A volume-based description of transport in incompressible liquid electrolytes and its application to ionic liquids.

Transference numbers play an important role in understanding the dynamics of electrolytes and assessing their performance in batteries. Unfortunately, these transport parameters are difficult to measure in highly concentrated liquid electrolytes such as ionic liquids. Also, the interpretation of their sign and magnitude has provoked an ongoing debate in the literature further complicated by the use of different languages. In this work, we highlight the role of the reference frame for the interpretation of transport parameters using our novel thermodynamically consistent theory for highly correlated electrolytes. We argue that local volume conservation is a key principle in incompressible liquid electrolytes and use the volume-based drift velocity as a reference. We apply our general framework to electrophoretic NMR experiments. For ionic liquid based electrolytes, we find that the results of the eNMR measurements can be best described using this volume-based description. This highlights the limitations of the widely used center-of-mass reference frame which for example forms the basis for molecular dynamics simulations - a standard tool for the theoretical calculation of transport parameters. It shows that the assumption of local momentum conservation is incorrect in those systems on the macroscopic scale.

Local Volume Conservation in Concentrated Electrolytes Is Governing Charge Transport in Electric Fields.

While ion transport processes in concentrated electrolytes, e.g., based on ionic liquids (IL), are a subject of intense research, the role of conservation laws and reference frames is still a matter of debate. Employing electrophoretic NMR, we show that momentum conservation, a typical prerequisite in molecular dynamics (MD) simulations, is not governing ion transport. Involving density measurements to determine molar volumes of distinct ion species, we propose that conservation of local molar species volumes is the governing constraint for ion transport. The experimentally quantified net volume flux is found to be zero, implying a nonzero local momentum flux, as tested in pure ILs and IL-based electrolytes for a broad variety of concentrations and chemical compositions. This constraint is consistent with incompressibility, but not with a local application of momentum conservation. The constraint affects the calculation of transference numbers as well as comparisons of MD results to experimental findings.

Superionicity in Ionic-Liquid-Based Electrolytes Induced by Positive Ion-Ion Correlations.

In ionic-liquid (IL)-based electrolytes, relevant for current energy storage applications, ion transport is limited by strong ion-ion correlations, generally yielding inverse Haven ratios (ionicities) of below 1. In particular, Li is transported in anionic clusters into the wrong direction of the electric field, requiring compensation by diffusive anion fluxes. Here, we present a concept to exploit ion-ion correlations in concentrated IL electrolytes beneficially by designing organic cations with a Li-coordinating chain. 1H NMR and Raman spectra show that IL cations with seven or more ether oxygens in the side chain induce Li coordination to organic cations. An unusual behavior of an inverse Haven ratio of >1 is found, suggesting an ionicity larger than that of an ideal electrolyte with uncorrelated ion motion. This superionic behavior is consistently demonstrated in both NMR transport/conductivity measurements and molecular dynamics (MD) simulations. Key to this achievement is the formation of long-lived Li-IL cation complexes, which invert the Li drift direction, yielding positive Li+ ion mobilities for the first time in a single IL-solvent-based electrolyte. Onsager correlation coefficients are derived from MD simulations and demonstrate that the main contributions to the inverse Haven ratio, which induce superionicity, arise from enhanced Li-IL cation correlations and a sign inversion of Li-anion correlation coefficients. Thus, the novel concept of coordinating cations not only corrects the unfortunate anionic drift direction of Li in ILs but even exploits strong ion correlations in the concentrated electrolyte toward superionic transport.

Li Coordination of a Novel Asymmetric Anion in Ionic Liquid-in-Li Salt Electrolytes.

We analyze the influence of the asymmetry of the anion on coordination and transport processes in a Li salt/ionic liquid system. The relatively new asymmetric 2,2,2-trifluoromethylsulfonyl-N-cyanamide (TFSAM) anion was investigated in Pyr14TFSAM(1-x)LiTFSAMx over a broad concentration range (up to x = 0.7 Li salt) and was compared to the well-known bis(trifluoromethanesulfonyl)amide (TFSA) anion. In contrast to the TFSA-based system, the system with TFSAM has no phase transition over the whole concentration range. Raman spectroscopy and NMR chemical shifts elucidate the Li coordination in detail. Up to x = 0.3, the asymmetric anion coordinates to Li+ only via the cyano group. With increasing Li salt fraction, the contribution of Li-oxygen coordination increases. This coordination effects influence the transport properties of the system, as examined via pulsed-field-gradient NMR (PFG-NMR). Although the overall diffusivity of both systems is decreasing because of viscosity effects, the relative diffusivity of the Li cation is increasing with x. This suggests a change in the transport mechanism depending on the Li salt fraction. Interestingly, the contribution of structural diffusion at high Li salt concentrations (x ≥ 0.6) seems to be higher in the TFSAM system, influenced by the nonsymmetric coordination, while in the TFSA system, the vehicular transport seems to be still predominant at x ≥ 0.6.

Improved lithium ion dynamics in crosslinked PMMA gel polymer electrolyte.

Since PMMA-based gel polymer electrolytes could substitute PVDF-HFP based gels currently used in Li-ion batteries at lower financial and environmental costs, we investigate here the solvation and transport properties of the lithium ions in a crosslinked PMMA-based gel polymer electrolyte by a combination of thermal and electrochemical methods, Raman spectroscopy, pulse field gradient (PFG) and electrophoretic NMR (eNMR) techniques, as well as ab initio calculations. The conductivity of the gel containing 10 wt% polymer is only reduced by 14% relative to the liquid electrolyte. In addition, the co-solvation by polymer functional groups, a priori expected to slow lithium transport relatively to the anion, has instead a positive effect on lithium transport. Indeed, the ester groups not only participate in lithium solvation and increase ionic dissociation, but since this interaction is rather weak, rather than lowering the lithium diffusion relatively to other species, it mainly decorrelates lithium transport from anionic mobility. Compared to its liquid fraction, the gels show, at the same time, better dissociation and a higher lithium transference number, which results in a higher cationic conductivity, despite the overall conductivity loss.