Fig. 3: Structural insights, reaction mechanisms, and performance of molten salt and ionic liquid electrolytes.

a Tetrahedral phase diagram representing the quaternary alkali-metal-based inorganic molten salt electrolyte system, elucidating the compositional design space and electrolyte formulation strategies. The presence of high-order chloroaluminate clusters (Al_nCl_3n+1−) with extended charge delocalization highlights their pivotal role in ionic conductivity and electrochemical stability. b Structural representations of key chloroaluminate anionic clusters—AlCl₄⁻ (black), Al₂Cl₇⁻ (cyan), Al₂Cl₆ (yellow), and Al₃Cl₁₀⁻ (red)—demonstrating the structural diversity of Al-based speciation in molten salt environments. Cationic species (Li⁺, Na⁺, K⁺) are omitted for visual clarity. c Rate performance of Al|Al symmetric cells employing quaternary alkali melt and ionic liquid-based electrolytes under varying current densities from 0.5 to 5 mA cm⁻², maintaining a fixed plating/stripping areal capacity of 1 mAh cm⁻², demonstrating electrolyte robustness and kinetic compatibility. d Sulfur K-edge X-ray absorption near-edge structure (XANES) spectra collected at various states of discharge and charge, showing the evolution among major sulfur species: neutral sulfur (S⁰, black), Al₂(S_n)₃ intermediates (red and blue), and terminal Al₂S₃ (orange). These spectral fingerprints provide mechanistic insights into the redox pathways and reversibility¹². e Voltage-capacity profiles of Al–Se batteries using both molten salt and ionic liquid chloroaluminate electrolytes, with performance compared under various charging rates at a constant discharge rate (D/10) at 180 °C, revealing the impact of electrolyte composition on rate-dependent behavior. f Operando Se K-edge XANES spectra (μ(E) vs. incident photon energy E) of the selenium cathode during discharge. Left: stacked spectra indicating evolution of electronic structure; right: contour map visualizing spectral transitions. The magnified region highlights isosbestic points, evidencing a clean two-phase reaction with preserved stoichiometry across initial discharge states⁵. g Radial distribution functions (RDFs) derived from ab initio molecular dynamics (AIMD) simulations, illustrating coordination environments and atomic correlations in the molten electrolyte: Al–S, S–Li, and S–S pair interactions⁴². h UV–vis absorption spectra of discharged sulfur cathodes at a capacity of 350 mAh g⁻¹ in different electrolyte configurations: (A) Li | LiCF₃SO₃ (DME/DOL) | S cell, (B) Al | [EMIM]Cl–AlCl₃ | S cell, and (C) hybrid Al | Li⁺–[EMIM]Cl–AlCl₃ | S system. The spectra reveal solvent- and ion-specific electronic transitions correlating with polysulfide speciation⁴². i Proposed redox reaction pathway schematic for the sulfur cathode and aluminum anode, encompassing the sulfur oxidation and reduction steps in molten-salt-facilitated Al–S batteries. The mechanistic flow delineates intermediate phase transitions, ultimately yielding reversible conversion between S⁰ and Al₂S₃³⁵.