Quantum and Classical Molecular Dynamics of Ionic Liquid Electrolytes for Na/Li‐based Batteries: Molecular Origins of the Conductivity Behavior

Abstract

Compositional effects on the charge-transport properties of electrolytes for batteries based on room-temperature ionic liquids (RTILs) are well-known. However, further understanding is required about the molecular origins of these effects, in particular regarding the replacement of Li by Na. In this work, we investigate the use of RTILs in batteries, by means of both classical molecular dynamics (MD), which provides information about structure and molecular transport, and ab initio molecular dynamics (AIMD), which provides information about structure. The focus has been placed on the effect of adding either Na(+) or Li(+) to 1-methyl-1-butyl-pyrrolidinium [C4 PYR](+) bis(trifluoromethanesulfonyl)imide [Tf2 N](-) . Radial distribution functions show excellent agreement between MD and AIMD, which ensures the validity of the force fields used in the MD. This is corroborated by the MD results for the density, the diffusion coefficients, and the total conductivity of the electrolytes, which reproduce remarkably well the experimental observations for all studied Na/Li concentrations. By extracting partial conductivities, it is demonstrated that the main contribution to the conductivity is that of [C4 PYR](+) and [Tf2 N](-) . However, addition of Na(+) /Li(+) , although not significant on its own, produces a dramatic decrease in the partial conductivities of the RTIL ions. The origin of this indirect effect can be traced to the modification of the microscopic structure of the liquid as observed from the radial distribution functions, owing to the formation of [Na(Tf2 N)n ]((n-1)-) and [Li(Tf2 N)n ]((n-1)-) clusters at high concentrations. This formation hinders the motion of the large ions, hence reducing the total conductivity. We demonstrate that this clustering effect is common to both Li and Na, showing that both ions behave in a similar manner at a microscopic level in spite of their distinct ionic radii. This is an interesting finding for extending Li-ion and Li-air technologies to their potentially cheaper Na-based counterparts.

Molecular Dynamics Analysis of Charge Transport in Ionic‐Liquid Electrolytes Containing Added Salt with Mono, Di, and Trivalent Metal Cations

“…We have previously demonstrated that the depletion of the conductivity upon the addition of salt is caused by the formation of aggregates or clusters composed by the metal cations and [Tf 2 N] − anions . These clusters induce significant changes in the microscopic structure of the electrolytes, hindering the mobility of ions and consequently reducing the conductivity.…”

“…The increasing requirements and power of these batteries, and disadvantages such as degradation or high flammability, make essential the search of alternative materials . Thanks to the high abundancy of Na as a raw material, Na‐ion batteries have attracted intense attention as potential candidates for the replacement of Li‐ion batteries . Unfortunately, Na‐ion batteries exhibit poor cycle stability and face a trade‐off between power density and energy.…”

“…Despite the above‐mentioned studies, the microscopic structure and the transport properties of electrolytes for alternative metal‐ion batteries are still poorly understood. In our previous work, we used molecular dynamics (MD) simulations to study the microscopic behavior of [C 4 PYR] + [Tf 2 N] − IL‐based electrolytes for Li‐ion and Na‐ion batteries. We identified the progressive formation of Li + /Na + complexes as the origin of the anomalous depletion of the conductivity upon salt addition.…”

Electrochemical Reduction of Oxygen in Aprotic Ionic Liquids Containing Metal Cations: A Case Study on the Na–O2 system

“…The molecular models for PYR14TFSI based on Lennard-Jones and Coulombic interactions were taken from Vicent-Luna et al [40,41] who demonstrated the ability of these models to reproduce the behavior of ILbased electrolytes is. The molecular models for PYR14TFSI based on Lennard-Jones and Coulombic interactions were taken from Vicent-Luna et al [40,41] who demonstrated the ability of these models to reproduce the behavior of ILbased electrolytes is.…”

Electrolytes, SEI Formation, and Binders: A Review of Nonelectrode Factors for Sodium‐Ion Battery Anodes

“…The same attributes that make organic‐based solvents suitable electrolytes, such as high ionic conductivity, low viscosity, and high charge transfer numbers, are also ones that ILs lack at room temperature. As demonstrated in a MD‐based paper by Anta and co‐workers a lot of these less favorable transport properties are due to increased ionic correlation, which leads to larger clusters, and as a result, more sluggish diffusion in the bulk . This is especially true if the IL is composed of ions with long alkyl chains, as they promote micelle formation, which negatively affects the transport …”