Interface and Safety Properties of Phosphorus-Based Negative Electrodes in Li-Ion Batteries

Abstract

Phosphorus is considered as a promising candidate for the replacement of graphite as the active material in Li-ion battery electrodes owing to its 6-fold higher theoretical specific charge. Unfortunately, phosphorus-based electrodes suffer from large volume changes upon cycling, leading to poor electrochemical performance. Furthermore, red phosphorus (P_red) is known to release phosphine gas (PH₃) once in contact with water (even at the ppm level), and thus, its safety profile needs to be assessed. In this context, the electrolyte/electrode interface of a P_red electrode during the first lithiation is fully investigated using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and online electrochemical mass spectroscopy (OEMS). The XPS analyses reveal that, at potentials higher than 1 V vs Li⁺/Li, the P_red starts to react via the outermost surface layer, which is mainly composed of the native oxide, P₂O₅, to form H₃PO₄. Once this surface oxide is consumed, the P_red reacts with moisture and the electrolyte, resulting in the re-formation of H₃PO₄ and the release of the toxic PH₃ as identified by OEMS. At potential lower than 1 V, a solid electrolyte interphase (SEI) develops on the top of H₃PO₄ as identified by XPS analyses. This SEI prevents further degradation of the P_red and inhibits PH₃ release. Following the lithiation, the reaction of Li with Pred generates particle fracture (identified by SEM) and the transformation of H₃PO₄ into Li₃PO₄ is also noticed. Understanding and monitoring the role of the decomposition products and processes within the battery is crucial to further improving battery performance and safety.