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

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 mas's 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 P-red 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.