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, LixP. We provide evidence of a gradient of LixP species throughout the solid electrolyte interphase and propose a growth mechanism in which the rate-determining step is the diffusion of lithium through LixP. We predict continuous solid electrolyte interphase growth as long as metallic lithium is present and a LixP 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.
