Ionic
liquids (ILs) show a promising future as electrolytes in
electrochemical devices. In particular, IL-based electrolytes bring
operations at extreme temperatures to realization that conventional
electrolytes fail to accomplish. Although IL electrolytes demonstrate
considerable progress in high-temperature applications, their breakthroughs
in devices operating at low temperatures are still very limited due
to undesirable phase transitions and unsatisfying transport properties.
In this study, we present an approach where, by tuning molecular interactions
in the system, the designed electrolyte of an IL-based mixture can
reach a lower operating temperature with improved transport properties.
We have discovered that the incorporation of the IL, ethylammonium
nitrate ([EA][N]), can contribute to reforming the molecular interactions
within the system, which effectively resolve the crystallization accompanied
with the excess of water and retain a low glass transition temperature.
The reported liquid electrolyte systems based on a mixture of 1-butyl-3-methylimidazolium
iodide ([BMIM][I]), [EA][N], water, and lithium iodide exhibit a glass
transition temperature below −105 °C. Furthermore, the
optimized electrolyte system shows significant viscosity reduction
and ionic conductivity enhancement from 25 to −75 °C.
The influence is also noticeable on the increased ionicity, which
made the developed electrolyte comparable with other good ILs under
the Walden rule. The electrochemical stability of the electrolyte
system is revealed by a steady and reproducible profile of iodide/triiodide
redox reactions at room temperature over a proper potential window
via cyclic voltammetry. The results from this work not only provide
a potential solution to applications of the iodide/triiodide redox
couple-based electrochemical devices at low temperatures but also
show a practical approach to obtain tailored properties of a mixture
system via modifying molecular interactions.
Imidazolium-based ionic liquids are well known for their versatility as solvents for various applications such as dyesensitized solar cells, fuel cells, and lithium-ion batteries; however, their complex interactions continue to be investigated to further improve upon their design. Ionic liquids (ILs) are commonly mixed with co-solvents such as water, organic solvents, or other ionic liquids to tailor their physiochemical properties. To better predict these properties and fundamentally understand the molecular interactions within the electrolyte mixtures, molecular dynamics (MD) simulations are often employed. In this study, MD simulations are performed on ternary solutions containing ionic liquids of 1-butyl-3-methylimidazolium iodide ([BMIM][I]) and ethylammonium nitrate ([EA][NO 3 ]) with increasing concentration of water. As previously reported, these ternary solutions displayed a wide temperature window of thermal stability and electrochemical conductivity. Utilizing MD simulations, the complex intermolecular interactions are identified, and the role of water as a co-solvent is disclosed to correlate with changes in their bulk properties. The MD results, including simulation box snapshots, radial distribution functions, and self-diffusion coefficients, reveal the formation of heterogeneous regimes with increasing water concentration, hydrogen bonding between iodide−water, iodide−[EA] + , and a change in IL ordering when in mixtures containing water. The simulations also display the formation of water aggregates and networks at high water concentrations, which can contribute to the thermal behavior of the respective mixtures. As the design of IL-based electrolytes grows in demand with increasing complexity, this work demonstrates the capability of MD simulations containing multiple constituents and their necessity in material development through identification of microscopic structure−property relationships.
A designed low-temperature electrolyte of [BMIM][I]/BuCN/LiI extends the liquidus range down to −150 °C. The complex interactions between imidazolium/iodide ions and nitrile solvent molecule results in enhancement of thermal and transport properties.
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