The Role of Phosphorous 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 a potential well above the full reduction to Li3P, forming partially lithiated phosphorus species LixP. We provide evidence of a gradient of LixP species throughout the SEI thickness, leading to a diffusion-limited growth. We predict continuous SEI growth as long as lithium metal is present.