Probing and understanding performances of conductive organic radical salts as positive electrode materials for anion-ion and dual-ion batteries

Abstract

We report here our investigations to evaluate the potential use of conductive 2 : 1 (TMTTF)₂X radical salts (TMTTF = tetramethyltetrathiafulvalene; X = PF₆⁻, ClO₄⁻, BF₄⁻, AsF₆⁻) as positive organic electrode materials for energy storage applications. To do so, a scaled-up electrocrystallization method as well as a large-scale chemical oxidation process were developed to obtain these salts in large quantities (>0.5 g per batch) needed for such applications. A new polymorph of the neutral TMTTF and a new 1 : 1 bromide phase isolated from large-scale chemical oxidation are also described. These crystalline 2 : 1 radical salts exhibit a high electronic conductivity (0.25–4.60 S cm⁻¹) originating from π-electron delocalization within the organic molecular stacks formed by mixed-valence TMTTF salts. Galvanostatic cycling experiments, conducted with or without a carbon additive were combined with X-ray diffraction measurements and Fourier-transform infrared spectroscopy, allowing for highlighting and monitoring the structural and redox changes occurring during cycling in a dual-ion configuration. This study demonstrates that, beyond the need for electronic conductivity, other parameters and properties need to be combined to obtain high-performance organic materials, paving the way toward more efficient organic electrode materials.