Investigating the densification of Li6PS5Cl solid electrolyte through multi-scale characterization techniques

Li₆PS₅Cl (LPSCl) has recently gained attention as a promising solid-state electrolyte for batteries. However, the mechanisms underlying the “cold sintering” process in LPSCl remain poorly understood. In this study, we performed in situ densification of LPSCl while simultaneously measuring electrochemical impedance spectroscopy and micro-tomography to gain deeper insights into “cold sintering” process and to correlate the ionic conduction with the three-dimensional microstructure of the solid electrolyte. We observed that the large (secondary) particles are fracturing, while the grain boundary (GB) conductivity is improving due to better contact between grains. We have found that during the pressure application (up to 510 MPa from 76.2 MPa) at room temperature, the conductivity increases 2.45 times (up to 1.66 mS cm⁻¹). From an in-depth electrochemical impedance and quasielastic neutron scattering (QENS) investigation, we show that the conductivity enhancement primarily arises from improved GB contact, with the bulk material remaining largely unaffected – that is, unsintered. However, the Li⁺conductivity is not limited by bulk but by GB resistance. The electrolyte’s conductivity without any GB contribution is estimated from QENS results with the Nernst-Einstein equation to be 5.3 mS cm⁻¹, giving us the maximum conductivity that could be reached with shaping without modifying the bulk of the material.