In Situ Investigation of Interphase and Microstructure Effects on the Chemo-Mechanics of Thiophosphate Solid Electrolytes

Lithium thiophosphates (Li3PS4, LPS) are promising solid electrolytes for safe, energy dense solid-state batteries. However, chemo-mechanical transformations within the bulk solid electrolyte and at solidjsolid interfaces can lead to lithium filament formation and fracture-induced failure. The interdependent role of kinetically stable interphases and electrolyte microstructures on the onset and propagation of fracture is not clearly understood. Here, we investigate the effect of interphase chemistry and microstructure on the chemo-mechanical performance of LPS electrolytes. Kinetically metastable interphases are engineered with iodine doping and microstructural control is achieved using milling and annealing processing techniques. In situ transmission electron microscopy reveals how iodine diffuses to the interphase and upon electrochemical cycling pores are formed in the interphase region. Pores/voids formed in the interphase are chemo-mechanically driven via directed ion transport. In situ synchrotron tomography reveals that interphase pore formation drives edge fracture events which are the origin of through-plane fracture failure. Active Li metal has a tendency to fill the fracture region. Cycling lithium in fracture sites leads to localized stress within the solid electrolyte which accumulates and ultimately leads to catastrophic failure. Fractures in thiophosphate electrolytes actively grow toward regions of higher porosity and are impacted by heterogenity in solid electrolyte microstructure (e.g. porosity factor).