From partial to complete neutralization of 2,5-dihydroxyterephthalic acid in the Li–Na system: crystal chemistry and electrochemical behavior of Na2Li2C8H2O6vs. Li

Abstract

The 2,5-dihydroxyterephthalic acid (H₄-p-DHT) is of special interest in the field of materials science because of the two symmetric sets of oxygen donor functional groups (i.e., β-hydroxy acid moieties). In its partially (–2H) or totally (–4H) deprotonated form, it is an efficient organic ligand making the synthesis of numerous coordination polymers possible. In addition, it exhibits dual redox-active properties, which make it very interesting as electrode material when combined with alkali and alkaline-earth elements. Herein, we report on a specific study on alkali salts obtained from partial to complete neutralization of 2,5-dihydroxyterephthalic acid in the Li–Na system, which notably enable us to prepare the mixed phase Na₂(Li₂)-p-DHT (Na₂Li₂C₈H₂O₆) and to test it electrochemically vs. Li. Depending on the experimental conditions, two Li-polymorphs and one Na compound, containing H₂-p-DHT²⁻ ligands with deprotonated carboxylic groups, were obtained, namely, M₂(H₂)-p-DHT(H₂O)₄ M = Li (1, 2) and Na (3) as well as the first mixed alkali metal compound, i.e. Na₂(Li₂)-p-DHT(H₂O)₈ (4). The single crystal structure analyses showed that all compounds display various inorganic motifs with discrete LiO₄ tetrahedra (1, 2), Li₂O₆ dimers of edge-sharing LiO₄ tetrahedra (2), ¹_∞[NaO₄]⁻⁷ 1-D chains of edge-sharing NaO₆ octahedra (3) and [Li₂Na₂O₁₄]²⁴⁻ clusters of LiO₄ tetrahedra and NaO₆ octahedra (4). The electrochemical assessment of the anhydrous Na₂(Li₂)-p-DHT compound measured in Li half-cell revealed however poor performances compared to Li₄-p-DHT (Li₄C₈H₂O₆) due to cationic disorder of the two alkali ions in the crystal structure giving rise to the progressive conversion of the pristine phase to Li₄-p-DHT upon cycling.