Electrolytes for Aluminum Ion Batteries: A Molecular Dynamics Study

A vital component in the fight against climate change is the development of highly efficient energy storage for renewable resources. The aluminum ion battery (AIB) is a promising technology, but there is a lack of understanding of the desired nature of the batteries’ electrolytes. These properties cannot simply be extrapolated from other metal ion batteries, as the ionic charge carriers in these batteries are not simply Al3+ ions but the anionic AlCl4− and Al2Cl7−, which form in the electrolyte. This study aims to illustrate the effect of mole ratios and organic solvents to improve the AIB electrolytes with the aid of computational techniques. To this end, molecular dynamics simulations were carried out on varying ratios forming acidic, neutral and basic mixtures of the AlCl3 salt with 1-ethyl-3-methylimidazolium chloride (EMImCl) ionic liquid (IL) and an organic solvent electrolyte (dichloromethane (DCM) or toluene). The data obtained from both viscosity and diffusion calculations indicate that the solvents could improve the transport properties. Both DCM and toluene lead to lower viscosities, higher diffusion coefficients, and higher conductivity. Detailed calculations demonstrated solvents can effectively improve the formation of AlCl3···Cl (AlCl4−) and AlCl4− …AlCl4− (Al2Cl7−) especially in acidic mixtures. Densities which were averaged around 1.25 g/cm3 for pure electrolyte mixture of AlCl3-EMImCl were of comparable values to the experimental reports. These results are all in agreement with experimental findings, and strongly suggest that DCM in acidic media with AlCl3 and EMImCl might provide a promising basis for battery development.