc. Once the sintering process will be fully understood, the solid electrolyte fully optimized will be then transferred to half-cell configuration (vs. Li metal) and the same methodology will be applied. Neutron imaging technique will be added to the pool of operando techniques to follow the Li ion transport as a function of cell cycling. Results from morphology/structure/chemistry gathered at this stage will be used to develop proper coating strategy and buffer layer to ensure electrochemical/chemical/morphological/structural stability. Again, once this goal will be reached, full cell investigation combining high voltage cathode vs. Li metal will be undertaken following the same operando-based approach. The results collected and obtained through this OpInSolid project will shed light on a forgotten player, the solid electrolyte. Based on the outcome, several strategies will be developed to tackle chemo-mechanical issues in sulfide-based solid state batteries.