Quaternary Electrolyte Mixtures for Rechargeable Aluminum-Sulfur Batteries
Authors: Bhalekar Snehal Sakharam , Messinger Robert J. 
Published: December 2024

Rechargeable aluminum-sulfur (Al-S) batteries hold great potential due to the abundance, safety, low cost, and high gravimetric capacity of Al metal and elemental sulfur (2980 mAh g-1 and 1675 mAh g-1, respectively). Despite these advantages, technological development of Al-S batteries faces challenges due to the limited number of electrolytes that not only enable the reversible electrostripping of Al metal and concomitant electroreduction of sulfur, but also yield small polarization losses. Currently, Lewis acidic mixtures of AlCl3 in 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) are the state-of-the-art electrolytes used in Al metal batteries, including Al-S batteries, though polarization losses in Al-S cells are large at room temperature. Recently, alkali chloroaluminate molten salts (NaCl-KCl-AlCl3) were tested in Al-S batteries at 110 °C, resulting in a large decrease in polarization compared to AlCl3-[EMIm]Cl electrolytes, even at elevated temperatures [1]. However, the link between chloroaluminate anion speciation and dynamics, temperature, and overpotential for sulfur reduction and oxidation is poorly understood.
Here, we prepare quaternary electrolyte mixtures composed of AlCl3, NaCl, KCl, and either [EMIm]Cl, urea, or LiCl to reduce the liquid temperature window and study how ion speciation and dynamics affect polarization losses in rechargeable Al-S batteries as a function of temperature. All mixtures were compared to AlCl3-[EMIm]Cl (1.5:1 molar ratio) and NaCl-KCl-AlCl3 (26:13:61) molar ratio mixtures as baseline electrolytes. Differential scanning calorimetry (DSC) was performed to measure thermodynamic phase transitions and the liquid temperature windows of these electrolytes. Liquid-state 27Al single-pulse and relaxation nuclear magnetic resonance (NMR) measurements were performed to characterize chloroaluminate anion speciation environments, and dynamics up to 110 °C. In addition, liquid-state 1H single-pulse, relaxation, and pulsed-field-gradient (PFG) NMR experiments were performed to measure the local environments, dynamics, and diffusion coefficients of organic species (e.g., [EMIm]Cl, urea).
The electrolytes were tested electrochemically in rechargeable Al-S batteries by performing galvanostatic cycling and cyclic voltammetry up to 110 °C, revealing the effects on cell polarization as well as specific capacity and cyclability. Furthermore, solid-state 27Al NMR measurements on discharged sulfur electrodes containing residual electrolyte [2] revealed how electrolyte composition affects the average environments of rapidly exchanging electrolyte-soluble sulfur species (SxAlCl4)y−). Overall, the results yield design strategies for electrolyte design in rechargeable Al-S batteries to reduce the operating temperature, reduce the polarization loss, and enhance energy efficiency.

References

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“Soluble electrolyte-coordinated sulfide species revealed in Al-S Batteries by nuclear magnetic resonance spectroscopy,” Rahul Jay, Ankur L. Jadhav, Leo W. Gordon, Robert J. Messinger, Chem. Mater., 2022, 34, 4486-4495.