Lithium Transference Numbers in PEO/LiTFSA Electrolytes Determined by Electrophoretic NMR

Recent literature shows a great controversy on the lithium transference number in salt-in-polymer electrolytes, namely Li-bis(trifluoromethanesulfonyl)amide (LiTFSA) in polyethylene oxide (PEO), as different experimental methods and theories yield strongly contradictory results [I. Villaluenga, D. M. Pesko, K. Timachova, Z. Feng, J. Newman, V. Srinivasan and N. P. Balsara, J. Electrochem. Soc. , 165 , A2766 (2018) and K. Pozyczka, M. Marzantowicz, J. R. Dygas and F. Krok, Electrochim. Acta , 227 , 127 (2017)]. Here, we introduce the method of electrophoretic NMR (eNMR) as a model-free approach to shed light on the controversy. Electrophoretic mobilities of the ions in PEO/LiTFSA electrolytes of varying salt concentration are measured in order to directly obtain lithium transference numbers. At the field strengths involved, the mobilities depend on the electric field, due to nonlinear effects which can be cancelled out by an extrapolation to the low field regime. In all cases, the drift direction of lithium is opposite to the TFSA anion and a high effective charge of both ions (> 0.7) indicates mostly uncorrelated ion movement. The resulting lithium transference numbers are around 0.2 and are almost independent on salt concentration. They thus confirm the literature results from Bruce-Vincent methods, but not those from a Newman approach.