Solid-state lithium-ion batteries are promising candidates as next generation of energy storage devices. Unlike conventional lithium-ion batteries, they comprise a solid electrolyte (SE) instead of a liquid organic one. The use of SEs allow an increase in the battery energy density, which is required for automotive applications [1]. Among SE categories, sulfide based solid electrolytes (SSEs) have several advantages including a high ionic conductivity (up to 10-2 S/cm2) and good mechanical properties [2, 3]. However, they also have some limitations and one of the most critical being their high sensitivity towards humidity. In fact, when SSEs are exposed to a humid atmosphere, they are severely degraded and release toxic H2S gas upon their reaction with water [4, 5]. This feature leads to significant process cost and potential safety issues associated with mass production of solid-state lithium-ion batteries with SSEs.
This study provides an exhaustive description of degradation processes occurring when Li3PS4 (LPS) SSE pellet is exposed to humid Argon (47% RH). A homemade flow-through setup was developed and used for precise quantification of H2S evolved during Li3PS4 reaction with humidity. Two synchrotron radiation-based techniques, X-ray diffraction–computed tomography (XRD-CT) and X-ray nanotomography were used for evaluation of three-dimensional structural and morphological degradation of the pellets.
Constant in-situ and online H2S measurements evidenced a two-stage reaction between LPS pellet and humid Argon: a fast surface reaction (peak) followed by a slower diffusion-controlled reaction (low plateau).
X-ray nanotomography revealed severe morphology degradation for humid Argon-exposed LPS pellet. Multiple cracks and pores were formed from surface towards the bulk of the pellet. The morphological mean degradation front after 120 min of humidity exposure reached 50 µm.
XRD-CT allowed identification of main chemical degradation phases produced upon humid Argon exposure of LPS pellet. The spatial distribution of these degradation phases within the LPS pellet were also determined. XRD-CT also evidenced the presence of multiple strains within humidity –degraded LPS pellet that drive the morphology degradation.
X-ray nanotomography and XRD-CT ultimately allowed us to identify a cyclic and self-sustaining chemo-morphological degradation mechanism for the degradation of LPS pellet exposed to humid Argon. This mechanism consists of an interplay between LPS hydrolysis and cracks and pores formation/growth following the hydrolysis.

Refs
[1] Nature Materials, VOL 18 1280, 1278–1291; [2] Energy Storage Mater. 2018, 14, 58–74; [3] Nano Energy 2021, 83, 105858; [4] Solid State Ion. 2011, 182 (1), 116–119; [5] Chem. Mater. 2020, 32 (6), 2664–2672.