Transference Number in Polymer Electrolytes: Mind the Reference-Frame Gap

The transport coefficients, in particular the transference number, of electrolyte solutions are important design parameters for electrochemical energy storage devices. The recent observation of negative transference numbers in PEO-LiTFSIunder certain conditions has generated much discussion about its molecular origins, by both experimental and theoretical means.However, one overlooked factor in these efforts is the importance of the reference frame (RF). This creates a non-negligible gapwhen comparing experiment and simulation because thefluxes in the experimental measurements of transport coefficients and in thelinear response theory used in the molecular dynamics simulation are defined in different RFs. In this work, we show that, byapplying a proper RF transformation, a much improved agreement between experimental and simulation results can be achieved.Moreover, it is revealed that the anion mass and the anion-anion correlation, rather than ion aggregates, play a crucial role for thereported negative transference numbers