  The transport coefficients, in particular the transference number, of
electrolyte solutions are important design parameters for electrochemical
energy storage devices. Recent observation of negative transference numbers in
PEO-LiTFSI under 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 gap when comparing experiment and
simulation, because the fluxes in the experimental measurements of transport
coefficients and in the linear response theory used in the molecular dynamics
simulation are defined in different RFs. In this work, we show that by applying
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 the reported negative transference numbers.
