Interphases at Interfaces in Solid-State Batteries

Licentiate thesis, 2026

Lithium-metal solid-state batteries offer compelling energy density and safety advantages over conventional lithium-ion batteries, but their practical implementation is limited by interfacial instability. The interface between the lithium-metal anode and sulphide-based solid-state electrolytes (e.g. Li₆PS₅Cl) is thermodynamically unstable, and the interphase that forms through chemical and electrochemical degradation governs the long-term cycling performance. In this work, we investigate interphase formation and reversibility at the Li/Li₆PS₅Cl interface, combining electrochemical characterization with multimodal X-ray characterization. Coulometric titration time analysis is used to quantify interfacial side reaction kinetics and resolve the electrochemical signatures of individual stages of interphase formation, including the role of phosphorus species and their lithiation state. To directly correlate electrochemical signatures with structural and chemical changes, micro- and nano-X-ray computed tomography were combined with X-ray diffraction computed tomography. This revealed that degradation products do not form homogeneously at the interface, but instead accumulate locally. At the nanoscale, this locally formed interphase is composed of distinctly different phases. The results demonstrate that chemically and electrochemically induced degradation produce distinct chemical and morphological differences at the interface. These findings provide new mechanistic insight into interphase growth at the lithium metal/sulphide solid-state electrolyte interface.