Impact of UHMW PEO on the ionic speciation and electrochemical properties of EMIC-AlCl3 gel electrolytes

Abstract

Polymer gel electrolytes based on EMIC-AlCl₃ (1:1.5) and ultra-high molecular weight polyethylene oxide (UHMW PEO M_v = 8 × 10⁶ g mol⁻¹) were synthesised with PEO concentrations of 2, 3.5, 5, 10 wt% without the use of auxiliary solvents, by melting the polymer into the electrolyte. This method produced elastic gels with progressively increasing elastic modulus. The speciation of the chloroaluminate anions in the gel electrolytes was characterized by NMR and Raman spectroscopy, revealing a decrease in Al₂Cl₇⁻ and corresponding increase in AlCl₄⁻ with rising PEO content. In particular, the gel with 10 wt% PEO showed no detectable Al₂Cl₇⁻. Electrochemical activity of these gels was evaluated using two configurations: a platinum disc electrode and parallel planar aluminium foil electrode, the latter mimicking a practical electrochemical cell operating geometry. Surprisingly, all gels exhibited electrochemical activity, even in the absence of the Al₂Cl₇⁻ species. Increasing PEO concentration led to reduced current densities but enhanced coulombic efficiency. The study discusses the influence of the molecular weight of PEO on ionic speciation and electrochemical performance of the polymer gel electrolytes, providing insights into the interplay between polymer content, ionic speciation, and electrochemical behaviour. These findings contribute to the development of safer, more sustainable aluminium batteries.