The role of phosphorus in the solid electrolyte interphase of argyrodite solid electrolytes

The solid electrolyte interphase that forms on Li₆PS₅Cl argyrodite solid electrolytes has been reported to continually grow through a diffusion-controlled process, yet this process is not fully understood. Here, we use a combination of electrochemical and X-ray photoelectron spectroscopy techniques to elucidate the role of phosphorus in this growth mechanism. We uncover how Li₆PS₅Cl can decompose at potentials well above the full reduction to Li₃P, forming partially lithiated phosphorus species, Li_xP. We provide evidence of a gradient of Li_xP species throughout the solid electrolyte interphase and propose a growth mechanism in which the rate-determining step is the diffusion of lithium through Li_xP. We predict continuous solid electrolyte interphase growth as long as metallic lithium is present and a Li_xP percolation pathway exists, highlighting the importance of understanding and engineering solid electrolyte interphase composition and nanostructure in solid-state batteries. We believe that this growth mechanism would apply to any solid electrolyte interphase that can contain partially lithiated phosphorus, or potentially any lithium alloy.