Phosphorus Enrichment as a New Composition in the Solid Electrolyte Interphase of High-Voltage Cathodes and Its Effects on Battery Cycling

Immersion of a solid into liquid often leads to the modification of both the structure and chemistry of surface of the solid, which subsequently affects the chemical and physical properties of the system. For the case of the rechargeable lithium ion battery, consquence of such a surface modification is termed as solid electrolyte interphase (SEI) layer, which has been perceived to play a critical role in the stable operation of the batteries. However, the structure and chemical composition of the SEI layer and its spatial distribution and dependence on the battery operating conditions remain unclear. Using aberration-corrected scanning transmission electron microscopy, coupled with ultrahigh-sensitivity energy-dispersive X-ray spectroscopy, we probed the structure and chemistry of the SEI layer on several high-voltage cathodes. We show that layer-structured cathodes, when cycled at a high cut-off voltage, can form a Phosphorus-rich SEI layer on their surface, which is a direct evidence of Li-salt (LiPF 6 ) decomposition. Our systematical investigations indicate that such a cathode/Li-salt side reaction shows strong dependence on the structure of the cathode materials, the operating voltage, and the temperature, indicating the feasibility of SEI engineering. These findings provide valuable insights on the interaction between the high-voltage cathode and the electrolyte, as well as the interface evolution upon battery cycling.