Polymerization of aluminate monomer in its initial nucleation stage of organic alkali solution revealed by ReaxFF molecular dynamics simulation

Properties and composition of the spent electrolyte for premium circulation mediated by Al-air batteries

Al-air batteries can serve as a bridge for high-quality cyclic utilization of aluminum resources. However, limited insights into the spent electrolyte are challenging to accurately adjust the recovery process to obtain premium Al-containing products. Herein, the properties and composition of the spent electrolyte were explored through experiments and theoretical calculations. The results demonstrate that the viscosity of the spent electrolyte increased with the rise in discharge current density, time and temperature under highly alkaline conditions, while the ionic conductivity and causticity obviously decreased. Al(OH)₄^- was the prime and balanced aluminate species when the battery was discharged at 25 °C and coexisted with a bit of [Al₂O(OH)₆]^2-, [Al₂O₂(OH)₆]^4- and Al(OH)₆^3- ions. Especially, the characteristics of the spent electrolyte were mainly dominated by the discharge time and temperature when the current density was continuously increased. There was only Al(OH)₄^- in the electrolyte at a higher discharge temperature. The DFT results also reveal that the polynuclear aluminate ions were produced by the interaction between the mononuclear aluminate ion Al(OH)₄^- and OH^-. This work manifests a profound insight into the spent electrolyte from Al-air batteries for the efficient recycling of aluminum resources.

Theory-Guided Inelastic Neutron Scattering of Crystalline Alkaline Aluminate Salts Bearing Principal Motifs of Solution-State Species

Aluminate salts precipitated from caustic alkaline solutions exhibit a correlation between the anionic speciation and the identity of the alkali cation in the precipitate, with the aluminate ions occurring either in monomeric (Al(OH)₄^-) or dimeric (Al₂O(OH)₆^2-) forms. The origin of this correlation is poorly understood as are the roles that oligomeric aluminate species play in determining the solution structure, prenucleation clusters, and precipitation pathways. Characterization of aluminate solution speciation with vibrational spectroscopy results in spectra that are difficult to interpret because the ions access a diverse and dynamic configurational space. To investigate the Al(OH)₄^- and Al₂O(OH)₆^2- anions within a well-defined crystal lattice, inelastic neutron scattering (INS) and Raman spectroscopic data were collected and simulated by density functional theory for K₂[Al₂O(OH)₆], Rb₂[Al₂O(OH)₆], and Cs[Al(OH) ₄]·2H₂O. These structures capture archetypal solution aluminate species: the first two salts contain dimeric Al₂O(OH)₆^2- anions, while the third contains the monomeric Al(OH)₄^- anion. Comparisons were made to the INS and Raman spectra of sodium aluminate solutions frozen in a glassy state. In contrast to solution systems, the crystal lattice of the salts results in well-defined vibrations and associated resolved bands in the INS spectra. The use of a theory-guided analysis of the INS of this solid alkaline aluminate series revealed that differences were related to the nature of the hydrogen-bonding network and showed that INS is a sensitive probe of the degree of completeness and strength of the bond network in hydrogen-bonded materials. Results suggest that the ionic size may explain cation-specific differences in crystallization pathways in alkaline aluminate salts.