General Trend of Negative Transference Number in Li Salt/Ionic Liquid Mixtures

Abstract

We show that strong cation-anion interactions in a wide range of lithium-salt/ionic liquid mixtures result in a negative lithium transference number, using molecular dynamics simulations and rigorous concentrated solution theory. This behavior fundamentally deviates from the one obtained using self-diffusion coefficient analysis and agrees well with experimental electrophoretic NMR measurements, which accounts for ion correlations. We extend these findings to several ionic liquid compositions. We investigate the degree of spatial ionic coordination employing single-linkage cluster analysis, unveiling asymmetrical anion-cation clusters. Additionally, we formulate a way to compute the effective lithium charge that corresponds to and agrees well with electrophoretic measurements and show that lithium effectively carries a negative charge in a remarkably wide range of chemistries and concentrations. The generality of our observation has significant implications for the energy storage community, emphasizing the need to reconsider the potential of these systems as next generation battery electrolytes.<br>

Ion Transport and the True Transference Number in Nonaqueous Polyelectrolyte Solutions for Lithium Ion Batteries

“…However, the distinct terms typically decrease both the conductivity and cation transference number relative to the ideal case. 78,79 While an analysis of the conductivity in this manner is more common for conventional, low-molecular-weight salt electrolytes, the framework here is consistent with that often used in the polyelectrolyte community in which the total conductivity is expressed as the product of the ideal solution conductivity and an interaction parameter capturing interionic friction and ion pairing effects. 80 It has been shown by Vink 81 that these expressions for polyelectrolyte conductivity can be derived from linear irreversible thermodynamics, the same starting point for deriving the Green−Kubo relations on which the conductivity analysis in this work is based.…”

“…However, the distinct terms typically decrease both the conductivity and cation transference number relative to the ideal case. 78,79…”

“…In this case, t NMR will correspond to the true transference number. However, the distinct terms typically decrease both the conductivity and cation transference number relative to the ideal case. , …”