19F and 31P Solid-State NMR Characterization of a Pyridine Pentafluorophosphate-Derived Solid-Electrolyte Interphase

Solid-state NMR spectroscopy was used to characterize the solid-electrolyte interphase (SEI) formed on the graphite electrode surface from the electrolyte additive pyridine pentafluorophosphate (PPF). The PPF-derived SEI was prepared in a full Li(Ni0.4Mn0.4Co0.2)O-2/graphite cell, rather than in a coin cell or using a bench-top synthesis approach. F-19 and F-19 -> P-31 cross-polarization measurements provide direct evidence that F atoms and P-F bonds are present at the graphite surface. By comparing rinsed and unrinsed samples, an insoluble SEI component was differentiated from residual LiPF6 from the dried electrolyte. The SEI species contains F and P chemical environments that are similar to, but distinct from, the additive starting material and the lithium hexaflurophosphate (LiPF6) electrolyte salt used in this work. The results are consistent with the previously proposed formation of a dilithium 4,4'-bipyridine-N, N'-bis(pentafluorophosphate) salt. LiF was also observed and is attributed to decomposition of the PPF-derived surface layer and/or the LiPF6 electrolyte salt. (C) The Author(s) 2017. Published by ECS. All rights reserved.