Thiophosphate solid electrolytes (SE) are promising candidates for solid state battery systems as they exhibit both reasonable ionic conductivity and ease of processability. Unfortunately, several challenges still hinder their commercialisation, one of them is linked to the control of the interfaces, and another one is linked to their electro-chemo-mechanical degradation. For the former, coating techniques can play a role to better handle the interfaces, whereas the second one is still unclear as it is declared that those degradation are electrochemical driven, when they could be there during the cell assembly, during the densification for example. Indeed, densification is a key parameter to control since it impacts the ionic conductivity, and ensures the contact between the active materials and the electrolyte¹. Finally, proper densification of the SE could limit the propagation of dendrites. Investigating the densification is far from trivial, and here we rely on novel advanced in situ synchrotron techniques to provide new insight into the sintering of the electrolyte.

The densification of two thiophosphate electrolytes (one amorphous and one crystalline) was investigated using the UToPEc set-up at Psiché beamline of Soleil synchrotron. This unique set-up² allowed for the simultaneous acquisition of absorption X-ray tomography, X-ray diffraction, density measurement and impedance measurements whilst applying a compressive force of up to 4 GPa meaning we can follow both structural and morphological changes during densification. As can be seen in Figure 1, the amorphous solid electrolyte can lose up to 53% of its own volume during the densification whereas the crystalline one could only be reduced by 20%.

Following the densification, we cycle the solid electrolyte to track the electro-chemo-degradation using X-ray diffraction computed tomography (XRD-CT) carried out at ID15a at the European Synchrotron Radiation Facility. It allows localizing the extend of the electrochemical and chemical degradation after long-term cycling and reveal that the thiophosphate solid electrolyte (LPSCL) decomposes electrochemically in known products such as S, Li2S, LiCl³ and that the chemical decomposition at the Li interface is more prominent than the electrochemical one.

Both advanced techniques reveal the difficulties in disentangling the extent of processing in solid state batteries.