From partial to complete neutralization of 2,5-dihydroxyterephthalic acid in the Li-Na system: crystal chemistry and electrochemical behavior of Na(2)Li(2)C(8)H(2)O(6)vs. Li

The 2,5-dihydroxyterephthalic acid (H-4-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., beta-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 Na-2(Li-2)-p-DHT (Na2Li2C8H2O6) and to test it electrochemically vs. Li. Depending on the experimental conditions, two Li-polymorphs and one Na compound, containing H-2-p-DHT2- ligands with deprotonated carboxylic groups, were obtained, namely, M-2(H-2)-p-DHT(H2O)(4) M = Li (1, 2) and Na (3) as well as the first mixed alkali metal compound, i.e. Na-2(Li-2)-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 infinity[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 Na-2(Li-2)-p-DHT compound measured in Li half-cell revealed however poor performances compared to Li-4-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 Li-4-p-DHT upon cycling.